A N D 233 PA, FOLLOWING NEUTRON
CAPTURE
P R O E F S C H R I F T
TER VERKRIJGING VAN DE GRAAD VAN DOCTOR IN DE TECHNISCHE WETENSCHAPPEN AAN DE TECH-NISCHE HOGESCHOOL DELFT, OP GEZAG VAN DE RECTOR MAGNIFICUS DR. IR. C. J. D. M. VERHAGEN, HOOGLERAAR IN DE AFDELING DER TECHNISCHE NATUURKUNDE, VOOR EEN COMMISSIE UIT DE
SENAAT TE VERDEDIGEN OP WOENSDAG 19 FEBRUARI 1969 TE 14.00 UUR DOOR P1301 6196 WILLEM HOEKSTRA Natuurkundig Ingenieur geboren te Apeldoorn
# - 69 " %
1969 " ^ £ e L ^ l > Drukkerij J. H. Pasmans - 's-GravenhageC10038
57529
BIBLIOTHEEK TU Delft P 1301 6198
Summary
Chapter Introduction
1. Introduction on thermal neutron capture gamma rays 9
2. The nuclear reactor as neutron source 9 3. The detection of neutron capture gamma rays 10
4. The Nilsson model 10
Chapter II The experimental arrangements
1. Introduction 14 2. The internal target geometry 14
3. The external target geometry 15
4. The Ge(Li)detector 18 5. The electronic system for the Ge(Li)detector 20
6. The pair spectrometer 24
Chapter III ^^Cl(n,7)3^Cl 1. Introduction
2. The exponential-pulse generator 3. Measurements 4 . Conclusions 27 27 28 31 Chapter IV 27Al(n,7)28Al 1. Introduction 2. Measurements 3. Discussion 33 33 33 Chapter V ^^S.187^^(^^^186.188^6 1. Introduction 2. Measurements 3. Discussion 40 40 41
Aan Jeannette en Kathelijne
1. Introduction 47 2. Measurements 47 3. Discussion 47 Chapter VII L e v e l s in 23 3pQ 1. Introduction 53 2. Sources 53 3. Experimental arrangements 54
4. Gamma ray measurements on ^ N p 55 5. Electron measurements on Np 56 6. Gamma ray measurements on '^'^^Th 57 7. R e s u l t s of the measurements 58 8. The decay scheme of ^^^Np 70 9. The ground state of ^^^Jh 73 10. The decay scheme of ^^^Th 74
11. Discussion 75 Appendix 80
References 83
A N D 233 PA, FOLLOWING NEUTRON
CAPTURE
Summary
The development of the lithium-drifted germanium detector made it possible to combinate very good energy-resolution for gamma ray detection with the efficient data-accumulation, a s used earlier only together with scintillation counters. The use of this detector in the study of nuclear structure with the aid of neutron capture gamma rays represents a major breakthrough in this fielH
At the Reactor Institute in Delft (H.O.R.), a neutron capture gamma ray experiment was arranged at the tangential beam tube of the reactor. With the aid of the lithium-drifted germanium detector single gamma ray spectra have been measured of the reactions ^^Al(n,7)'^^Al, *«5'i87Re(n,7^i86,i88Rg ^ d 233Th(n.7i233Th. The reaction ^^Clin.-))^^ CI was used for the calibration of the experimental arrangement.
The results of the (n,7)-reactions on " Th and Re have such a complex character that coincidence measurements are necessary to make a decay scheme. The new experimental set-up, installed for this purpose, had not yet yielded results at the time of writing of this t h e s i s . Yet, it was possible to propose values for the total decay energies of t h e s e r e a c t i o n s .
It appeared that on the basis of the known data no reasonable Nilsson model assignment could be made for 22 m Th, i.e. one of the product nuclei of these (n,7)-reactions. A possible explanation which assumed the e x i s t e n c e of an isomeric state of similar half-life was disproved by experiments, showing no trace of any isomer with a half-life above 0.1 second. Therefore the decay of ^^^Th and that of ^^^Np, leading to the same final nucleus ^^^Pa, was studied with Ge(Li) gamma counters, Si(Li) electron detectors, surface barrier alpha spectrometers and coincidence equipment.
Interpretation of the results made possible a unique '/4 [6,3,1] assignment to 22 m ^^''Th and indicated the existence of a K = 3/2 band in ^'^'^Pa; this band had a rather uncommon deformation resulting from Coriolis interaction with K = /4 and K = 3/2 bands.
Chapter I INTRODUCTION
I, I Introduction on thermal neutron capture gamma rays
In interaction with matter a neutron can be captured by a nucleus. The energy balance, if the neutron has a kinetic energy E^, i s E +M + M . = M . . , + ( 0 + E ) . M . M , and M. ., represent the
n n A A + 1 *^ n' n' A A + 1
energy-equivalents of the m a s s e s of respectively the neutron, the initial nucleus and the product nucleus. The energy within the brackets i s the total excitation energy of the product nucleus. This excitation energy is released very rapidly ( < 1 0 ~ ' ' ' sec) as one or more gamma quanta and a small recoil energy of the product nucleus. The product nucleus is then left in its ground s t a t e or in an isomeric s t a t e (1).
If thermal neutrons are used, E (<0.1 eV) can be ignored with respect to Q (>1 MeV). The neutron separation energy Q of the product nucleus i s now equal to the sum of the energy of the ground s t a t e gamma ray (E ) and the recoil energy E / 2 M ^ ^ j of the product n u c l e u s . If there i s no ground s t a t e decay s u c c e s s i v e gamma quanta can pro-vide the same information (2,3,4).
Even more important than the reaction energy balance is the information on the structure of the levels fed in c a s c a d e t r a n s i t i o n s . This information can be obtained from the study of feeding and decay of these levels in neutron capture gamma ray spectra. Several of these spectra have been measured already and have been d i s c u s s e d (e.g.5, 6,8,9). The spectra of nuclei in the spherical region show a number of discrete transitions (see chapter III and IV). The spectra of nuclei in the nonspherical region are typified, using current techniques, by an unresolved continuum, sometimes with resolved transitions at the high- and low-energy end of this continuum (7). The spectra of the reactions lö5''8^Re(n.'>^^ö^''8ÖRe and ^^^Th{n.yf^^Th (resp. chapter V and VI) show this characteristic.
1,2 The nuclear reactor as neutron source
The source strength in a neutron capture gamma ray experiment is given by Bcrcp (neglecting self-shielding of the target), in which B represents the total number of atoms in the target, cr the capture c r o s s -section of the target material and <^ the flux of neutrons.
The energy spectrum of the neutron flux in a water reactor has the form E~^ with a peak in the fast region (2MeV) and a peak in the
thermal region (10). The capture cross-section a l s o depends on the energy of the neutron and is given by the Breit-Wigner formula (1). Outside the resonances o-is proportional to E~'^. Thus thermal neutron capture i s favoured and only a strong resonance absorption in the lower part of the epithermal region ( l e V < E <100 keV) can compete (11). When measuring thermal neutron capture gamma ray spectra with the aid of a nuclear reactor the target can be placed either in an external beam or in a high-flux region beside the core. The gamma ray detector must be outside the reactor shielding (1).
'Background radiation from the core can be avoided by using a tangential beam tube. In an internal target geometry the detector system is seen through a small solid angle; the flux at the target position, however, is large. In the external target geometry the neutron flux is much lower, but the solid angle is much larger. (See chapter 11,2 and 3).
1.3 The detection of the neutron capture gamma rays
Earlier high-resolution gamma ray detection in neutron capture experiments has been carried out with the aid of magnetic Compton spectrometers (1,12,13,14). In more recent experiments preference has been given to the lithium-drifted germanium detector (Ge(Li)detector) in combination with a data-accumulating system, e.g. a multichannel pulse-height analyser (15) (See 11,3). The best resolution obtained with a Compton spectrometer was about 0.25% above 2 MeV (1), the Ge(Li)detector can give a resolution ranging from 3 keV at 2 MeV to 8 keV at 8 MeV (15) (See fig. 1,1).
Using an internal target geometry, single gamma ray spectra only can be measured. With this geometry, target changing cannot be carried out at short notice because of the radioactive strength of the target material due to the daughter nucleus of the (n,7)-reaction and because of a s p e c t s due to the reactor geometry. Coincidence exper-iments are possible thanks to an external target, but background radiation must be avoided in this c a s e (See chapter 11,3).
The small volumes of the first Ge(Li)detectors make coincidence experiments unattractive. In the meantime the volumes of the Ge(Li)-detectors grow by factor 50, but an efficient coincidence experiment
s t i l l needs a complex data-accumulating system.
1.4 The Nilsson model
In the nuclear shell model the nucleons are moving independently in an isotropic, averaged potential. The experimentally observed discontinuities a s s o c i a t e d with the so-called magic numbers could be
«MalB/ltk O
E;f.VV^V?><
J^<y^ ViXi;^- •-^X.y
1
35 fig. 1,1. A part of the gamma ray spectrum of the reactionCl(n,'>')-CI, measured with respectively the compton spectrometer of Groshev (a), the compton spectrometer at Los Alamos (b) and our first GeLl)detector (c) (1).
understood when a rather strong spin-orbit coupling was assumed. Nuclei with proton and neutron numbers very different from those magic numbers have large deformations. This influences the potential and thus the motion of the individual nucleons. Moreover the collective motion of the nucleus should now be considered. The interplay between the collective modes of motion and the individual particle motions forms the basis of the unified nuclear model (86). This " N i l s s o n m o d e l " describes the motion of the last odd nucleon in the field of a deformed rotating core (88).
Because the most of our measurements (chapter V, VI and VII) have been carried out with isotopes in the non-spherical region we shall d i s c u s s in brief the Nilsson model and the rotation (and vibration) modes.
Instead of each (2j + 1 ) fold degenerate level with spin j in a spherical nucleus, the shell model now yields (2j + l ) / 2 l e v e l s , each twofold degenerate. They can be labelled with a new quantum number K, which, in the c a s e of slow collective rotation, can be defined as the projection of the particle angular momentum on the nuclear sym-metry axis (K < j). In addition to K, other new quantum numbers, n^ and A are introduced in the Nilsson model. The number of oscillator quanta along the symmetry axis i s represented by n^ (the total number, N, i s known from the shell model). The component of the total orbital angular momentum along the symmetry axis is represented by A. The difference ^ = K — A, i . e . the component of the intrinsic spin of the l a s t particle on the symmetry a x i s , i s limited to values 0^2. Only in the limit of large deformations are n^ and N good quantum numbers. Diagrams of energy levels for odd protons and odd neutrons as a function of the deformation have been published (2,59,84,88,89). In these diagrams the levels are indicated by asymptotic quantum numbers in the following way: K'^L N.n^.A] 21, the last symbol given as an arrow upwards for +/4 and downwards for—/4, for instance 3 / 2 ^ [ 6 , 5 , 1 ] t . It often happens that 2 is not written, since it can be derived from the other quantum numbers. The parity can be derived too: TT = (—1)^. Knowing the deformation one can find the expected ground s t a t e level of the last nucleon in these diagrams.
The deformed nucleus may rotate with preservation of shape and internal structure. Thus a band of rotational s t a t e s is generated with spins I ^ K , and with energies:
Here E represents a constant depending on the intrinsic structure of the nucleus, J is the effective moment of inertia and -fi is P l a n c k ' s constant divided by 27T. TWO special c a s e s exist. If K=0, all levels in the band have either even spins or odd spins. In the c a s e K=!^, a correction term is present, which represents a decoupling of the spin angular momentum from the rotational motion. The constant a is called the decoupling constant.
Rotaüon-particle coupling c a u s e s a deviation from the 1(1 + 1) law. This coupling (Coriolis interaction) is usually small, but in Chapter VII an example of a strong interaction can be found.
The energy a s s o c i a t e d with the excitation of the lowest vibra-tional s t a t e s is in the order of 1 MeV in the majority of the regions where the nuclei are strongly deformed. Therefore the low-energy levels can be described by the Nilsson model and the rotational law only. No systematic treatment has yet been found to explain level structures that have to be interpreted as being the result of coupling with vibrational modes.
Chapter II
TÜE EXPERIMENTAL ARRANGEMENTS
11.1 Introduction
The reactor in question i s that of the Reactor Institute Delft (H.O.R.). The fact that the reactor power had been raised from 200KW via 500 KW to 2 MW was of great advantage for the experiments.
The tangential beam tube used p a s s e s through a box between the reactor core and the thermal column (fig. 11,1). The thermal flux at 2 MW near the core in this tube i s about 10 s e c ~ cm~ . The fast flux is more than two times lower.
In the beginning of the experiments an internal target geometry was applied, but future a s p e c t s about coincidence measurements with two Ge(Li)detectors, using a P . D . P . 9 computer on line, may make an external target geometry more a t t t a c t i v e .
11.2 The internal target geometry
The aluminium inside tube is placed within the aluminium lining of the tangential beam tube. There i s a shield of water between the inside and the outside tube.
The target can be installed from the left-hand side; on the right-hand side the detector system together with the gamma ray collimator is positioned (fig. 11,1 and 2).
tongentiol beom tube
fig. n , l b . A vertical c r o s s - s e c t i o n of the reactor (H.O.R.).
The target material is contained in an aluminium cylinder attached to an arm, which arm in turn i s connected to the top of an aluminium rod. The arm and the cylinder are connected flexibly. The rod can be pushed through a hole on top of the left-hand shieldings (fig. 11,2), consisting of a specially shaped graphite plug near the basin and paraffine. As soon a s the target h a s p a s s e s these shieldings it falls into the right position: the centre of the inside tube. The shutter on this left-hand side is normally kept closed. Corrections to the target position can be made by rotating the rod with the aid of a flexible axis bypassing the shutter (fig. 11,1).
At the right-hand side the gamma rays from the target are colli-mated by a system of three lead diaphragms (fig. 11,4). Graphite and paraffine plugs are added to limit the outgoing neutron flux (16,17). The inside tube i s maintained at a lower air pressure in order to limit the effect of possible s p i l l s .
11,3 The external target geometry
In coincidence experiments the source strength i s limited by the resolving time of the electronic equipment. A large solid angle of the detectors is n e c e s s a r y for a good efficiency (18). Thus coin-cidences on neutron capture gamma rays can be measured only with an external target geometry.
A scattering device is placed near the core inside the tangential beam tube in order to get an external neutron beam. The scattering material c o n s i s t s of demineralized water contained in a box of 1 mm thick aluminium. The thickness of the scatterer i s about 1 cm and it is placed at an angle of about 20° to the axis of the tangential beam tube to get the highest possible flux of thermal neutrons in that directions (19,20) (fig. 11,5).
+ B t C
fig. II, 3. Vertical croBS-aection of the rlqht-hand part of the inside tube
of the tangential beam tube with the external target geometry. CT>
fig. II, 2, Vertical cross-section of the inside tube of the tangential beam tube with the Internal target geometry.
The power absorption of the scatterer material due to the gamma radiation of the reactor core makes cooling n e c e s s a r y . This i s effected by circulating the water through a heat exchanger. Before the water leaves the reactor shielding it is stored for a few minutes in a tank in order to allow decay of ^^N activity. This activity r e s u l t s from the reaction 0(n,p)'^N and has a half-life of 7.6 s e c o n d s . This tank i s positioned between the reactor shielding and the shutter.
The shieldings on the left-hand side are the same as in the internal target geometry experiment. The water is brought to and from the scatterer through aluminium tubes which are positioned inside a rod on top of the left-hand shieldings.
To the right the neutron beam is collimated with the aid of paraffine and boroncarbide. Lead collimators are added to limit the outgoing gamma rays (fig. 11,3). With the reactor at a power of 2 MW, a thermal neutron flux of 5 x 10 s e c " cm"^ a s been measured at the end of this collimator. The gamma ray intensity in the beam was a few hundreds of roentgen per hour and the fast neutron flux i s estimated
to be 5 X 10^ cm~2-sec~^ (20).
Between the end of the collimator and the beam catcher the beam p a s s e s through a cylinder of paraffine with a high concentration of boroncarbide; around the target a cylinder (2 cm inner and 10 cm outer diameter) containing lithiumfluoride i s used which h a s a large thermal capture cross-section, but which produces no gamma rays.
The beam catcher c o n s i s t s of a 3 cm thick layer of boroncarbide attached to an iron rod of 80 cm length surrounded by paraffine and boroncarbide. Behind the catcher i s 10 cm of lead, around the catcher 20 cm of concrete.
fig. II, 4, The diaphragms of the gamma ray collimator for the internal target geometry. The diaphragms A,B and C can be changed e a s i l y .
oluminium
fig. II, 5. T h e s c a t t e r e r , c a u s i n g an e x t e r n a l neutron b e a m .
11,4 The Ge(Li)detector
Gamma rays mainly interact with matter through three p r o c e s s e s : the photon-, pair- and Compton effect (see fig. 11,10). It i s p o s s i b l e to determine the energy of a gamma quantum (E ) due to the fact that the two first p r o c e s s e s give rise to electrons (one and two respectively) with a total energy (E^) determined uniquely by the gamma ray energy. In the pair effect, where this energy E is divided over a positon and a negaton, the first one gives by annihilation rise to two gamma quanta of 511 keV, emitted in opposite directions. This effect can be used a s described in section 11,6.
Gamma ray measurement with high energy-resolution and very reasonable efficiency was recently made possible by the advent of the lithiumdrifted germanium detectors (Ge(Li)detectors). The d e s -cription of its properties by E wan and Tavendale opened a new period in the research of nuclear physics (15,21,22,23).
The Ge(Li)detector i s a semiconductor diode with a reversed b i a s voltage. It c o n s i s t s s u c c e s s i v e l y of a p-, an i- and a n-type layer of germanium. The i-type layer is created by drifting lithium ions into p-doped germanium (24,25). An electric field is present in the i-type layer. The electron(s), caused by the interaction of a gamma quantum within this layer, will create electron-hole pairs, which are collected by the e l e c t r i c field. The collected charge i s directly proportional to E .
e
The relative error in the number of electron-hole pairs is equal to 2.36 / F . e . / E \ Here e represents the energy n e c e s s a r y for the production of one pair (Ge : 2.85 eV) and F i s the Fanofactor, which i s a correction for the n o n - s t a t i s t i c a l nature of the production of electron-hole pairs (F<0.3)(26,27). The low values of F and e make a semiconductor detector much more attractive than the previous popular scintillation counters, which otherwise have very similar properties. The leakage current of the detector c a u s e s a contribution in the s t a t i s t i c a l error in the number of the produced electron-hole p a i r s .
/ j
(I M i l )
^
potythtnt
stoinltn stttl
fig. II, 6. The cryostat for the Ge{Ll)detector, used in combination with the pair spectrometer»
The surface leakage current i s minimized by cleaning the detector thoroughly. P a r a l l e l leakage currents through connectors, e.g. must be avoided. The leakage current due to thermal generation of pairs can be made small by using the detector at low temperature. Therefore the detector i s placed in a cryostat and is kept at liquid nitrogen temperature (77°K) (fig. 11,6 and 8).
Different detectors have been used for the experiments. An encap-sulated 0.5 cm'' planar detector of B.C.A. has been warmed up and drifted several times between the experiments in order maintain a good energy resolution. A 5 cm^ coaxial detector of Princeton Gamma Tech was delivered in its own cryostat and is kept cool. Thereafter an u-type drifted detector of 0.5 cm'' was fabricated at the Institute (28). We a l s o used a 13 cm^ Ge(Li)detector of Princeton Gamma Tech and a 5 cm^ detector made in the Reactor Institute.
11,5 The electronic system for the Ge(Li)detector
A charge-sensitive low-noise preamplifier is connected to a Ge(Li)detector (29). The noise in this preamplifier i s added to the s t a t i s t i c a l error in the number of produced electron-hole pairs. This preamplifier noise contains two components: the noise without detector at its input and the noise i n c r e a s e due to capacitance added at its input. Thus a low capacitance detector i s advantageous.
Preamplifiers with electron tubes were used first (23,30,31,32), but later on the input stage was made with a field effect transistor ( F . E . T . ) (33,34,35,36).
A preamplifier h a s been built with an EC 1000 triode in a c a s -code input s t a g e (37) (fig. 11,7). Its noise without detector i s equi-valent to 2 keV, its slope a s a function of the capacitance 0.045 keV/pf. Later Tennelec TC 130 and TC 135 preamplifiers were used. Their input characteristics are better, and they are more stable and have l e s s microphonic effects.
An input stage has been mounted inside a cryostat in order to avoid the capacitance due to connectors and cables between the Ge(Li)detector and the input stage of the preamplifier. This has an additional advantage: the noise of the field effect transistor d e c r e a s e s with decreasing temperature. The best value for the temperature of the field effect transistor is 130°K (35). The input stage of a Tennelec TC 130 preamplifier is used. The field effect transistor is put in the middle of a heat r e s i s t a n c e between the cold finger and the wall of the cryostat (fig. 11,8). Thus, the Reactor Institute 0.5 cm^ Ge(Li)-detector (28) yielded a resolution of 1.4 keV at 100 keV (fig. 11,9) and of 2.5 keV at 1 MeV.
O+300V.
det. bios o OUT
O I j i ^ F
I
soof.
40n 1 4= O.ln 1k2 2xEC100rx
E88CCfig. II, 7. The electronic scheme of the low-noise charge-sensitive pream-plifier, having an EC 1000 triode in its input s t a g e .
The charge-sensitive low-noise preamplifiers are connected to main amplifiers (fig. 11,12). The frequency response of the main amplifier is limited by a differentiating and an integrating network in order to get a better signal-to-noise ratio (38,39,40).
The output pulse of the main amplifier i s fed to the input of the analog-to-digital converter of the pulse-height analyser either directly or via a biased amplifier. The latter serves the following purpose. A gamma ray spectrum with energies up to 10 MeV at a resolution of about 10 keV" requires at the very least 2000 channels of the pulse-height analyser since one should notice a peak in certainly not l e s s than two adjacent channels. Only Nuclear Data 512 channels and
otuminium
F E T O—»WOU of lh« cryostot 2N3823 i
fig. II, 8. Vertical c r o s s - s e c t i o n of the cryostat with the 0.5 cc Ge(Li)-detector and the cooled field effect transistor. Also the first stage of the TC 130 preamplifier, partly built inside the cryo-s t a t , i cryo-s cryo-shown, 4.»«-».»• i V SCO 700 -chonntl numbw
fig. II, 9. A gamma ray spectrum of a Co source, measured with the 0,5 cc Ge(Ll)detector in the position as shown in figure 11,8.
Laben 1000 channels pulse-height analysers were available, so that a biased amplifier is n e c e s s a r y in order to expand the s p e c t r a .
In the beginning we used our own main amplifier together with the RIDL 30-21 biased amplifier. Later on the main amplifier 1410 and the biased amplifier 1460 of Sturrup were used.
1711 h« 17»4" 1 H « S - • 1 1 ( 1 N
•v^.-V*^-•••]
- CHANNEL N U M M Rfig. 11,10. The high-energy part of the s i n g l e - and pair-spectrum of a 24
Na source. The energies are according to (59), The s i n g l e -e s c a p -e p-eak is indicat-ed by 2754', th-e doubl-e--escap-e p-eak by 2754", The measure time of the pair spectrum is a factor
| - ^ photomultip(i*rtubK
fig. 11,11. Horizontal and vertical c r o s s - s e c t i o n of the pair spectrometer.
11,6 The pair spectrometer
A spectrum of gamma rays can be simplified by using the pro-perties of the pair effect. A single gamma ray spectrum of a mono-energetic gamma source (E >1.1 MeV) c o n s i s t s of a Compton conti-nuum with single- and double-escape peaks superimposed on it and a photo peak above its end. Only the double escape peak i s obtained (fig. 11,10) when a pair spectrometer i s used.
A pair spectrometer can be arranged with the aid of three detectors (41,42,43). Pair production takes place in a Ge(Li)detector positioned in the centre (44,45). Two N a J ( T l ) c r y s t a l s , placed beside this Ge(Li)-detector (fig. 11,11), are used to count a 511 keV gamma quantum, caused by the annihilation of the positon. The electronic equipment will extract the right interactions (fig. 11,12).
The 3 x 3 inch N a J ( T l ) c r y s t a l s are mounted on photomultiplier tubes (E.M.I. 9531 AF). The output pulse of the anode is fed to a clipping circuit, the output pulse of the last dynode to an inverter (fig. 11,13).
The clipped pulses of the photomultipliers and those of the Ge(Li)detector are fed to tunneldiode discriminators (46) (fig. 11,12 and 13). The uniform narrow output p u l s e s of these tunneldiode discrimi-nators are transferred to a fast-coincidence circuit, giving an output pulse if three input pulses arrive within 100 nanoseconds.
The p u l s e s of the inverters are fed to two differential discrimi-nators giving an output pulse if the input pulse is equivalent to total absorption of a 511 keV gamma quantum in the N a J ( T l ) c r y s t a l .
The output pulses of the fast-coincidence circuit and the differ-ential discriminators are fed to an and-circuit. The output of this circuit is sent to the coincidence input of the analog-to-digital con-verter or to the gate input of the biased amplifier. A pulse of the
G.(li)[ d e t e c t o r / / NoJdO p m . Ü I » /l—i-—1 _^ »toJ(TO p.m. 1 • • A B B A -^ -L 1 J » » » » n . B D.O. — T.D. TB. T.D. F C C V S . .. -'m .
•
A.D.C A C 1 •• _fig. 11,12, Block scheme of the electronic equipment used in combination with the pair spectrometer.
p.m. A B D.D. T.D, F , C . C . A.C. I II III A.D.C. P.H.A. P . G . photomultiplier tube inverter
limiter, delay-line clipper differential discriminator tunneldiode discriminator fast-coincidence circuit and-circuit
charge-sensitive low-noise preamplifier main amplifier
biased amplifier
analog-to-digltal converter pulse-height analyser exponential-pulse generator
Ge(Li)detector can be analysed only if the andcircuit h a s been a c t i -vated.
In this way the peak-to-background ratio of the double-escape peak could be improved by factor 7. The efficiency of the detector equipment will then d e c r e a s e by factor 55. These factors are very similar to those achieved by E wan and Tavendale (15). The first factor depends on shape and volume of the Ge(Li)detector, the second on shape and efficiency of the surrounding c r y s t a l s .
1M»F WOpF m ^ TO WVEIITER ^ 3 1 k p F H p P
Hhdb-n^'M
^1—GOUT Ó'^^joa'i'T"'"'' DM»* nkpF • i l v -tjv -1IV «fiv - L - « V i i i | j — o » «©
1000 4-\\-1000 j - o - I I ' V * 7 A . . „ 1 0 0 ^ ! « . --jt-IM.t14 1 0 0 0 ^ |itoor^^^iE?f^
S
T U M N E l l V7 DiooE m -TTH-OUHl-fig. II, 13. The electronic schemes of the clipper (a), the tunneldiode discriminator (b) and the fdst-coincidence circuit (c),
Chapter III 35ci (n,y)^^Cl
111.1 Introduction
Many radioactive sources with rather good half-lifes and well-known gamma ray energies up to 3 MeV are available, but these energies are not high enough for the energy calibration of the Ge(Li)-detector system in the neutron capture gamma ray experiments. Gamma rays of a known (n,y)-reaction must be used for this purpose.
Most experiments are carried out with the source material in an aluminium container, so that the ^^Al capture gamma ray at 7723.8keV can be used for this calibration (see chapter IV too) (9,44,47,48). This source h a s the disadvantage, however, that no other strong tran-sitions are present for testing the linearity of the device. This test has been carried out in a separate experiment using the gamma rays of the reaction '^^Cl(n,7)''^Cl. This reaction has been studied previous-ly with sufficient accuracy (9,49,50,51,52).
Target changing cannot be carried out at short notice in the internal target geometry. The method of energy calibration is there-fore the following: pulse heights from a pulse generator are compared with those caused in the Ge(Li)detector by the reaction ^^Cl(n,7)^^Cl. Later on the pulses of this pulse generator can be used again to con-struct the energy calibration curve of other gamma ray spectra.
111.2 The exponential-pulse generator
An exponential-pulse generator can induce a charge on the input of the preamplifier of the Ge(Li)detector through a small capacitance
(lpf)(53). The induced charge can be compared with the charges caused by the interaction of gamma rays with the Ge(Li)detector (see chapter II,d).
The precision exponential-pulse generator c o n s i s t s of a well-stabilized D.C. voltage source connected to the ends of a ten-turns potentiometer. The divided voltage is fed to a relay with mercury contacts, switched 50 times per second (fig. 111,1). When the contact of this relay is in the position A the capacitor C is charged, in the position B this charge is fed to the test-input of the preamplifier (fig. II, 7,8a^. The decay time of this pulse depends on RC (R defined a s in fig. 111,1), which is chosen so that the electronic system behind the input stage of the preamplifier cannot distinguish between a p u l s e
fig. III, 1. The exponential-pulse generator.
of the _Ge(Li)detector and a pulse of the exponential-pulse generatoij a modified model 414 of the Radiation Instr. Co.
It i s found that the decay time of the pulse out of the generator must be at l e a s t 1 millisecond in order to avoid ambiguities in the relation between the potentiometer reading of the pulse generator and the detected gamma ray e n e r g i e s . These ambiguities depend on the level of the biased amplifier.
The relation between this potentiometer reading and the detected gamma ray energies has been approximated by polynomials using the TR4 computer of the Technische Hogeschool. It was found that a linear relation agreed with the data of the measurements of Groshev et al. on the reaction ^^Cl(n,7)^^Cl (50). Higher order curves did not provide improvement.
This linear relation facilitates the construction of the energy calibration after changing a part of the electronic equipment. More-over the linear response between the input and output pulse heights of the electronic equipment is not very important. Only the gamma ray energy of one transition i s needed for a good energy calibration. The energy of this transition must be a s high as possible in order to de-crease the error.
111,3 Measurements
A target of 10 gram NaCl was u s e d . When the reactor was down, the pair spectrometer could be tested with the remaining ^''Na activity (fig. 11,10). During irradiation only the gamma rays of the reaction '^^Cl(n,y)^^Cl were measured: the capture cross-sections of respec-tively 23Na, 3^01 and ^ ' C l are 0.5, 30 and 0.005 barn.
Several single spectra have been taken with different Ge(Li)-detectors and different amplifier s y s t e m s .
In fig. 1,1 and III.2 and 3 spectra of the reaction ^^Cl(n,7)^®Cl, measured with the pair spectrometer are shown. The efficiency curve of the pair spectrometer with the smallest Ge(Li)detector in its centre was calculated with the aid of the intensities of the gamma ray energies
2910' 302t'
1
5 iv\ \. • ; . • * • • • . • . • • • • ' • ' • •• • , • •• " t o U3 • chonnal numkerfig. Ill, 2, A l o w - e n e r g y part of the gamma ray spectrum of the r e a c t i o n
Table III, a
Gamma rays from •'^Cl(n,r)''®Cl.
E k 8573 ± 7786 ± 7410 ± 6974 ± 6621 ± 6423 ± 6344 ± 6266 ± 6110 ± 6028 ± 5959 ± 5902 ± 5715 ± 5585 ± 5516 ± 5246 ± 5016 ± 4981 ± 4945 ± 4825 ± 4757 ± 4732 * 4613 ±
r
sV 4 5 5 5 5 83) 82) * 6 5 lO^' * 6 5 4 5 4 5 4 4 * 5 5I) 52) 4 4 rel. 3.1 a2 12.7 2.7 13.6 0.3 0.3 0.6 25.2 0.2 0.3 0.3 6.1 0.8 1.5 0.6 0.6 4.2 1.4 0.3 0.3 I.O 0.7 keV 4589 ± 5 4547 ± 8 * 4522 ± 5 4500 ± 5 ^ ' 4444 ± 4 4417 ± 6 4298 ± 4 * 4203 ± 5 * 4165 ± 5 ^ ) 4138 ± 8 4080 ± 4 4053 ± 4 3980 ± 4 3957 ± 7 3822 ± 4 3742 ± 5 ^ ) 3596 ± 5 3 566 ± 4 3510 ± 4 * 3435 ± 4 * 3383 ± 5 3338 ± 5 3121 ± 4 3067 ± 4 rel. 0.3 0.4 0.6 0.3 l.I 0.5 0.5 0.2 0.1 0.2 0.7 0.7 1.2 0.3 1.8 0.4 0.5(1.2) 1.2(0.8) 0.8(0.6) 1.0 0.5 0.8 0.9 3.6 keV 3024 ± 4 3002 ± 5 2980 ± 4 2896 ± 5 2868 ± 3 2852 ± 4 2810 ±4^) 2681 ± 3 2628 ± 4 2535 ± 7 2498 ± 3 2427 ± 3 2422 ± 4 2235 ± 3 2033 ± 7 1957 ±33) 1949 ± 3^)1 1636 ±32)' 1597 ±32) 1329 ± 22) U 6 5 ± 2 792 ±22) 518 ±22) rel. 0.8 1.4 1.4 0.6 5.9 0.8 0.7 1.5 0.5 0.7 0.5 0.5 0.5 0.8 1.2 10.6 18.5 l.I 3.2 1.3 27.5 18.0 20, , Transitions not placed in a decay scheme.
) Transitions not seen by Groshev et al. (50); the i n t e n s i t i e s are derived „. from the present measurements.
A Not seen in our measurements. ^' Broad line.
1.0 o.a 0.6 o.t 0.2 1 2 3 i S t 7 • 9 HlV
fig. Ill, 4, The efficiency curve of the 0.5 cc Ge(Li)detector of R.C.A. used in combination with the pair spectrometer.
given by Groshev et a l . (50). The result is shown in figure 111,4. The curve indicates that the pair spectrometer has adequate sensivity above 2.0 MeV (54,55).
111,4 Conclusions
The energies and i n t e n s i t i e s of the gamma rays of the reaction 3^Cl(n,7)'^®Cl are tabulated (table III,a). In a few places our results indicate slightly different v a l u e s . Also some new transitions have been found, which can be placed in the decay scheme published by Groshev et a l . (50) and by Endt and van der Leun (51).
The 6028 keV transition can be ascribed to a 6032 ± 8 keV level found in the (d,p)-reaction on CI; this level may be fed by the 2535 keV transition. It is not possible for the 4945 keV gamma ray to originate in the reaction ^^Ciniy) C a s proposed by Groshev et a l . (50). It is almost impossible for his experimental arrangement to have the same intensity of background radiation. Moreover this tran-sition has not been found in the spectrum of the Al(n,7)2^Al reaction (see chapter IV). This transition i s not placed in the decay scheme. The 4732 keV transition reported by Groshev was not found. Instead we found a 4828 keV transition. This gamma ray may be the ground state transition from the 4834 ± 8 keV level found from the (d,p)-reaction and, also according to Bartholomew et a l ' s compilation (9), fed by the 3742 keV transition. Earlier authors suggest a 3742 keV level.
Levels on 4504 ± 7 keV and 4413 ± 8 keV are known from the (d,p)-reaction on CI. These levels are probably involved in 4080-4500*keV and 4165*-4417 keV c a s c a d e s , of which the gamma rays indicated by stars are seen here for the first time.
4522 4569 4500*4547" 4613" " " " 4«15' $5I«' ^ J«:A?:v ••>: 5617 "No 6467" • chonntt number 100
fig. Ill, 3, A high.energy part of the gamma ray spectrum of the reaction o r o c
Chapter IV 2 7 A 1 (n,y)28Al
IV, 1 Introduction
The target material in other measurements is encapsulated in an aluminium cylinder. The gamma quanta radiated by the cylinder material were studied separately.
The three available measurements on the reaction 2^Al(n,7)2^Al do not agree very well (5,6,9). The last r e s u l t s (9) were not yet avail-able when the present measurements were commenced.
IV,2 Measurements
The capture cross-section of 2^A1 is 0.2 barn. An aluminium rod of 10 cm length and 1 cm diameter did not provide sufficient activity for measurements with the aid of the pair spectrometer.
We have measured single gamma ray spectra with the Princeton Gamma Tech 5 cc Ge(Li)detector using the internal target geometry
(fig. I V , l , a , b ) .
It was possible to carry out the energy calibration with the aid of the ground s t a t e decay gamma rays of the reaction 2^Al(n,7)2^Al and with the aid of the exponential-pulse generator (see chapter III).
An average efficiency curve of the pair effect in the 5 cc Ge(Li)-detector was constructed with the aid of the intensities as given by Groshev et al, (5). The efficiency curve agrees with the shapes of such curves given by other authors (54,55). Moreover, our data seem to be in good agreement with those of Rasmussen et al.(9) (fig. IV,2).
IV,3 Discussion
In table IV,a the data of our experiment can be compared with those of the other authors. The levels known from the reaction 2^Al(d,p)2^Al are mentioned too, if the excitation energy i s close to a detected gamma ray energy. Rasmussen et al.(9) have measured very many gamma rays of which only the more intense transitions can be found in table IV,a.
Several of the high-energy gamma rays can be assigned to sitions from "a known high-energy level to the ground s t a t e . The tran-sitions of the capturing level to these levels have not been detected. Even Rasmussen et al.did not detect these transitions.
r> \*, > . • . . ^ • ' '. ' - = ^
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..-^ s o 1 1 1 1 1 1 1 1 1 1 " o K w D -ct < j ~ -_ -1 11
O • O . s s ^ o f « o e f w 3560 -CHANNEL NUMBER « V s ..•„»<>, J t 7 7 ' 3141' 4259' 4692' 3994'«OJJ- jl fl UU' 4736" •''••^VrvWvïtv. I A • ^^ ' CHANNEL N U M K R
fig. IV, 1. A high-energy and a low-energy part of the gamma ray spectru of the reaction ^"^Al{n,y)^^Al, mec
detector of Princeton Gamma Tech 27 28
Ge(Li)-T a b l e IV, a (1) 27 28 G a m m a r a y s from A l ( n , ' y ) A l . Bartholom keV 7724 ± 6 7340 ± 4 0 6980 ± 4 0 6770 ± 2 0 6610 ± 3 0 6500 ± 3 0 6330 ± 2 0 6130 ± 2 0 6010 ± 5 0 5880 ± 4 0 5780 ± 3 0 5600 ± 20 5410 ± 3 0 5310 ± 3 0 5210 ± 2 0 4940 ± 50 4790 ± 2 0 4660 ± 50 4450 ± 2 0 sw(6) r e l . 20 0.45 0.5 0.8 0.23 0.9 1.5 0.44 0.5 0.8 1.2 1.2 0.55 1.5 0.8 4.3 2.5 1.4 Groshev ( keV 7730 ± 1 5 6760 ± 2 0 6350 ± 2 0 6130 ± 2 0 (5880 ± 3 0 ) 5450 ± 30 5140 ± 1 5 4910 ± 2 0 4730 ± 1 5 4660 ± 20 4420 ± 20 5) r e l . 24 1.7 2.3 3 0.8 1,6 3,9 2.4 8 5 1 R a s m u s s keV 7723.8 7694.0 6978,4 6710,6 6622,8 6440.7 6316,2 6199.0 6101.6 6019.5 5926.4 5810.3 5765.9 5709.3 5585.8 5411.1 5301.8 5209.3 5134.2 4903,3 4766,2 4734,1 4690,6 4660,4 4427.2 en (9) rel. 16,6 0,83 0,08 0,33 0,08 0.37 1.47 0.48 1.79 0.21 0.06 0.10 0.33 0.40 0.53 1.11 0.21 O.Il 1.63 1.84 0,4 5.4 4.6 1.28 0.59 p r e s e n t r keV 7722 ± l ' ' 7697 ± 2 6 7 1 1 ± 3 6626 ± 3 6618 ± 3 * 6440 ± 3 6316 + 3 6200 ± 3 6102 ± 3 6021 ± 3 5927 ± 3 5767 ± 3 * 5712 ± 3 5586 ± 3 5445 ± 5 5412 ± 3 5136 ± 3 4905 ± 3 4766 ± 3 4750 ± 5 * 4736 ± 3 4692 ± 3 4659 ± 3 4440 ± 3 * 4414 ± 3 s s u l t s
V
r e l . 23 1,8 (0,8) 0,2 0,35 0,5 2.3 0.7 2,7 0.4 0.4 0,65 0,55 1.1 0.9 2,5 3.5 3,7 0.8 0.3 7,0 6,0 1,9 0,8 0,8 l e v e l s found in (d,p). reactions E keV 7700 ± 10 7345 ± 10 6970 ± 10 6719 ± 10 6626 ± 1 0 6446 + 10 6322 ± 1 0 6201 ± 1 0 6012 ± 10 5931 ± 1 0 5802 ± 1 0 5766 ± 10 5445 ± 10 5405 ± 10 5138 ± 1 0 4906 ± 10 4767 ± 10 4741 ± 10 4685 ± 1 0T a b l e IV, a (2) 27 2 8 G a m m a r a y s from Ai{n,y) Al,
Bartholome ^y keV 4290 ± 2 0 4160 ± 2 0 4060 ± 40 3880 ± 2 0 3620 ± 2 0 3460 ± 20 3290 ± 2 0 3020 ± 2 0 2840 ± 3 0 2260 ± 30 970 ± 30 »w (6) rel. 4.3 3 2 4.4 2.5 1.5 2 10 4.6 14 10 Groshev ( keV 4260 ± 15 4130 ± 2 0 3880 ± 2 0 (3800 ± 3 0 ) 3600 ± 15 3470 ± 15 (3340 ± 3 0 ) 3040 ± 10 2960 ± 15 2820 ± 2 0 2610 ± 1 0 2280 ± 10 2120 ± 2 0 5) r e l . 6.1 6 6 1,3 4,5 5,2 0,6 5,1 8 2 5 5,1 3 R a s m u s s keV 4381.5 4259.9 4133.7 4017.7 3935,9 3876.3 3849.6 3825,1 3790,2 3591.7 3564.8 3465.5 3393.0 3347.3 3304,7 3267.5 3034,4 2960.4 2821,5 2590,7 2283.7 2139.9 2107.6 1778.5 1623,1 2014.0 983.4 ien(9) r e l . 0,22 4.07 4.26 0.31 0.19 1.28 1.43 0.20 0.85 2.83 0.41 4.30 0.23 0.53 0.83 0.46 5.82 6.16 2.58 1.03 1.66 1.47 1.18 88,17 3.22 1.52 3.92 p r e s e n t ri keV 4384 ± 3 4258 ± 3 4133 ± 3 4022 ± 3 3984 ± 3 * 3877 ± 3 3848 ± 3 3796 ± 3 * 3695 ± 3 * 3589 ± 3 3464 ± 3 3303 ± 3 3032 ± 3 2960 ± 3 2822 ± 3 ^ s u l t s r e l . 1.6 7.5 7.3 0.8 0.'8 2.5 2,1 0.8 1.2 4.5 6,1 1.1 7.5 8.3 2.5 l e v e l s found in (d.p)_ r e a c t i o n s E keV 4383 ± 10 3935 ± 10 3878 ± 10 3591 ± 1 0 3461 ± 10 3347 ± 10 2589 ± 1 0 2279 ± 1 0 2143 ± 1 0 1633 ± 1 0 1017 ± 5 973 ± 5
1) Energy according to (47), u s e d for the energy calibration. * T r a n s i t i o n s not p l a c e d in t h e d e c a y sc h eme (fig. IV,3).
1.0 0.8 0.6 0.4 0.2 2 3 A 5 6 7 8 MeV
fig. IV, 2. The efficiency curve for the double-escape peak of the 5 cc Ge(Li)detector of Princeton Gamma Tech.
In the energy range from 2822 keV to 4905 keV several tran-s i t i o n tran-s are atran-stran-signed here to feeding and decay of known l e v e l tran-s . (See fig. IV,3, in which a level scheme i s proposed). Sometimes two solu-tions are possible forplacing transisolu-tions in a level scheme. The 5445keV transition can be the decay of a level at 5445 keV, but can be the feeding of the 2279 keV level too. The 4384 keV transition can a l s o be the decay of a level of this energy or a transition of the capturing level to a level at 3347 keV.
The low-energy levels of 2 8A1 are know from the beta decay of 2^Mg too. (In the level scheme of figure IV, all the known low-lying levels up to 2147 keV are mentioned). Only a few high-energy gamma rays can be assigned to transitions of the capturing level in ^^Al to these known l e v e l s .
In this way 33 of the 40 transitions observed are placed in a level scheme.
The intensity calculations have been carried out with the aid of data of Groshev et al., because Bartholomew et al.did not publish two of the most intense transitions: the 5140 and the 2960 keV transition. The higher resolving power in the present experiment allows the correction of some intensity ratios as given by Groshev. The inten-s i t i e inten-s of Bartholomew deviate rather much from our reinten-sultinten-s and thointen-se of Rasmussen et al.
rel. eff.
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-1
^ .n -— ï 5 S S ^ „ -^ ', -o O to » « <n " ro i n ü • * • o : -»n ! i n ï -2 U 7 k « V 1633 1375 1017 974 _ 3 1 " A l 22.6 minV
Chapter V 185,187R,(„^y)186,188R,
V,l Introduction
Gamma ray energies from the (n,7)-reaction on ^^ Re and ^^'^Re were published by Groshev et al.only (6). The binding energies of the last neutrons in these isotopes were calculated from a variety of experimental data (4), but t h e s e values do not agree with the data of Groshev et al.(see table V,b). We tried to measure the neutron capture gamma ray energies with a better accuracy.
The expected gamma ray spectrum due to these (n,7)-reactions will c o n s i s t of a continuum with discrete transitions at the high-energy end (see chapter 1,1). Some low-high-energy levels are known in
^^^Re and ^®^Re from conversion measurements on the gamma rays of the (n,'y)-reactions (63). Unfortunately, as will appear below, these measurements did not help very much in unravelling the decay scheme.
We avoided the high c o s t s of targets of separated rhenium i s o -topes. We carried out our measurements with a target consisting of natural rhenium.
V,2 Measurements
A target of 25 gram rhenium powder was used. The target con-struction was the same a s that of Th, mentioned in chapter VI,2.
p \-L 5467' ^ COUNTS ••••^
- 1
L 1 5672" /I 5634" 5623" 1 5790" 5762" 5833" \ ft 5874"'••••-MJA
'.*.* :• 1 1 6165' — CHANNEL NUMBER '• * - ^ . V / . 400 500 600fig. V, 1. A high-energy part of the single gamma ray spectrum of the reaction ' ^^'^^'^Re(n,y) ^ ^ ^ ' ^ ^ ^ R e , taken with the 5 cc Ge(Li)-detector of Princeton Gamma Tech,
A spectrum measured with the Reactor Institute 5 cc Ge(Li)-detector, used in combination with the pair spectrometer (see chapter 11,6), is shown (fig. V,2). We also show a part of a single gamma ray spectrum measured with the 5 cc Ge(Li)detector of Princeton Gamma
Tech, using the internal target geometry (figure V , l ) .
The capture cross-section of ^^^Re i s 100 barn, that of ^^''Re 63 bam. The natural abundance of these isotopes is respectively 37% and 63%o, which means that the capturing ratio of t h e s e isotopes in the target is nearly one. In table V,a the results of the measure-ments are shown. The i n t e n s i t i e s are calculated with the aid of the efficiency curve in figure IV,2; the gamma ray energies have been calibrated with the aid of the exponential-pulse generator and the ground state decay of the reaction ^"^Al{n,'y)^^Al (see chapter 111,2).
V,3 Discussion
a) General aspects
Thermal neutrons are captured with zero orbital angular momentum. If the spin of the target nucleus is I, the spin of the compound nucleus is either I + '/^ or I — /4. The compound nucleus has the same parity as the target nucleus (1).
Re and Re have 75 protons and an even number of neutrons. The ground s t a t e of t h e s e two isotopes is determined by the last odd proton and was found to be 5/2 , belonging to the [4,0,2] rotational band (59) (the explanation of these asymptotic quantum numbers can be found in chapter 1,4). Therefore the capturing level in Re and in Re will have a spin 2 or 3 with even parity.
The ground s t a t e s of these two isotopes were found to be 1~ in both c a s e s (60). This can be explained by assuming that the proton probably s t i l l belongs to the 5/2 [4,0,2] rotational band and the neutron in both c a s e s to the 3/2 [5,1,2] rotational band; this means that these two particles are coupled in agreement with the Gallagher-Moszkowski rule (61,62).
When the capturing level is 2 , the ground s t a t e decay has an E l character and one may expect to find it in the gamma ray spectrum. If the capturing level is 3 the ground s t a t e decay has an M2 character and one should not expect to see it.
Data have been published recently by Egidy et al. (63) on low-lying levels in the two isotopes mentioned. T h e s e authors measured conversion electrons of the (n,7)-reactions on Re and R e . They propose a level scheme up to 380 keV of each product nucleus. We
fig. V, 2. A high-energy part of the gamma ray spectrum of the (n,y)-reactlon on rhenium, measured w>ith the Reactor Institute 5 cc Ge(Li)detector used in combination with the pair spectrometer.
will try to decide below the relation between these data (see fig. V,4) and our gamma ray spectrum, but first need information on the reaction energies of the capture p r o c e s s e s .
Values of the binding energy of the last neutron in ^ ° R e and Re can be calculated with the aid of cycles of reaction energies (see figure V,3). They are compared in table V,b with measured v a l u e s . The values of Andreeff and Sheline were reported after our measure-ments.
b) ^^'^Re(n.y}^^^Re
The Q-value (see chapter 1,1) of the reaction ^^^Re(n,7)'^^Re is calculated to be 5716 ± 25 keV. This has been carried out with the aid of the r e s u l t s of the reaction ^^^0s(d,p) Os and the decay of ^^^Re and ^^^Re (see fig. V,3). The data of the decay of ^ ^ ' R e and ^^^Re are known very accurately (59). It should be mentioned that the product nucleus '^®0s may be created in its first excited s t a t e at 155 keV (59).
We checked whether some of our gamma ray energies could be due to decay of a capturing level to several of the low-lying excited s t a t e s . The best results of this investigation were:
185^, (y,n) 186,, (n.y) ^ 187^ 429 ± 2 - 7 2 8 0 ± 6 0 _. 5464 ± 1 /3-185p,^ (n,y) E G 1315 ± 5
r
186 R e (cl,t) 187 R e (n.y) 188 -1055 ± 2 5 2,6 ± 0,1 R e 2113 ±3 187 O s (d.p) - * 1 8 8 o , 5602 ± 20 (+ 155)fig. V, 3. The e n e r g i e s (in keV) of the r e a c t i o n s and d e c a y s are from ref. 66. To get the binding energy of the last neutron with the aid of a (d,p)-reaction, 2224.6 keV has to be added to the reaction energy. In the c a s e of a (d,t)-reaction —6257,4 keV must be added to the reaction energy. (These two e n e r g i e s represent v a l u e s equivalent to the binding energy of the l a s t
2 3
neutron in r e s p e c t i v e l y H and H (4)). In the reaction 187 188
Os(d,p) Os the product nucleus is expected to be created in i t s first excited s t a t e (at 155 keV).
1° 5790 - 0, 5725 - 64, 5634 - 156, 5623 - 179, 2° 5672 - 0, 5445 - 231, 5418 - 257, 5382 - 290 and
3° 5634 - 0, 5478 - 156, 5467 - 169, 5382 - 257, 5276 - 362. The second choice is closed to the reaction energy derived above (5716 ± 25 keV). Yet we are somewhat reluctant to accept this a s a proof of the correctness of the value stated a s long as it is a l s o not understood why none of the gamma rays observed can be interpreted as transition to the 2~ rotational level of the 1~ ground s t a t e (very probably either the 64 or the 94 keV level; s e e figure V,4).
For reasons unknown to us Andreeff accepted 5870 ± 10 keV as the reaction energy. This value is close to the one calculated
assum-Table V, a
185 187 Gamma rays from t h e (n,'y)-reactions on Re and R e .
E y keV 6165 ± 3 5938 ± 5 5874 ± 3 5833 ± 3 5790 ± 3 5762 ± 3 5725 ± 5 5672 ± 3 5634 ± 3 5623 ± 3 5478 ± 3 5467 ± 3 5445 ± 5 5418 ± 3 5382 ± 5 ^' 5365 ± 5 5330 ± 3 5276 ± 3 5190 ± 5 5170 ± 5 5137 ± 3 5100 ± 3 5068 ± 3 5043 ± 5 4966 ± 3 4934 ± 3 4910 ± 5 4873 ± 3 4849 ± 5 ly rel. 1.3 < 1 4.2 7,0 3.5 3.2 < 1 26.0 12.0 8.5 8.5 5.5 < 1 3,2 4,5 < 1 3,0 5,2 < 1 2.0 12.0 3.5 6.0 1.5 6.0 14.0 2.0 19.0 2.5
1
Ey
keV 4830 ± 3 4809 ± 3 4787 ± 5 4750 ± 5 '^ 4720 ± 5 4693 ± 5 4670 ± 3 4642 ± 5 4600 ± 5 ^' 4585 ± 5 1' 4554 ± 5 4523 ± 5 4486 ± 3 4455 ± 3 4430 ± 3 4370 ± 10 1) 4350 ± 10 I) 4330 ± 10 1) 4265 ± 10 4240 ± 5 4220 ± 5 '' 4120 ± 5 4055 ± 5 4020 ± 5 '' 3970 ± 5 3944 ± 5 3922 ± 5 3890 ± 5 3700 ± 5 ly rel. 10.0 16.0 5.0 6.5 3.0 3.0 14.0 2.5 8.0 8.0 < 1 1,5 9.0 4.5 3.0 2.0 3.0 3.0 1.5 5.5 1.5 4.0 1.5 2.5 4.0 1.5 3.0 2.5 1.5 I) Broad lineing that Os is created in the (d,p)-reaction in i t s first excited s t a t e : 5871 ± 25 keV. If t h e s e r e a s o n s are correct, we can improve the value for the binding energy of the l a s t neutron i/i ^^^Re to 5874 ± 3 keV.
In this last c a s e one other transition, the 5623 keV one, can be interpreted as a decay of the capturing level. It feeds the 257 keV level, known from E g i d y ' s work. This interpretation would agree with his assignment of a spin 2~ to this level.
c.) ^^^Re(n.y)^^^Re
The calculation of the binding energy of the last neutron in Re i s based on the data of several reactions and decays ( s e e fig. V,3). The binding energy of the last neutron in ^°^W is known with certainty: the result of the (n,7)-reaction 5464 ± 1 keV agrees very well with the value found with the (d,p)-reaction on ^^®W, 5460 ± 20 keV(4). The reaction '^^Re(d,t)^^^Re gives 7312 ± 25 keV for the binding energy of the last neutron in Re (57), which is in good agreement with the result of 7320 ± 60 keV from the reaction ^^^Re(7,n)^^^Re (4,56). B e c a u s e the beta decay of ^^^W to ^^^Re is a l s o well-known, the Q-value of the electron capture process ^®®Re-*^®®W is known: 533 ± 25 keV.
The biggest uncertainty in the further calculation is caused by the data of the reaction '^^W(7.n)^^^W: - 7 2 8 0 ± 60 keV (65). With
379.2 316.4 314.0 210.7 U6.3 99.4 59.0 0 I n 2- 1,2-J.3* 1,2-3" 1.2J-2" r Inv 362.4 290.7 2U.7 256.9 231.0 171.9 In 1,2" ' < » • 4" 2-?,3' 6" BTZ T 1S6.0 9 * 2 63.6 3" 3" r 0 r
Table V, b
Binding energy of the last neutron in:
u
k e V 6318 ± 70 6140 ± 30 6243 ± 10 6179 ± 3 6164 ± 25 6165 ± 3 '«Re ref. Reaction cycle (66) (n,y) Groshev et al. (6 (n,y) Andreeff (58) (n,y) Sheline (57) (d.p) Sheline (57) Present work '^^He k e V 5871 ± 25 5951 ± 20 5870 ± 10 5874 ± 3 ref. Reaction cycle (66) ( n , y ) , Groshev et al.(6) ( n , y ) , Andreeff (58) Present workthe aid of the 0-value of the beta decay of '^^W, we get 6318 ± 70 keV
for the binding energy of the last neutron in ^® Re.
Information on the reaction cycles presented in figure V,3 can a l s o be obtained from mass doublets (4,67). The result of an adjust-ment combining all t h e s e data i s an increase of the input value - 7 2 8 0 ± 60 keV for the reaction energy of ^^^W(7,n)^^^W to an output value —7213 ± 43 keV. Considering the relative weights, this means that without the input value mentioned the output value would have been around —7140 ± 60 keV. This makes us suspect that in reality the above-mentioned (7,n)-reaction feeds the 160 keV level known in ^ W. T h e s e data suggest a binding energy for the l a s t neutron in ' ^ ^ R e of 6160 ± 60 keV. The 6165 ± 3 keV gamma ray i s either the ground s t a t e transition or the transition to the 59 keV level (see fig. V,4). As shown in table V,b the first assumption would be more in line with the interpretation of the other authors with the exception of Andreeff.
Looking at the level scheme of Re in figure V,4 we do not find any other transition that we can interprete. We found one possible sum rule: 5938 - 0, 5874 - 59, 5833 - 99, 5790 - 146, 5725 - 210, 5623 — 315; the resulting binding energy, though, is not very accept-able.
An efficient coincidence experiment would make it possible to determine more certain values for the (n,7)-reaction energies.
Chapter VI 232Th(n,y)233Th
VI, 1 Introduction
Gamma rays from the reaction ^^'^lh{n,'y}^^^Th. have been published only by Groshev et al.(6) and by Burgov et a l . (68). Each author published six t r a n s i t i o n s , two of which agreed (one at 3.73 MeV and one at 3.54 MeV). We tried to measure the gamma ray energies
with much better accuracy.
The binding energy of the last neutron in Th has been calcu-lated from reaction cycles (see fig. VI,1); Mattauch et a l . (4) give a value of 4956 ± 27 keV, corrected later to 4789 ± 5 keV (69). The highest gamma ray energy of Groshev e t a l . and Burgov et al. are r e s p e c -tively 4.92 and 5.11 MeV.
VI,2 Measurements
The target c o n s i s t s of 25 gram thorium powder, e n c a p s u l a t e d
in an aluminium cylinder. The open end of this cylinder was closed
with the aid of a screw having a fine thread and an o-ring, both fixed with the aid of araldit. T h e s e precautions have been taken to avoid spills of 233pa activity. This activity results through decay of '^^^Th, which has a half-life of 22 minutes (see chapter VII too). Moreover precautions have to be taken with 232'pi.j powder, which has a toxity
100 times l e s s than that of 226^^ (71),
The capture cross-section of Th i s 7 barn. In figure VI,2 the high-energy part of a spectrum measured with the 5 cc Ge(Li)detector of Princeton Gamma Tech i s shown.
VI, 3 Discussion
The gamma rays of the reaction '^^^Th{n,yi'^^^Th. can be found in table VI,a. In this table the data of Wasson et al. (73) and Kernbach et al. (72) can a l s o be found. The energies of the gamma rays have been calibrated with the aid of the exponential-pulse generator and the transitions due to the reaction 2''Al(n,7)2^Al (see chapter 111,2). The i n t e n s i t i e s have been calculated with the aid of figure IV,2.
The gamma ray spectra of Wasson et al.(73) were measured with the aid of thermal neutrons and with neutrons at resonances at 21.8, 23.4, 59.5 and 69.1 eV. The gamma ray energies found in these spectra
Table VI, a (I)
Gamma rays from the reaction •^Th(n,y) Th. Present k e V 5000 4950 ± 8 4930 ± 8 4810 4766 ± 3 ^' 4643 ± 8 ^' 4625 ± 8 4605 ± 8 4545 ± 5 4520 ± 5 4490 ± 5 4455 ± 5 4445 ± 5 ^' 4355 ± 5 4305 ± 5 4245 ± 5 4215 ± 8 4200 ± 8 4170 ± 8 4153 ± 5 4075 ± 8 4065 ± 8 ' ' 4042 ± 5 4020 ± 8 3972 ± 5 3965 ± 8 3945 ± 3 3897 ± 5 3862 ± 5 3850 ± 8 ^' 3840 ± 8 ^^ 3827 ± 5 work rel. < 5 < 5 < 5 < 5 '^10 5 5 < 5 10 10 10 12 15 7 < 5 35 < 5 50 10 8 10 10 30 15 10 10 80 < 5 15 10 10 < 5 Wasson e ^ y keV 4888.8 4861.7 4243.8 4212.8 4198.6 4069.6 4043.8 4033.5 3967.7 3951.0 3891.6 3877.4 3856.7 3833.0 t al. (75) rel. < 8 < 8 78 ± 2 0 81 ± 15 78 ± 10 55 ± 12 60 ± 15 < 8 < 8 130 ± 15 < 1 0 77 ± 13 82 + 13 88 ± 13 Kernbach et al. (76) ^ y keV 4895 4810 4780 4765 4730 4680 4650 4545 4520 4485 4410 4355 4240 4215 4200 4170 4100 4065 4040 3965 3940 3860 rel. 1 1 3.3 11 4.5 4.4 1.8 2.3 4.5 13 2.9 5.5 21 10 29 6.9 16 30 32 12.5 85 11
Table VI, a (2)
Gamma rays from the reaction 232rp^jjj^^j233,j,jj_
P r e s e i t k e V 3817 ± 5 3805 ± 5 3795 ± 8 ^' 3750 ± 8 3737 ± 5 3728 ± 5 ^ 3720 ± 5 3705 ± 5 3600 ± 5 3580 ± 5 3568 ± 5 3545 ± 5 3525 ± 3 3505 ± 3 3468 ± 3 3445 ± 3 3432 ± 3 3410 ± 5 3393 ± 3 3370 ± 5 3337 ± 5 3320 ± 5 3282 ± 5 3240 ± 5 3188 ± 5 3164 ± 5 3140 ± 5 3122 ± 3 3109 ± 5 3080 ± 5 3018 ± 5 2954 ± 5 work r e l . < 5 16 < 5 17 30 12 < 5 12 18 12 12 20 120 50 200 60 60 30 100 40 30 35 20 25 80 75 120 75 25 (30) (30) (30) Wasson e k e V 3733.9 3535.0 3501.3 3473.3 3459.1 3430.0 3393.3 3349.5 3336.5 3319.8 3305.6 3261.7 3145.0 3113.4 2979 2965 t al,(75) rel. < 8 67 ± 12 36 ± 10 135 ± 2 0 < 1 0 75 ± 10 20 ± 10 < 1 0 17 ± 10 < 1 3 < 1 3 15 ± 8 55 ± 10 < 8 < 8 < 8 Kernbach e ^ y keV 3800 3740 3730 3675 3630 3590 3580 3520 3500 t aU(76) rel. 11 5.3 6.5 13 10 39 18 63 100 1) Broad line
236^ (d.p) 237^ (d,t) 238^ / 2898 ± 8 116 ± 6 ' / / a 4573 ± 3 /3-a , 4268 ± 5 514 ± 3 2 3 7 Np 232^^ (n,y) 233-^^ 23 3 P a ai 4956 ± 3 I 1245 ± 3 > 233 U 569 ± 3 (d.p) 4559 ± 29 2 3 4 234 /S 234 T h 260 ± 5 P a 2240 ± 10 U
fig. VI, 1. The reaction c y c l e s for determining the binding energy of the 233
l a s t neutron in Th. The data have been taken from the ref. 59,69, and 70. In this way 4789 ± 5 keV can be calculated for this binding energy, which is in agreement with the data of Erskine et al (70) for the reaction 232.pj^ ^^^^^233,^,^^ . 2567 ± 7 keV (see a l s o the comment on figure V,3).
can be found too in table VI,a; the intensities are given for the c a s e of the capture of thermal neutrons. The intensity ratios were very different in the c a s e of neutron absorption at the thermal capturing level and the other resonance capturing levels.-Wasson et al.did not give a value for the binding energy of the last neutron in ^^^Th.
In the meantime also Kernbach et ai.(72) published data of the reaction Th(n,7)2'^'^Th. T h e s e authors suggest a binding energy of about 5.26 MeV, but express some doubts about the correctness of this value.
The data of all authors begin to show agreement at 4245 keV. Wasson and Kernbach both report a line at about 4890 keV, not seen in this experiment. But it should be mentioned that at one side the most intense gamma ray of Kernbach (at 4765 keV) is not seen by Wasson and on the other side Wasson's 4962 keV line is not observed
by Kernbach. The 4765 keV gamma ray almost coincides with an 28^1 gamma ray, but it is definitely present in our spectrum.
The spectrum of Kernbach shows that this author had a lower background at the high-energy end than we did.
t . i ^
-*.«'
4US"
-CHANNEL NUHBER
fig. VI, 2. Tlie high-energy part of the single qamipa ray spectrum of the reaction ^^'^Th{n,y)^^^Th, measured with the 5 cc Ge(Ll)-detector of Princeton Gamma Tech.
The spin of the ground s t a t e of 232^^ i s zero (59) at even parity. The compound nucleus can be only '/4'*' (see chapter V,3a). The ground s t a t e of 233i'h is expected to be ¥2* or 7 / 2 ~ , regarding 235^ ^^^ 2''^Pu where the ground s t a t e is 7 / 2 ~ and an isomeric s t a t e just above it is l^"*" (59). T h e s e levels belong to the [6,3,1] and [7,4,3] rotational band (see chapter 1,4). In chapter VII,9 a further discussion about the ground state of 233'p]^ ^^.j j-,g found; there it will be pointed out that we expect it to be Vi .
In this c a s e the ground s t a t e decay of the capturing level will have an M1/E2 character, which means that it may be seen in the gamma ray spectrum, though it probably will have to compete with stronger El transitions (84).
Because no excited s t a t e s are known in Th we just look at the •^3 5y nucleus to get an idea of where levels may be expected in
233Th. The 3 / 2 and 5/2 members of the [6,3,1] rotational band are 14 and 52 keV above the 'A'^ member in 235u^ Levels with negative (59) parity and low spin (5^,3/2) are known only at 652 and 658 keV in
235u.
In common with the other authors, we did not observe a gamma ray energy of 4789 keV: the calculated value of the binding energy of the last neutron in 233^).^^ Kernbach found a gamma ray of 4780 keV, and 15 keV lower he found the 4765 keV transition, which we found too.
It may be expected that the 4780 keV transition is the ground state decay of the capturing level and that the 4765 keV one feeds the 3 / 2 member of the groundstate rotational band. The level spacing between '/^ and 3 / 2 members would then be in good agreement with the spacing of t h e s e same levels in 235u^
The transition at 4245 and 4200 keV may feed the negative spin l e v e l s . These levels would then be at 535 and 585 keV in 233'p|.j. ^^^ 233u such levels are found at 652 and 658 keV.
Our data suggest many more levels in 233^^1. Their interpretation will probably require rather complex theories involving coupling between particle levels and collective vibrations.
Chapter VII Levels in ^-^^Pa
VII, 1 Introduction
A study of the decay of 23 7i^p Q^d 233^];^ provides information about the levels in 233pa_*)
23^Np transmutes to 233p|-, j^y alpha decay. This alpha decay has been studied already by Baranov et al,(74) and by Asaro et al.(75, 76). The results of their measurements of the alpha spectrum are almost identical though Baranov, perhaps not always correctly, suggests that some of his peaks may be complex.
On the basis of these data it was suggested that the ground s t a t e and the 57 keV level in ^^^Pavjere levels with a spin of 3/2 and 5/2 of a band with Nilsson model parameters i 4 ~ [ 5 , 3 , 0 ] (see chapter 1,4). The level with 1 = ^2 should be at about 7 keV (74,75). An 87 keV level, decaying to the ground s t a t e and the 57 keV level, was suggested to be the 5/2'^[6,4,2] level.
According to unpublished work of Freedman et a l . , quoted by Hollander et al.(77), the beta decay of 233'pjj feeds the ground s t a t e of 23 3pQ ^j^ gyy^ Q£ ^Yie d e c a y s . Some five higher levels are fed by the remainder. T h e s e data suggest too that the level at 87 keV i s fed for about 8% (75) of the total decay.
Thus both the [5„3,0] and [6,4,2] band are fed by beta decay of either allowed hindered or first forbidden unhindered transitions (87). The Nilsson model (see chapter 1,4) does not contain any reasonable orbit that could then be suggested for the 23 3^1^ ground s t a t e .
The following s t u d i e s on the levels in 233pa fed by the alpha and beta transitions were carried out to find a solution for this puz-zling problem. The development of the Ge(Li)detector resulted in a much more significant study of this problem than was hitherto possible
(see chapter 11,4 and 5).
VII,2 Sources
The sources of 23 ^^p have been prepared at I.K.O. in Amsterdam. Sources electroplated on aluminium foil were used for the detection
of electrons or alpha p a r t i c l e s . The area of these sources was 15 cm^, the strength about 5 ^tCi.
*
