o o» ^-<o o ^ o O" u> VII 4»
liiliil
liilllllhlli Uil II l l i l i IjjjilSIiniliil
•o CD IJl BIBLIOTHEEK TU Delft P 1250 3284 C 346946BIBLIOTHEEK DER TECHNISCHE HOGESCHOOL DELFT
THE CONSTRUCTION OF
AND MEASUREMENTS
WITH A BETA-RAY SPECTROMETER
FOR ELECTRON-GAMMA
CORRELATIONS
THE CONSTRUCTION OF
AND MEASUREMENTS
WITH A BETA-RAY SPECTROMETER
FOR ELECTRON-GAMMA
CORRELATIONS
P R O E F S C H R I F T
TER VERKRIJGING VAN DE GRAAD VAN DQPTOR IN DE TECHNISCHE WETENSCHAPPEN AAN DE TECHNISCHE HOGESCHOOL DELFT OP GEZAG VAN DE RECTOR MAGNIFICUS IR. H. J. DE WIJS, HOOGLERAAR IN DE AFDELING DER MIJNBOUWKUNDE, VOOR EEN COMMIS-SIE UIT DE SENAAT TE VERDEDIGEN OP DONDERDAG
2 MAART 1967 TE 14 UUR.
door
HANS VAN KRUGTEN
^ ^ ^ - ; • - « « e n t . ^ natuurkundig ingenieur / ,< * o \ geboren te 's-Gravenhage "' X J.2- -,•':-„•• J. 01 c^ ' • Lri.t "
1150
Z^Q'^ '^
OFFSET-DRUKKERIJ . L U N A . - DELFT '^j.,t ODit proefschrift is goedgel<eurd door de promotor Prof. dr. A. H. Wapstra.
Aan mijn ouders. Aan Jacqueline,
CONTENTS. Chapter I. Introduction
1. Requirements
2. Choice of a spectrometer Chapter IL Theory of the spectrometer
1. General theory for a H = ^.Q/T field
2. Special theory for a spectrometer with a boimdary curve 3. Focusing properties
4. Fringing fields
Chapter IE. Design, construction and investigation of the spectrometer 1. Introduction
2. Description of the sp)ectrometer 2 . 1 . Magnet system
2. 2. Source holder and sources 2 . 3 . Detector assembly
2.4. Vacuum system 2 . 5 . Power supply 3. The focusing
3 . 1 . Shaping of the boimdary curves of the pole faces 3. 2. Focus forms and particle trajectories
3 . 3 . Baffles and shields
4. Final behaviour of the spectrometer 4 . 1 . Resolution and transmission 4. 2. Hysteresis effects
4 . 3 . Effect of stray field upon the second si)ectrometer 5. Magnetic field measuring system
5 . 1 . Measuring apparatus 5.2. Temperature stabilization
Chapter IV. Vacuum deposition apparatus and source techniques 1. Introduction
2. Construction of the apparatus 3. Pe rforman ce
Chapter V. Measurements with the spectrometer 1. Theory
1.1. Introduction 1.2. Beta decay
1.3. Coincidence measurements 1.4. Correlation experiments
2. Electronics
3. Measurements of beta continua in the decay of ^^"Ac and the daughter products of ^^^Ac
4. Investigation of the shape of the beta continuum from the decay of •*^^^Au
5. Measurements on the decay of •'^^^Eu 5 . 1 . Introduction
5.2. Beta decay
5 . 3 . The K/L and L/M ratios of the 123 keV transition 5.4. Y (1278 keV) - Y_(123 keV) correlation
5.5. Y (1278 keV) - 0^(123 keV) correlation and Y(1278 keV) - eL(123 keV) correlation
5. 6. Particle parameters for the e~(123 keV ) transition
Summary
Ned. Samenvatting References
CHAPTER I INTRODUCTION 1^. Requirements.
The intention was to build a magnetic beta ray spectrometer suitable for several purposes. In the first place the apparatus must be suitable for mea-surements of beta continua and internal conversion electron lines. Second-ly, it should be possible to perform coincidence measurements. These should cover coincidences between gamma rays and positons, negatons or internal conversion electrons as well as coincidences between beta rays and conversion electrons. Furthermore, the possibility of electron-gamma or electronelectron directional correlation measurements should be p r o -vided. These different tjrpes of measurements determine the properties which such a spectrometer should have..
First, since one asks a reasonable counting rate for most .of the above mentioned experimental methods, the transmission has to be relatively high. This is especially of importance in the case of the coincidence set-up, since the coincidence counting rate is proportional to the transmission of the spectrometer. The strength of the source is in general limited by the ratio of true to accidental coincidences (see chapter V).
To limit the statistical e r r o r s one has to collect a certain minimum amount of counts per point, prescribed by each individual experiment. As to avoid very long measuring times (which in the case of isotopes with a rather short half-live would fully exclude the experiment) the counting rate in the beta channel has to be sufficiently high. This can be reached by using strong sources. However, many times the source strength is limited. Thus, one of the most important requirements of the spectrometer is a high transmission.
Another feature we need is the separation in the apparatus of positons and negatons, which is of special interest when we have an isotope that emits positons as well as negatons. This is of importance, when measuring in-ternal conversion lines in the decay of a positon emitting isotope: it will lower the "background" and therefore improve the detection and m e a s u r e -ment of weak lines.
For coincidence measurements it is also necessary to place a second sjjec-trometer (Nal(Tl) crystal, or an anthracene-or a plastic scintillator equipped with a photomultiplier) near the radioactive source. Since the gain of a photomultiplier is very sensitive to strong magnetic fields it is advantageous to use a beta ray spectrometer in which the source is placed outside the direct field. Remaining fringing fields can be made unobjection-t able by u s i i ^ light-pipes and mu-metal shields in order to decrease the in-fluence of these fields on the photomultipliers.
To record directional correlations it will be necessary to be able to place the gamma- or the electron-spectrometers at various positions aroimd the source to allow measurements at several angles with the mean direction of electrons detected in the beta ray spectrometer.
Finally, we have to consider the resolution of the spectrometer. This is of importance for the recording of close-ljong conversion lines. When measuring coincidences or directional correlations the resolution plays a role too. With a good resolution it is possible to select a
lar transition from a decay scheme; thus more specific information can be obtained (see chapter V).
Summarizing, the principle characteristics needed a r e : a. reasonably high transmission
b . comparatively good resolution
c. effective way of positon-negaton separation
d. source position outside the direct magnetic field (for coincidence mea-surements etc).
e. possibility for turning the second spectrometer around the source. 2. Choice of a spectrometer.
Using the requirements of paragraph 1 of this chapter, we can exclude immediately several types of spectrometers.
First of all^ '"2 double focusing spectrometers since they have the source inside the spectrometer field and ho possibility for angular correlation measurements.
Secondly all types of lens-spectrometers. The long lenses have the source inside the spectrometer field and vacuumtank and so no possibility for mea-suring correlations. The short lenses have a poor transmission. Medium thick lens spectrometers can be used (and are used), but their transmis-sion is poorer *) than that of the sector field instruments. Sector field in-struments are much more suitable. Of these the ones with a uniform magne-tic field and with a double focusing field have been used for correlation pur-poses (Sakai et al (Sa 60)). Prismatic spectrometers could be used too. However, generally they need smaller sources which is in most cases a disadvantage (this will be discussed in chapter IH paragraph 2.2.). Further-more, the transmission is lower than of one of the type we have chosen. A sectorfield spectrometer with inclined pole faces seemed to meet all our r e -quirements, as mentioned in paragraph lof this chapter. A transmission ranging from 1% to 2% with corresponding resolutions of about 1% to 2% seemed possible. The next point which has to be decided on is whether to construct an iron containing spectrometer or an iron-free one. The last type, however, implies a very careful shielding from external magnetic fields, such as iron containing material in the neighbourhood etc. For really good working conditions in that case we should need an iron-free building. This is very hard (and very expensive) to realize, thus we deci-ded to build an iron containing instrument.
*) A few years ago (after we started constructing our spectrometer) Siegbahn (Si 63) designed a lens-type spectrometer which seems to suit most of the requirements very well.
1.2. Special theory f or a st)ectrometer with a boundary curve.
If we assume a field satisfying equations (1) we obtain the radius of curva-ture (P) of the orbits from the Lorentz-force and the centrifugal force
B e v = S x ' (8)
Thus,
' - ^ = h - ^ - <9)
in which
•^=11? (10)
Here P is proportional to r , and the constant K depends only on the ratio of the momentum p and the magnetic field B.
For the trajectories of the electrons we have the foUowii^ differential equation
By integrating this equation we get / log ^ dq
4K2-(loga)2|. o
where a and z are integration constants.O
If a suitable value for K is chosen it can be shown that the electrons in such a field perform loops parallel to the z-axis (see fig. 3). The curves described by equation (12) can easier be represented by parameter formu-las in the angle e between the z-axis and the tangent of the curve:
z = a K U ( K , e ) + z (14) -K cose ,,,.,
r = a e (15) with U (K, e ) = lcosx.e~^°°^^dK (16)
Here z = z for e = n. It appears that a = r for 9 = ^ and
-K +K
r . = ae . r = ae min ' max (17) So = e and is only depending on K.
max
9 =7r
- • z
fig. 3 An electron trajectory in a field B = B . / r with K = 0. 6.
fig. 4 A symmetrical set of orbits with K = 1.
From equations (14) and (15) we find that the orbits of the electrons in the field are periodic in the direction of the z-axis.
z( 9 - 2iT) = z( 9 ) =-2TriaKJ^(iK) = 2 4
= 2 T r a K 2 ( l + ^ + ^ 2 ^ ) ^®) Here J., is the first order Bessel function of the first kind.
The parameters a and K describe the form of the particle trajectories. For the computation of the function U(K, 9 ) we can expand the latter in a series of Bessel-functions:
U(K, e ) = i J ^ ( i K ) ( ^ 9 )
-°° »
TZ n <i)°"'{jn-l (^K) -Jn.l ^"^j «^ ' (1»)
n = 1
These U functions are tabulated by Jaffey et al (Ja 60). Similar functions were tabulated by Burgov et al (Bu 61).
If we take a set of orbits for which z at the point of r = rjnax is ^ function of r^jja^x' i- ®' ^ function of a, then we can denote these orbits by y(a) E z(for r = rjnax)- ^^^ ^ symmetrical set of orbits y(a) = o. This converts formulas (14) and (15) into:
z = a K U (K, 9 ) + y (a) -Kcos e
(20)
r = a e 1.3. Focusing propertie s'.
If we put the electron-emitting source in the point z = z^ (see fig. 4) and we choose a one-parameter set of curves determined by z , we can construct the boimdary curve by drawing the tangents from z„ to each of the curves. This boimdary curve will correspond to the pole pieces of the spectrometer. The coordinates (z, r) of this boundary (in the symmetrical case) can be found as a function of 6 : KU(K, 9 ) ^ • K U ( K , e ) - c o t g e . e - ^ « ° ^ ö (21) -K cos 9 r = z ^ ' K U ( K , 9 ) - c o t g 9 . e " ^ ° ° ^ ö 15
fig. 5 Boundary curve for a symmetrical spectro-meter with K = 0.6.
In fig. 5 such a boundary curve is given for a symmetrical spectrometer with coordinates z / z . and r/z„ with K = 0.6.
Only part of the solid angle can be used since there is a critical angle d e -fined by the common tai^ent of the curves described by (21). For K = 0.6 this angle is 36 and electrons emitted from the source with smaller angles can never be focused.
For the image formation we consider an electron emitted from the neigh-bourhood of the point z„... This elctron arrives at the boundary curve at
(z.., r . ) under an angle e. +6 9, if an electron emitted from z„ a r r i v e s under an angle e, • The motion rs then defined by the function y(a) + 6y(a)
(instead of y (a) ).
From (20) we get now by a variation method:
iy(a) = 6 e^ (a)K ( z^^ - y(a) + a - ^ y (a)) sin e ^ (22)
We do the same for the focus side of the spectrometer (denoted by index 2) In the case of a symmetrical arrangement ( y(a) = 0 , z^, = Zro and 9, (a) =
e 2^^) ) w® ^ ° ^ t^'^^ *^® positive direction of the z-axis in the opposite di-rection and replace y(a) by -y(a). It then follows from (22) that 6 9, (a) = -&.6_2 (a).
This meads that there is first order image formation with a magnification of 1. Let us now consider particles emitted a little outside the z,r-plane through the symmetry axis (emitted at a distance d^ and passing on the fo-cus side at a distance d from the z-axis).
Then conservation of momentum requires
d.. sin ^ = d 2 s i n i (23)
In the case of a symmetrical spectrograph ( 9 , = 9,) this implies that the image formation holds for all particles emittea from the immediate neigh-bourhood of the source.
To look at the dispersion we consider particles with moments slightly dif-16
ferent from those of the electrons focused. This means a change of 6K in the quantity K,
From (20) and (21) we get then in our symmetrical case 6 9 ^ = - 26K
K ^ - | K U ( K , 9 ) C O S 9 + U ( K , 9 ) + K - ^ U ( K , 9 ) | (24) Zjsm
If we call 6 X the shortest distance between the focus and the varied path we used in obtaining formula (22), then it is possible to calculate from (24) the dispersion factor (óx/z^/Jsp/p).
In table I this dispersion factor is given as calculated (for the case of K = 0. 6 and 60 <9 < 150 ) by Kofoed-Hansen et al (Ko 50). For comparison: with the same parameter values the dispersion factor for a semicircular spectrometer (with a homogeneous magnetic field) is 2.
Table I.
e
60° 75° 90° 105° dispersion factor 21.1 9.1 5.1 3.3 0 120° 135° 150° dispersion factor 2.8 1.8 0.3From this it is obvious that if we use not too big values of 9 , we get a great advantage over the conventional semi-circular spectrograph. This idealized situation is hard to realize since perturbations occur by some instrumental effects.
Two of these effects are;
1. finite extension of the counter at the focus. If we call the width of the counter window d, then the resolving power will be about;
(25)
in which f is the average dispersion factor.
2. deviations from axial symmetry of the field between the pole shoes. This will give a contribution R to the resolution.
1.4. Fringing fields.
The effect of the fringing field can be taken into account by the following procedure. We assume that the stray fields at the narrow side of the gap between the pole-shoes are the most important ones. If we consider now 17
the fringing field in a certain point of the boundary of the pole-shoes, it is possible to separate this field into several components (see fig. 6).
Fringing field at the boundary curves of the pole-fa ces.
Due to these components we can have the following perturbations; 1. The Bil'-component gives rise to an extra deflection of the particles
leaving the source at an angle 9 . This is equivalent to having a larger field region resulting in a practical boundary curve outside the mate-rial one. This extra 9 -deflection also depends upon the angle of rotat-ion i|) .
2. The component B, shows up, when we separate B (Ijang in the r , z, 'I' plane) into B. ^along the trajectory) and B. (perpendicular to the t r a -jectory). This È. acts as a cylindrical lens and transforms a point source into a sort of line image. This effect is of course a function of t and i|) . This line image (or focus) lies in a plane perpendicular to the r , z-plane. Hence, the best detector slit to be used should have the shape of a distorted line.
Most of these effects are approximately of the same magnitude and are working in the same direction at the focus side of our symmetrical spectrometer. In a first order approximation they can be linearly super-imposed. Kofoed-Hansen et al (Ko 50) give a rough estimate of their magnitude. Their influence can be minimized by shaping the boundary 18
curves. This procedure is actually followed in practice to obtain a spec-trometer with the best properties i . e . best combination of resolution and transmission.
CHAPTER m
DESIGN, CONSTRUCTION AND INVESTIGATION OF THE SPECTROMETER 1^. Introduction.
From the theoretical investigations it appears that a one-gap toroidal spec-trometer would meet most of our requirements. They also show that only in the case of a symmetrical spectrograph, the characteristics (and in p a r -ticular the form of the boundary curve) will have a simple form.
Since it appears from experiences with the Copenhagen six-gap model (Ko 55) that the effect of the fringing fields is rather strong, it seems wise to choose a relatively simple form of the boundary curve. It would then not cause us too much trouble shaping the pole pieces in a form that gives the desired optimum conditions. Therefore we have chosen the symmetrical type of spectrometer. If we take a K-value of 0. 6, with focusing in an in-terval of angles 9 from about 45 to 115 , the boundary curves can be ex-pected to become nearly perpendicular to the particle trajectories. For the opening angle ^ one has to choose as large a value as possible because the transmission (to a first order approximation) is directiy p r o -portional to this angle «c .
However, when i" becomes too large, the field between the main part of the pole faces will not retain the desired form (B = B / r ) . So, one has to compromise between a high maximum transmission and a good field form (which is important for the resolution). Considering all this we have chosen an opening angle of "f = 20° which (combined with e ) corresponds to a maximum transmission of about 3% of 4 IT .
Now we have to decide about the linear dimensions. F i r s t of all it must be possible to put a reasonable amount of shielding material (e.g. lead) between the source and the electron detector. This is necessary to prevent direct gamma radiation from reaching the detector, since this radiation would increase the background radiation. For this shielding we have taken a lead cylinder with a height of 15 cm. This will attenuate gamma radiation up to a few MeV sufficiently. Also, the lead shield has to be placed at a certain distance from the source to prevent scattering of gamma quanta from the source via the shield into a second spectrometer in the case of electron-gamma correlation measurements.
Another important thing is the source strength. In general it is necessary to be able to use relatively strong sources (from a few y Ci's up to several hundreds of v Ci's). This becomes very important when measuring weak decays. As explained in chapter IV, it is necessary for beta ray m e a s u r e -ments to use very thin sources (at a certain specific activity) it must be possible to use sources with a surface of about 20 mm^. If the dimensions are chosen too small, this rather large source area will give rise to a bad resolving power. On the other hand a very large spectrometer would b e -come too heavy. By combining the above requirements we arrived at a dia-meter of the pole faces of 450 mm.
For a good balancing of the weight of the iron yoke we put the yoke in a ho-rizontal position, which means that the pole faces are placed vertically. This also has the advantage that the gamma ray spectrometer for electron-gamma correlations can move in a horizontal plane.
The coils providing the magnetic field are placed at the other side of the yoke, away from the pole pieces. This is done for two reasons:
Spectrometer, power supply and Hall-voltage measuring instrument.
a. it must be possible to place the gamma ray spectrometer in a 90° po-sition compared with the mean entrance direction of the electrons. b. for the same reason as mentioned above (scattering) the amount of
ma-terial in the vicinity of the gamma ray spectrometer has to be as small as possible.
2. Description of the spectrometer. 2 . 1 . Magnet system.
In paragraph 1 of this chapter we discussed the dimensions of the pole-pieces. We decided on an opening angle of the pole faces of 20° and a dia-meter of the pole pieces of 450 mm. Now we have to deal with the type of iron to be used. At the moment of design the only suitable iron which was available, was steel C. 15. With a carbon content of about 0.15%. It appear-ed that this percentage is sufficientiy low to provide a good magnet. For mounting the coils we used copper discs with a big center hole, fitting around the iron yoke. On each side of such a disc the coils were baked with Araldite (see fig. 7). These coils are wound of enameled copper strip with outer dimensions of 9.2 x 2.2 mm. Every coil consists of about 70 turns on each side of the mounting disc. By means of this baking proces a fairly good heat conduction is obtained from the coils to the copper plate. At the outer edge of this plate a copper cooling tube is soldered. In addition, an extra spiral cooling tube is mounted between the coils and the iron cylinder of the magnet yoke (see fig. 8 and 9).
There are 20 coilplates, so that the total amount of windings is about 2800. Each coilplate has a resistance of about 0.1 ohm. The possibility of pa-rallel or series connection of any of these coils is provided. When all the coils are used in series the total resistance is about 2 ohms.
Extra cooling spirals are mounted on the iron yoke near the pole shoes.
This is to prevent displacement of the pole pieces due to changing ambient
temperatures causing extension of part of the yoke. The pole pieces are
placed in an argon-arc welded aluminium vacuum tank. The iron yoke fits
in this tank with 0-ring seals. The vacuum tank has removable upper- and lower lids vacuum-sealed with 0-rings. These lids permit easy changing of baffles and detector slits. The whole spectrometer, as shown schematical-ly in figures 8 en 9 is placed on a wooden frame to avoid stray fields to be ejqpected from an iron construction. The vacuum system is placed below
the Éfpectrometer and will be described in paragraph 2.4 of this chapter. A
table can be mounted near the source for coincidence and correlation mea-surements with a second spectrometer CNal(Tl), anthracene, etc.) See fig. 10. For angular correlation studies the second spectrometer can be r o -tated either by hand or automatically around a line through the source and the focus and with the source as center of rotation.
Several other spectrometers can be mounted at various angles on the table. In that case the spectra of these spectrometers can be recorded simulta-neously in different parts of the memory of a multichannel analyzer. This system has the advantage that it requires less time to collect the desired
data than in the case of one rotating spectrometer. However, a
disadvanta-ge is the necessity of very careful adjustment of the electronics.
fig. 7 Side view and cross-section of coil plate.
<)\er:ill view of the spectrometer, (cross-sections :ire partially shown).
Qjb Ijf Ijl
Uprxr lid
To vacuum lysttm
2.2. Source holder and sources .
Special care has to be taken in constructing the source holder. In order to avoid scattering the amount of material near the source has to be as small
as possible. But not only the total amount of the material is important, but also the material itself. A high atomic number Z gives a high number of electrons per atom and thus a higher scattering probability for electrons and gamma-rays.
Since the scattered radiation can contribute to the background counting rate in both the beta-ray spectrometer and the second spectrometer, we tried to use as little material as possible, and with the lowest Z-value possible. As pointed out in paragraph 2 . 1 , the vacuum chamber is made of aluminium for the same reason. The cup of the source holder (shown in fig. 11) is also made of aluminium. The wall thickness of the cup is 0. 5 mm. and its dia-meter is rather big (86 mm). In this way the direct scattering of particles or electromagnetic radiation into the spectrometer is less probable. The rather thin wall has also the advantage of low absorption of the radiation to be detected in the second spectrometer. The source cup can easily be connected to the vacuum chamber by means of 4 screws. An 0-ring is used again as vacuum seal.
The actual source holder inside the cup (see fig. 11) was made of Incite. The construction is such that the source can be moved in all directions (viz. to and from the spectrometer, and in a plane perpendicular to this di-rection). In this holder the source ring (see fig. 12) can be placed and ad-justed to the correct position.
Aluminium
Source ring
fig. 12 Source ring (backing holder). It is possible to modify the source holder assembly in such a way that e l e c -tron-electron and electron-gamma coincidence experiments can simultane-ously be performed (see fig. 13).
Due to the self-charging of a beta-radioactive source, the holder has to have a grounding connection. Otherwise a considerable potential (up to
lOkV or even higher) can change the energy of charged particles leaving the source.
For the same reason the foils used as backing have to be rendered conduct-ive. This can be done in several ways. We used two methods:
WZA
Source\ZZZi
Branch pipe Clamping block To spectrometer Photomultiplierfilg. 13 Branching pipe for vacuum cconecUng of outer qpeotrometers
a. aluminizing the foils by vacuum evaporation
b . using thin aluminium foils (thicknesses of 0. 5 and 6. 0 microns). Very thin films with a low Z-value as backing material are used to avoid "back-scattering" of electrons from the source via the backing into the spectrometer and in addition to minimize the production of electrons by Compton- and photo-electric effect from electromagnetic radiation of the source.
Zapon-films (produced by dropping some zapon on lukewarm water) can be made much thinner than aluminium ones. Another important advantage is their resistance against acid (viz. HCl). This enables us to make sources on these foils by using the liquid drop method, because most radioactive isotopes are chemically easy to handle in the form of chlorides. Since zapon is an insulator, it is necessary to render these zapon films conduct-ive by evaporating a very thin layer of aluminium into them in a high vacuum bell-jar (see chapter IV).
Very important is the thickness of the sources. In order to decrease the self-absorption of the electrons in the source material they have to be thin. Especially low energy electrons (energies below 100 keV) will be absorbed or slowed down in the source if its thickness exceeds an amoimt of 50 to 100 micrograms per cm2 (depending also on the Z-value of the used isoto-pe). Therefore, it is important to use (if possible) sources with a very high specific activity when measuring low energy electrons.
For our calibration measurements we used the beta-continua and internal conversion electrons from the decay of -'^•^'Cs and •'•"°Au, and internal conversion electrons originating from the decay of ThB(2-'-2pb). The sour-ces of I'^'^Cs and l^^Au were prepared by using the liquid drop method (e.g. drying drops of active solution on a zapon film). ThB-sources were made by deposition of 216po obtained from an emanating RdTh-source onto an aluminium foil. As discussed in chapter II (paragraph 1.3.), we can use line sources in this spectrometer. Yet, in calibration measurements gene-rally (because of the easier way of preparation) circular sources were used with diameters from 2 to 3 mm.
2 . 3 . Detector assembly.
At the focus position an anthracene crystal is used as detector. This c r y s -tal is mounted on a Incite light pipe of 350 mm to keep the photomultiplier sufficiently far away from the magnetic fields. As an extra protection a mu-metal shield is used.
The light pipe is positioned by means of an aluminium holder with an 0-ring seal. This holder is screwed to the vacuum-chamber and sealed with another 0-ring, and can easily be exchanged for mounting other detectors (such as plastic scintillators or a GeigerMUller counter). The whole a s -sembly for an anthracene counter is shown in fig. 14. Fig. 15 shows the Geiger-MUller tube assembly. We used a Philips Geiger-MUller counter type 18506 with an 1. 5 milligram per cm^ mica window. This rather thick window will cause absorption of electrons below 100 keV. For electrons with higher energies this detector can be used very well. Its advantage is the lower backgroimd and noise level. This is especially important when measuring low intensity conversion lines or beta ray spectra. In the future a flow counter with an exchangeable very thin window will be used for low energy measurements.
N
y
O - r i n g seals . V y u - n n g seats
\vy/////z^
A n t h r a c e e n c r y s t a l Light pipe"X
\ \ \ D e t e t o r slit\ \ \ \ \ \ \ \ \ w \
^ . n - ^ V V V k V V V V V VTTTTl Photonriultiplier 2 Series of U e l e c t r i c a l connections to H a l l — g e n e r a t o r O 1 2 I • • 5 c mThe anthracene crystals used have different thicknesses (ranging from 0.5 to 5 mm) depending on the energy we want to measure. For low energy measurements a very thin crystal will be sufficient. For example the maximum range of electrons of 400 keV in anthracene is about 1mm. How-ever, when a 1 mm thick crystal is used for electrons of considerable higher energy, one gets a distorted differential spectrum. The spectra for various energies are shown in fig. 16. The shape of spectrum c (from a too thin a crystal) arises from the fact that part of the high energy electrons will give full absorption pulses. This is caused by the fact that at those higher energies the minimum electron range will exceed the crystal thick-ness. Such a spectrum is hard to correct for noise and background radiation. For the investigation of the best detector slit size and position (described in paragraph 3 of this chapter) we .made use of a special apparatus. By means of this instrument, shown in fig. 17, it is possible to change the dis-tance from the source to the detector slit and the disdis-tance of the slit to the spectrometer chamber. The size of the slit can be varied too. The focus form was already determined experimentally by taking photographs of the focus on nford X-ray film (see paragraph 3.2. of this chapter).
2.4. Vacuum system.
The vacuum system of the spectrometer consists of a combination of dif-fusion pumps and a rotary pump.
The rotary pump is a two stage one, manufactured by Galileo (type V2h) with a displacement capacity of 15 m^per hour. The final vacuum is 2 X 10~4 t o r r . On this backing pump an Edwards magnetic valve and air in-let are mounted. By means of valves this pump can be connected either to the vacuum chamber or to a vacuum tank. This tank supplies a rough vacuum to a mercury diffusion pump (Leybold Hg3) which sustains the backing pressure for the oil diffusion pump (Edwards F204). The latter is
connected to the vacuum chamber by means of a butterfly high vacuum valve. The pumping speeds of the mercury- and oil diffusion-pumps are 3 liters and 70 liters per second respectively.
The Hg3-pump acts as a booster to the oil diffusion pump.
The above construction makes it possible to disconnect the rotary pump for several hours from the system which is advantageous in the case of long measuring times.
The whole system is shown in fig. 18. All valves (except the butterfly valve to the spectrometer) are ball types (Argus "Kugel-hahne").
This system enables us to reach a vacuum of 10~4 t o r r within half an hour (starting from atmospheric pressure).
2 . 5 . Power supply.
For the current supply to the spectrometer coils we need a rather highly stabilized D. C. power supply. By using a current stabilizer capable of giving about 100 A at a voltage of 33 V we can choose a convenient magne-tic field r a i ^ e in the spectrometer by parallel or series connection of the coils (see paragraph 2 . 1 . of this chapter. If we want to measure internal conversion lines with a resolutioh of 0. 5%, then it must be possible to d e t e r -mine the peak position within 0. 05%. Thus the stability of the current must be about 0. 02%. For long term measurements (e.g. when measuring elec-tron-gamma coincidences) a stability of 0.05% will do, to make sure that
Aluminium
Scole
the counting rate at the maximum of a conversion peak will not change beyond the statistical e r r o r .
The stability also has to be sufficient at low currents for the measurement of electrons with energies down to 50 keV.
Electrons
Anthracene crystal -* Pulse height
I n t e n s i t y k -*• Pulse height I n t e n s i t y 1 -• Pulse height
fig. 16 Spectrum shapes originating from monoenergetic electrons entering a scintillator.
a, range R < crystal thickness D; b, R > D and c, R ><> D (In the spectra R denotes the full-absorption peaks).
Air inlet
— D X h
Penning ion. gougeO
Vocuumchomber of spectrometer Magn.valve H and oir inlet [ ^Rotary pump
B u t t e r f l y valve
Fore vacuum tank
fig. 18 Vacuum system of the spectrometer.
R.R. > Ref resistance MR. = Mognetcoil fig. 19 Block scheme of the current stabilizer.
DC = Electric Instruments DC amplifier (gain : lOOOx); K2W and K2P = Philbrick operational amplifiers. M.C. = magnet-coU.
For this purpose a current stabilizer was built by Lourens (Lo 67), deve-loped from a modified design of Brog and Milford (Br 60). A scheme of the electronics is given in fig. 19. The unregulated power is supplied by a 24-phase rectifier-transformer, built by N. V. Diode. The maximum power is 50 A at 66V or 100 A at 33V.
The magnet current is fed through a reference resistance made of manga-nine band (total resistance 0.1 ohm). The voltage drop over this resistance is compared with a very stable reference voltage. The difference between these voltages will change when the current through the magnet changes. This differential voltage is supplied to a D. C. -amplifier which controls the regulating power transistors. The current setting is done with a Kelvin-Varley divider (ESI dekapot), having a total resistance of 500 ohms. This voltage divider has a linearity of 0. 002% and a resolution of 0. 003%. For automatization purposes this dekapot can be replaced by a Colvern 40-turn potentiometer (Linearity: 2 x lO"'^, resolution: 3.5 x 10~5). To protect the stabilizer against electronic faults measures have been taken. When the collector-emitter voltage (VQE) 3-t the power transistors becomes too high, the danger exists that the breakdown voltage is passed. To prevent this the voltage over the power transistors is blocked by the voltage (s 55V) of a Zener-diode.
A second Zener-diode (6V) is used to prevent overloading of the regulating circuit by the output voltage of the operational amplifiers. To prevent a voltage surge over the coils (due to the induction of the coils) occurring after a break in the main current leads, a diode is placed across the refe-rence resistance and the spectrometer coils. The regulation is not influ-enced by this diode, for it is back-biassed. Finally to protect the power transistors from overheating when the cooling fails, a bi-metal switch cuts off the supply at a temperature of 450C. The power transistors as well as the reference resistance are cooled by kerosine. This kerosine is cooled via a heat-exchanger by the main water supply. In fig. 20 a scheme of the cooling system is given. It appeared from long term measurements on an internal conversion line (resolution setting at 0. 5%) that the stability was better than 0.05%.
3. The focusing.
3 . 1 . Shaping of the boundary curves of the pole faces.
The boundary curves were first shaped roughly according to the calculated form taking a K value of 0. 6 (see chapter 11). Due to fringing fields and the characteristics of the iron, we did not expect to find optimal values for the resolution and transmission. Using a rather narrow slit (about 1. 0 mm high and roughly shaped into the focus form) the best resolution was about 1.5% at a transmission of nearly 0. 5%. These results were obtained by measuring the internal conversion peak of the 661. 6 keV transition in ^"^'^Ba (from the decay of 137cs, see fig. 21. The resolution was defined as the r e -lative width at half height of the peak. For the transmission we took the top counting rate in a single conversion line divided by the total emission rate of these monoenergetic electrons.
To obtain information about possible improvements, photographic picture sof the focus and of the particle trajectories were taken in the spectrometer and near the detector. This procedure worked quite well and we learned from it that particles passing through different parts of the spectrometer 36
Tronsistor-panel
Wóler
fig. 20 Cooling system with heat exchanger for current stabilizer.
£
30 y. 137f^5 11/. 2J5min. 0.5UMeV,95% ^ / 1.180 MeV 5 % /• V •
stable iQ6616MeV 137Ba 137 fig. 21 Decay scheme of Cswere focused at rather different places of the detector. The best way to correct for these effects is to adjust the shapes of the boundary curves of the pole faces. There are various reasons why the application of extra coils to obtain a correction is less attractive e.g. reduction of aperture,
duction of extra electron scattering etc. To shape the boundary curves we constructed a series of baffles to divide the aperture of the spectrometer into 14 parts (see fig. 22).
Source
fig. 22 Aperture of the spectrometer divided into 14 "windows"
We now placed the source and detector at the calculated positions (viz.dis-tance from source to detector-slit: 340 mm, detector and source lying on the intersection of the planes in which the pole faces lie). By measuring the peak positions (viz. the current at which the electrons are focused) for the different regions of the spectrometer gap we got a picture of the deviations of the actual trajectories from the desired ones. We defined the peak posi-tion as the point of intersecposi-tion of the sides of the peak (this is a good mea-sure since the peaks appeared to be rather symmetrical). The removal of some iron at a place of the boundary curve corresponding to that particular part of the spectrometer which is studied, makes the distance which the electrons travel through the magnetic field shorter, and in that way chan-ges the focus position to a higher current. This procedure enables us to correct for the effect that electrons emitted in directions making different angles ^ with the central orbit are focused at different places. Electrons emitted in a direction making an angle ()> with the central plane cause a cur-vature of the line focus. This happens especially to the electrons with big emission angles 't». By shaping the slit of the detector the influence of this effect can be minimized. To perform these "window-curve" measurements it is necessary to reproduce the position of a certain peak with an accuracy of about 0. 05%. The normal procedure of demagnetizing the iron before each measurement was not accurate enough. Another alternative is to feed once or twice the maximum current through the spectrometer. This method was not satisfactory either. Therefore, we developed a new method. F i r s t we used a very high current. After that we adjusted the current just below the top of the peak of the F-line in ThB. Then we increased the current to
100 V. I
i
99 "A 100 %• II
99 %• J 99.8 %•I
fig. 23 Window curves : a before correcting, b after 2 times correcting,
£ final curve after 11 times correcting.
2 i 6 8 10 12 U Sl.tnumber Before correctmg 10 12 U Slitnumber After 2x correcting 2 i. 6 8 10 12 U Slitnumber After 11K correcting 39
a value which was slighüy above the peak. Then decreased it again to the lower value. This procedure was repeated about 40 times after which it was possible to reproduce the current setting of the peak within 0.05%. To e x -plain why the regular demagnitization methods did not work out, the current regiilation was checked. It appeared that the curcurrent is stable and r e -producable within 0.02%. This leads to the conclusion that the above effect is due to characteristics of the iron.
The measurements during the process of shaping the pole pieces were c a r r -ied out with the 148 keV electrons of the Thorium-F line (from the decay of ThB).
The transmission calibrations were performed with K-conversion electrons from the 661.6 keV transition in 137Ba from 137cg sources calibrated within
1% against a standard 137cs-source.
From the first window-curve it appeared that window 10 needed the highest focusing current (fig. 23a). We now decided to file away some iron at places of the boundary curve corresponding to all other windows. This had to be done very carefully since shaping of a certain part of the pole face edge in-fluences neighbouring p a r t s . After several corrections we obtained a win-dow-curve that deviated less than the original curve from a horizontal line. However, the windows 1 and 2 still required big corrections compared to the other windows. From fig. 24 it appears that windows 1 and 2 contribute together only 5% to the total transmission.
Relative Resolution
3-
1-fig. 24 Relative transmission curve for the different windows (slit numbers).
- I 1 1 1 1 1 1 1 1 1 1 1 T"
2 t, 6 8 10 12 U Slit number
In addition, an eventual correction of these windows could destroy the fo-cusing properties of all the other windows. We therefore decided not to use windows 1 and 2. The iron was filed away in 11 small steps. After each step the result was checked. Window curves taken before the correction procedure was started and taken after the second and after the final step of
this procedure are shown in fig. 23. Finally, a window-curve (fig. 23c) with a deviation of less than 0.1% from a straight horizontal line was obtained. The relative resolution of the various windows is presented in fig. 25. It appears that the resolution is better for the outer parts of the spectrometer. The line shape is better too (more symmetrical) for that region of the spec-trometer. Relative Transmission 5- L- 3- 21
-fig.25 Relative resolution curve.
— I — I — I — I — I — r —
10 12 U Slit number
Rvtotivr Rt«o(ution Relative Tronsmissron S400 mm 5t7S . .
5250 . ,
16 12 e I.
"I—r
• Varigtsn «f saurct-focut d i t t a n c tfig. 26 Relative resolution and transmission curves for various detector slit positions (1 = distance to spectrometer chamber).
To achieve the best resolution and transmission we used the movable detec-tor slit shown in fig. 17. The shape of the slit was designed according to new photographs taken after the last boundary curve correction. We changed the position a s well as the size of the slit in combination with the various entrance baffles (described in the next paragraph). Fig. 26 shows the relative transmission and resolution a s a function of the position of the slit for a certain entrance baffle. Our measurements enabled us to choose the best combinations of entrance baffle and position of the detector slit. 3. 2. Focus forms and particle trajectories.
After the final shaping we took photographs of the focus for the different entrance windows and combinations of them. In fig. 27 some of the results are drawn. It appears that the central parts of the spectrometer (window numbers 7-10) give the narrowest foci; however, they have the biggest cur-vature .
Window no 3 + i
fig. 27 Focus forms obtained for different combina-tions of the entrance
windows. (^=^ 5 + 6 7+8 Focus f o r m s <^^ 9+10 11 + 12 13+U Detector Slit no. 1
fig. 28 Detector slits.
Since they contribute most to the transmission we have to match the detec-tor slit to these foci. As a result of our investigation we arrived at slit forms as shown in fig. 28.
To get an impression of the particle trajectories in the spectrometer we put photographic plates nearly in the same plane as the r , z-planes of the electrons. We used these pictures to place the baffles at their correct po-sition in the spectrometer.
3 . 3 . Baffles and shields.
As was mentioned in par. 1 of this chapter we put a lead cylinder (height 15 cm, diameter 10 cm) between source and detector which both are outside the wall of the vacuum chamber. Therefore, the lead shield can be mounted outside the vacuum tank which has the advantage that the lead shield can easily be removed and be replaced by another one. In fig. 29 the various baffles to be placed at the entrance boundary of the pole pieces are shown. They are used in combination with the different detector slits.
fig. 29 Entrance baffles (3-14 are the numbers of the slits used in taking the "window-curve").
To diminish the background counting rate we placed two baffles about half-way in the spectrometer. They have to stop scattered electrons and second-ary electrons, originating from interactions of gamma rays with material of the spectrometer. Fig. 30 indicates their position which was selected by the photc^raphic method.
All baffles were made by sandwiching a 1 mm aluminium plate, a 1 mm b r a s s plate and again a 1mm aluminium plate. These plates were put t o -gether with araldite. After this the whole combination was covered with araldite and then baked in an oven.
fig. 30 Position of the stopping baffles in the spectrometer. These baffles can stop electrons with energies up to about 4 MeV and they still have a rather low probability of emission of secondary electrons by external conversion.
4. Final behaviour of the spectrometer. 4 . 1 . Resolution and transmission.
By using the entrance baffles and detector slits mentioned in the previous paragraphs we were able to select several satisfactory resolution and transmission combinations. The use of these various combinations depends on the experiment under consideration. Sometimes it will be necessary to resolve close-lying electron lines (e.g. internal conversion lines). Then one prefers a high resolution and has to accept a relatively low t r a n s m i s s -ion. In other experiments (especially in low Intensity measurements) one would rather have a high transmission (and consequentiy a poorer resolut-ion).
"/<
1 2 » Transmission / o
fig. 31 Graph of the different combinations of resolution and transmission.
Fig. 31 shows a graph of the merits of different combinations of entrance baffles and detector slits. Four fixed combinations were selected: see table n . The shape of conversion peaks (123 keV K, L and M electrons from the decay of 154EU) is shown in fig. 61.
Especially in coincidence measurements it is important to have very little scattering of electrons. To test the influence of the anti-scattering baffles we decided to measure the scattering of the K-conversion line of the 661. 6 keV transition in 137Ba (a decay scheme is given in fig. 21). This was done by measuring coincidences between the conversion electrons and the X-rays following the electron emission. Subsequentiy the coincidences between the X-rays and the electron background at two momentum settings (namely between the K and L conversion peak and also above the L-peak) were
mea-sured. We found that between K- and L- peak the scattering ratio i s 1 to (1.2 + 0.6) x 10-4, and above the L-peak 1 to (0.6 + 0.4) x 10-4. These data were taken with combination 2 (see table U).
Table II combination 1 2 3 4 transmission i n % 0.7 1.1 1.6 1.9 resolution i n % 0 . 5 0 . 8 1.2 1.7 entrance baffle ^2 b l bo bo detector slit height in mm | 3 4 6
9 I
454.2. Hysteresis effects
As was mentioned before (chapter IH paragraph 3.1.) it was impossible to return after a measuring run to the same point of the magnetization curve by ordinary demagnetization procedures. This fact can introduce e r r o r s in the energy calibration. It is also impossible to construct a momentum versus current calibration curve of general applicability. We solved this problem by applying a m a ^ e t i c field measuring device. This system is discussed in paragraph 5 of this chapter. It is still interesting to measure the linearity of the spectrometer. We used a ThB source, viz. the F-line
( B P - v a l u e : 1388. 44 gauss cm; 148. I k e V), the J-line (BP: 1811.11 gauss cm; energy: 234. 6keV), the L-line (BP : 2607.17 gauss cm; energy: 422. 8 keV), the P-line ( B P : 3931 gauss cm; energy: 773.5 keV) and the X-line
(BP : 9986.7 gauss cm; energy: 2526.3 keV). In this region the instrument was found to be linear within 0.1%.
4. 3. Effect of stray field upon the second spectrometer.
The stray field at the place of the movable detector on the turntable is rather low. Thus the influence on the multiplication of the photomultiplier is small. However, in the case of angular correlation work the position of the detector may be charged from 180° to 90° (rather close to the spectro-meter) and then the change in amplification of the detector system is not negligible. To diminish this effect several measures have been taken. First of all the detector holder is made of iron (with low carbon content). Secondly, a mu-metal shield covers the photomultiplier and finally, an ex-tra mu-metal shield fits over the detector holder. A schematic view is given in fig. 32. The photomultipliers used are of the Venetian blind type, which are less sensitive to magnetic fields than other tj^es . In this way no noticeable changes in amplification are left.
Aluminium window M u - m e t a l shields Iron holder
Venetian blind type
fig. 32 Cross-section of Nal(Tl)-spectrometer for coincidence or correlation measurements.
5. Magnetic field measuring system. 5 . 1 . Measuring apparatus.
It was pointed out in paragraph 3 . 1 . of this chapter that continuous magnetic
field measurements are necessary for accurate experiments. This might be done by using a rotating coil, inside the spectrometer, connected to another rotating coil in a reference field. By measurii^ the differential voltage over these coils an accurate field calibration can be obtained. Such a device, un-fortunately, requires a very precisely working mechanical system and a very careful shielding from external fields. Therefore, we decided to make
fig. 33 Hall-plate.
use of another possibility, viz. a Hall-generator which consists of a thin layer (mostly about 0.1 mm thick) of semi-conducting material of a special type. If a current Ig flows through the material (see fig. 33) and a magnetic field B is applied perpendicular to the layer a voltage Vjj is generated at the points indicated in fig. 33. The voltage Vjj is given by
V H = RnlgB (26) The Hall coefficient Rjj is a function of the temperature, but is nearly
in-dependent of Ig and B. If Ig is stabilized, an accurate value for B can be ob-tained by measuring Vjj. For this measurement an apparatus (Lo 67) is used as shown (block scheme) in fig. 34. The Hall generator X (type: FC 32 from Siemens) is fed by an alternating current generator A (1000 Hz). The refe-rence voltage (over transformer T2) is proportional to Ig. The Hall voltage (over transformer T^) is also proportional to Ig. When mese voltages (with opposite phases) are added, the setting with a zero differential voltage is independent on Ig. R^, R2 and R3 correct the null-effect.
The differential voltage is amplified by C and D. A selective amplffier E improves the signal-to-noise ratio. A phase-sensitive detector H gives the ou^ut signal. In the original set-up we tried to use the output voltage to control the current stabilizer of the spectrometer (see fig. 34). However, it appeared impractical to do this in our case since at sufficiently high am-plification (for high accuracy) the system (Hall-generator, measuring in-strument, main current stabilizer and spectrometer) started generating. Therefore, we decided to use the instrument as a field measuring device. We now supply the output voltage of detector H to an oscilloscope. If a c e r -tain magnetic field value is required, the calibrated potentiometer R is set at this value. After this, the main current is adjusted to obtain a minimum signal on the oscilloscope. The magnetic field in the spectrc«neter c o r r e s -ponds now to the desired value. To get the maximum signal at low field
settings, the Hall generator is placed at the region of highest field in the 47
P l ^
spectrometer.
We made sure that it could not block any particles. The four output leads (2 for VJJ and 2 for Ig) are soldered to the vacuumconnections in the d e tector holder of the spectrometer. Very close to the Hallgenerator a t e m -perature stabilizing circuit (see the next paragraph) is placed.
The whole system was tested by comparing the output voltage to an e x t e r -nal, highly stabilized, reference voltage. The accuracy appeared to be bet-ter than 0.1%. The system was tested in another way too. We put the spec-trometer setting on one of the sides of the conversion line of the 661.6 keV transition in ^^^Ba. The shift was less than 0. 037o over a period of 24 hours. After this test we made a calibration table using the already men-tioned conversion lines from ThB. The best fittii^ curve to the calibration points was calculated from a third degree polynomial approximation with help of the TR4 computer (see E.W.Koopmann (Ko 66)). We tabulated the values of B P in gauss cm, the relavistic momentum n in units mgC, the r e -lativistic energy e in units mjjC^ and the energy in keV's as a function of the reading of liie potentiometer of the measuring instrument.
5. 2. Temperature stabilization
The Hall-genera tor has a temperature coefficient of - 0 . 04%/°C for an open loop, and of - 0 . 2%/°C for a short circuited output. It is possible to keep the generator at a constant temperature by means of a heating spiral. However, this did not work quite well. Thus we decided to make a tempera-ture compensation circuit. In fig. 35 this circuit is given. We can write
, R
^H " ^H R + R,™„ (^'^) m NTC
Differentiation to the temperature gives :
^ ^ T C
3T Rin + % T C ^
(28)
From the specifications of the manufacturer it appears that Rj^ + R N T C ^ 13 ohms to keep the circuit linear. If it is possible to choose the tempera-ture coefficient of R N T C such that 3 V H / 3 T = 0^ then temperature stabili-zation is reached. Since an NTC-resistor with desired temperature coef-ficient was not available, we used a modified circuit given in fig. 36. R^\ R2 and R3 were determined experimentally. R^ = 2 ohms (-3%/°C), R2' = 3 ohms (-0. 02%/"3C) and R3 = 12 ohms. After testing this circuit we found a temperature dependence of less than 0. 003%/°C (Ko 66).
^NTC Ig 1 1
4-H X - ,
1 ^H
^ m r ( , VH ffig. 35 Possible circuit for temperature
compen-sation.
Ig
fig. 36 Modified circuit for temperature compen-sation. 1 i — X — 1
r'
11
R'3
H -1
VH 1-• t 50CHAPTER IV.
VACUUM DEPOSITION APPARATUS AND SOURCE TECHNIQUES.
1^. Introduction.
In beta-ray spectroscopy the preparation of the sources is very critical. The required properties of the sources are in general:
a. a very thin homc^eneous layer of active material in order to avoid a b -sorption and scattering of electrons.
b . a very thin backii^ (preferably of low Z material) to decrease back-scattering.
c. an adequate conductivity of the backing to prevent chaining of the source. d. a reasonable mechanical strength of the backing.
Requirement a can only be fulfilled if the radioactive material has a suffi-cientiy high specific activity, depending on the kind of experiments to be performed and the Z of the radioactive isotope itself.
The requirements (concerning specific activity) are not so stringent when dealing with low Z material or when measuring conversion lines with ener-gies above a few hundred keV. In general a source thickness ranging from 50 to 100 micrograms per cm^ will be the upper limit. It is hard to make a homogeneous source when applying the liquid drop method. The method can be improved by painting the backing with insuline before the active material
(mostly dissolved in HCL) is applied. In this way it is possible to decrease the effect of formation of a ring of crystals at the boundary of the source area. As backing material we generally use zapon films, produced by spreading a drop of zapon laquer (diluted in amyl-acetate) on lukewarm water. They can be made as thin as 10 micrograms per cm2 and since zapon is an organic compound the Z is rather low. They are rendered con-ductive by evaporating (in vacuum) aluminium onto the backside. The total electric current needed to discharge the source capacity, is very small. Thus the conductivity need not to be very good; a foil-resistance from 10^ to 10^ ohms is sufficient. Therefore, the amount of evaporated aluminium can be very small. We used thicknesses of a few micrograms per cm^. Another method to produce beta-ray sources is the vacuum deposition technique. The commonly used evaporation arrangement is shown in fig. 37. The greatest disadvantage of this arrangement is the large amount of activity necessary to produce a relatively weak source. This is mainly caused by the bad geometry of the arrangement.
In addition, condensation of activity at thé top of the boat, which h£.s gene-rally a lower temperature than the r e s t of the metal, will occur. The con-densed activity will subsequently evaporate into a solid angle of about 2-n. 2. Construction of the apparatus .
To improve the vacuum deposition technique we constructed a new type of crucible (see fig. 38). The active solution is dried in the small beaker. After evaporation the activity flows throi;^h a narrow channel (1 mm dia-meter) of the crucible towards the backing material where it condenses. Due to this narrow channel and the fact that no condensation takes place at the top of the crucible, sources with a very small diameter can be p r o -duced.
Source backing
_I
Mask
u m> } i f )
fig. 37 Old type of crucible for vacuum deposition of radioactive material.
Tantalum
Crucible
fig. 38 New type of crucible for vacuum deposition of sources.
Beaker ƒ Tantalum .or
" ' ' j m / / / /
yA'/x'
* ^ ' »'^'//z
Carbon oven fig. 39 Carbon oven.
The condensation at the top of the crucible is avoided by using a carbon oven (see fig. 39) of a special construction. By making a saw-cut in this oven close to the top, the current will cause there (at the place of the highestelectrical resistance) the highest temperature. Now the effect of evaporation into a solid angle of 2 w will not take place; thus, the efficien-cy of the arrangement can be very high (up to 90%). The source holder with the backing material is clamped between watercooled copper discs (see fig. 40).
Copper
/
Cooling tubes
fig. 40 Cooled source holder.
By placing the backing at various distances above the top of the crucible, sources of different diameters (down to about 2 mm) are obtained. As backing material 1 and 6 micrometer aluminium is used. Zapon or other organic backings usually don't stand the high temperature due to radiated heat. To obtain line-sources for double focusing spectrometers one can use a small drum rotating above the crucible. The backing is stuck on the drum and in this way it is possible to get very good line sources (Pa 59). The crucible is generally made of tantalum which has a very high melting point (2977°C). However, other types of material can be used too. When evaporating chemical compounds with low boiling points (below lOOO^C), we have had very good results with iron and chromium-iron crucibles. The advantage of these materials i s that they a r e much easier to machine. Molybdenum and platinum were used too and gave also good results. The isotope-containing beaker inside the crucible is mostly made of plati-num because of its chemical inertness since most of the activities are used in the form of chlorides. Another advantage of platinum is that its relative softness enables a gastight sealing of the top of the beaker to the crucible. This prevents the leakage of evaporated activity through the screw-thread at the bottom. However, when very high temperatures (above 1600°C) are needed the beaker has to be made of tantalum too, which will not give such a good seal as platinum does because of the hardness of tantalum. The ef-ficiency will in that case be lower.
During early experiments it appeared that a very high pumping speed in the bell-jar during the actual evaporation is necessary. Otherwise the source area becomes much too large. This is probably due to gases escaping at the moment the activity starts boiling. The resulting gas stream tiien spreads the activity over a large area on the backing.
The mean free path for air-molecules in air is about 50 cm at a pressure of 10~4 t o r r . Since the distance from the beaker to the backing is only a few centimeters a pressure of about 2 x 10~4 t o r r will be sufficient to en-sure littie scattering of the active atoms or molecules.
The crucible is mainly heated by radiation. Therefore, a lot of power is needed to provide the high temperatures in order to reach the boiling point of several hard to evaporate compounds. To meet these requirements a vacuum evaporation system was built. The vacuum circuit is shown in fig.
41.
r^
B e l l - j a r
High vac. valve A i r i n l e t
I 1
Oil d i f f . pump
C3
Penning ion. gauge
3 way valve
Fore vac. t a n k
Air inlet fig. 41 Vacuum system for the evaporation apparatus.
0
Rot .pump
An Edwards F603 oil diffusion pump (pumping speed 700 liters/second) is directly (without baffles) connected to the bell-jar at the base plate via a type of butterfly valve of own construction. Via a vacuum tank this oil dif-fusion pump is connected to a two-stage rotary pump (Galileo V2h) with a pumping speed of 15 m"^ per hour. This system provides the required high pumping speed and lowers the pressure in the bell-jar from 1 atm. to about 2 x 10~4 torr in about 8 minutes. Two pairs of electrodes are installed on the base-plate. One pair (C-D) is watercooled and is able to carry a maxi-mum current of 200 amperes. C-D are used for heating the oven and crucible. The power transformers for these electrodes get their supply from a 220 volts, 20 amperes variable transformer. The principle electri-cal circuit and electrodes are shown in fig. 42.
In order to be able to use all the power available (about 4 kilowatts) the resistance of the oven has to be about 0.1 ohms. Therefore, one has to use carbon with a high resistivity. The carbon should also have good machining properties in o r d t r to be able to give it the desired shape.
To protect the bcU-jar against the radiated heat an aluminium housing fits
^^a^-r^'i.^ V'^-'. ..^ ' ^ li^"^:'*-'C>"-"7tfcL^
Electrodes, oven and crucible for vacuum
evaporation, O = graphite oven; T = top of crucible. Vacuum deposition apparatus.
over the electrodes and oven. An extra nickel radiation shield i s put
direct-ly above the oven and crucible. Ondirect-ly the top of the crucible sticks out of
this nickel shield. A Pt-PtRh thermocouple is installed to measure tempe-ratures.
Main supply 220V a.c.
Power transf. 20 V 2D0Amax. fig. 42 Electrodes and electrical circuit.
3_. Performance.
The described apparatus proved to be a powerful instrument in making radioactive sources for a - and B-ray spectroscopy.
The first isotope we evaporated was 200x1. The activity was produced by the reaction 2()0Hg(d,2n)200xi in the I. K. O. synchrocyclotron in Amster-dam (No 59). After the chemical separation we got the activity in the form of chloride. It appeared that the activity still contained some mercury
tacking the aluminium backing. To get rid of this contamination we first heated the crucible to a temperature above the sublimation point of Hg2Cl2(302OC) and below the boiling point of T1C1(720°C). Next we took a new backing and evaporated the TlCl. In this way we removed all the m e r -cury without loss of activity. The total efficiency proved in this case to be over 90%. Using thin carbon foils (produced by sputtering) as backing, targets of mass separated 34s for a Van de Graaff accelerator were made. Good efficiencies were reached with many other isotopes of the elements P, Cs, Er, Dv, Ho, Zn, Se, mostly in the chloride form. Alpha-sources of 22lFr and 2l3Bi (being daughter products of 225AC) were also prepared by evaporation. Here the actinium activity didn't evaporate.
Only the daughter products came on the aluminium backing (Lo 67). The activity distribution was measured for thallium sources with an ab-sorbing shield with a pin-hole in front of a Geiger-MUller counter.
Relative intensity
, 1
r — I 1 1—'T
- 2 - 1 0 +1 +2 • (mm) DisiQiice fro;!, c e n t e r of source
fig. 43 Activity distribution of a vacuum evaporated source.
The resulting curve (see fig. 43) showed a continuous and rather uniform distribution.
A second pair of electrodes of the arrangement has been used for several purposes. Zapon backings can be aluminized by putting some aluminium on a tungsten wire. These electrodes can be provided with carbon rods to p r e -pare thin carbon foils for accelerator target backings. With a tantalum boat various materials can be evaporated to make thin films on zapon backings to do flux measurements in a nuclear reactor. With gold, thick-nesses ranging from 30 A to 2000 A were obtained. By burning up a tungsten wire it was even possible to produce tungsten films.
CHAPTER V. MEASUREMENTS 1. Theory. 1.1. Introduction.
Our spectrometer is suitable for several types of measurements:
a. for determinii^ intensities, endpotnt energies and shapes of beta-con-tinua .
b. for measuring coincidences and directional correlations between e l e c trons (beta's or conversion electrons) in the spectrometer and other r a -diations (gamma quanta, electrons or alpha rays) detected in a second spectrometer.
A brief summary of the theory for these types of experiments will be given. 1.2. Beta decay
Considering a single beta branch, we can express the distribution of e l e c -trons witli a momentum between n and n + d n as
N ( r i ) d n = ^—^ F(Z,e)n^(E - e ) ^ a ( Z , E ) S (E)dn (29)
In this formula the symbols have the following meaning: N(ri) = counting rate of electrons with momentum n
n = relativistic momentum (in units mgC) e = total relativistic energy (in units mQc2) g = coupling constant
0 ( E ) = screening correction F(Z,e) = Fermi-function
S ( E ) = shape-factor for an n times forbidden transition.
When plotting {N(n)/F(Z.e )n2a (Z, E )Sjj(e)) 2 against £ one gets a straight line. This is called the Fermi-Kurie plot. From (29) it is seen that this straight line will intersect the E -axis at the point e Q. This point is called the maximum energy or the endpoint energy of the electrons from the beta branch considered.
The values of F ( Z . E ) are tabulated (N.B.S. 52). In the early calculations of these values the extended nuclear charge distribution, nor the screening by atomic electrons was taken into account. Later on several corrections for these effects were calculated (Ro 51, Bh 62. Bu 63, Bu 65, Ya 66, Ma 66 and Dz 56).
Explicit screening corrections were given by Longmire and Brown (Lo 49) and Reitz (Re 50). The calculations of BUhring (Bu 63. 65) and of Matese and Johnson (IVIa 66) show in general a very good agreement with Reitz's r e -sults. The screening corrections are generally small except at low energies. In this low energy region (below 50 keV) a big difference occurs between Reitz's and Longmire and Brown's values (see chapter V, paragraph 4). Beta transitions may be classified according to their transition probability'