Measurements with (p,gamma) reactions

MEASUREMENTS

WITH (P,

y)

REACTIONS

PROEFSCHRIFT

TER VERKRIJGING VAN DE GRAAD VAN DOCTOR IN DE TECHNISCHE WETENSCHAP AAN DE TECH-NISCHE HOGESCHOOL TE DELFT, OP GEZAG VAN DE RECTOR MAGNIFICUS DR. R. KRONlG, HOOG-LERAAR IN DE AFDELING DER TECHNISCHE NATUURKUNDE, VOOR EEN COMMISSIE UIT DE

SENAAT TE VERDEDIGEN OP WOENSDAG 30 MAART 1960,

DES NAMIDDAGS TE 4 UUR

DOOR

KAREL JAN VAN OOSTRUM

NATUURKUNDIG INGENIEUR GEBOREN TE 'S·GRAVENHAGE

.",

GaAPlSCH BI!DRI'P A V ANTI - DELPT

---~

_BIBLIOTi-i

--

_EEK

---"

DER

TECHNISCHE HOGESCHOOL

DE

LFT

--.---DIT PROEFSCHRIFT IS GOEDGEKEURD DOOR DE PROMOTOR PROF. DR. A. H. W APSTRA

Aa" mijn oltders Aan mijn vroltw

CONTENT8

Introduction . . . 7

Chapter I. Apparatus concerning the proton beam

1.1 Brief description of the generator and the magnet 9 I. 2 The analyzed proton beam . . . . . 10 I. 3 Calibration of the proton energy . . . . . 14 Chapter 11. Energy spread of the bombarding protons

11.1 Introduction . . . . . 18 11.2 8hape of a resonance in the gamma ray yield

curve . . . . 11.3 Experimental procedure 11.4 Results . . . .

Chapter m; Gamma ray detection equipment 111. 1 The scintillation counters. . . . 111. 2 Electronic equipment. . . . m.3 Angular distribution arrangement .

Chapter IV. Determination of the crystal efficiency

18 20 21 24 28 30 IV. 1 Introduction . . . 33 IV. 2 Two-lines method . . . . . 33 IV.3 Application of annihilation radiation . . . . 36 IV.4 Determination via a semi-empirical method 39 Chapter V. The reaction 28Si(p,y)29p

V.1 Introduction . . . 41

V.2 Gamma ray yield curves 42

V.3 Gamma ray spectra 46

V.4 Q-value . . . 51

V.5 Angular distributions 51

V.6 Resonance strengths 63

Chapter VI. Discussion of excited levels in 29p and 298i VI.1 Comparison of experimental data . . . . . . 67 VI.2 The rotational collective model . . . . . 70 VI. 3 ~plication of the rotational collective model to

2 Pand 2981 . . . 73

Summary . . 80

Samenvatting 82

References 84

INTRODUCTION

The investigation of proton capture gamma rays forms a powerful tooI in the study of light nuclei. The proton energy required for the investigation of (p, Y) reactions can be obtained by rather simple proton accelerators such as Greinacher ("Cockroft-Walton") machines and Van de Graaff generators. Therefore, this type of nuclear reactions has been studied thoroughly in many nuclear physics laboratories . Almost all (p, Y) reactions are resonance reactions; they occur only near some resonance proton energies Eres . The first investigations in this field (for instance Ta46 , P140, Br47) dealt with this resonance character. Targets made of various elements of natural isotopic constitutions were bombarded by protons , the energy of which was varied by steps. The gamma' ray yield was measured for each proton energy . . Thus, a gamma ray yield curve was deter-mined from which the value of EreR and sometimes of the natural width

r

of aresonanee could be derived. Many tables of resonances in various elements resulted from these reports. They are very useful in identifying contaminations in targets. Some very strong and clear resonances were measured several times using an absolute proton energy determination. I These resonances are frequently used as

energy calibration standards.

When scintillation counters and various types of pulse height analyzers were developed, the study of the reaction gamma rays became possible. The reaction energy Q could be calculated from measurements of gamma ray energies and the resonance proton energy. The Q-values thus deter-mined delivered some useful links in the tables of nuclear masses. The nuclear level scheme for the nucleus produced by the reaction could be studied using the coincidence technique. The intensities of the gamma rays gave iJlforma-tion about level widths and decay probabilities for excited levels. The theory of angular distributions and correlations of reaction products allowed the interpretation of measure-ments of these features. They resulted in a determination of spins and parities of the levels.

Results obtained with the experimental techniques mentioned above were summarized several times in review articles by Ajzenberg and Lauritsen (Aj52, Aj55) for Z

=

1 till Z 10 and by Endt, Kluyver and Braams (En54, En57) for Z

=

10 till Z

=

20.

The features of excited levels in a nucleus became interesting wh en theories about nuclear structure were developed. The shell model was not completely successful in interpreting these features for the low Z nuclei studied with (p, y) reactions. The rotational collective model, however, s eems to be able to explain more in s ome cas es . The most successful example of such an interpretation is the mirror pair 25 Mg - 25 Al which are very thoroughly studied nuclei in this region.

The features described above made (p, y) reactions a suitable subject to be studied with the 2.5 ,MeV Van de Graaff generator built at the Delft High Voltage Laboratory. This thesis describes the construction of apparatus necessary for these measurements and the performance of them. Some of these measurements we re used to calibrate the gamma ray counting equipment and to study some features of the proton beam produced by the accelerator.

Firstly, some well-known resonancé energies were used to calibrate the energy of the protons produced by the generator and selected by the analyzing magnet (chapter I). Secondly, the shape of the gamma ray yield curve from a (p, y) resonance was measured with high accuracy in order to obtain an idea of the energy spread in the energy selected proton beam. The efficiency calibration of the Na! crystal counting equipment was made partly with a new method taking advantage of well-known properties of some resonances (chapter IV).

From the above it will be clear, that the investigation of (p, y) reactions is a useful tooI not only in the study of nuclear structure, but also in checking nuclear physics apparatus.

CHAPTER I APPARATUS CONCERNING THE PROTON BEAM

1. 1. BRIEF DESCRIPTION OF THE GENERA TOR AND THE

MAGNET.

The measurements described in this thesis were carried out using the proton beam of the Delft Van de Graaff generator built at the High Voltage Laboratory of Prof. Heijn. The maximum proton energy reached is 2.5 MeV. The machine stands upright in a high room the accelerator tube going through the floor into the experimental room. A pressure tank around the generator allows the high voltage to be insulated with nitrogen mixed with small quantities of freon. The ion source · in the top terminal is controlled by means of light signals (Bo56). The control panel and the electronic equipment for the nuclear physics experiments have been placed in an adjoining room. The accelerator tube is evacuated by a silicon oil pump system placed in the experimental room beneath the generator.

The proton beam is deflected over 90 degrees by an analyzing magnet. The current through the coils of this magnet is electronically stabilized. The magnetic field defining the energy of the protons coming through this deflecting field and an exit slit is controlled with a variabie resistance R in the electronic circuit stabilizing the magnet current.

The high voltage of the machine is stabilized by a variabie corona current from the top terminal to needles protruding from the tank wall. This current is controlled by a signal obtained from the two parts of the exit slit of the analyzing

. magnet.

A slow stabUizing circuit acting on the variac regulating the spray current to the belt of the generator is controlled by the same signa!. Thus, the high voltage is controlled by the magnetic field alone.

1. 2 THE ANALYZED PROTON BEAM

The proton beam deflected by the magnet must bombard the target in a way suitable to the nuclear physicist. The apparatus shown in figure 1. 1 has been constructed in order to satisfy all the experimental requirements.

The protons are focused horizontaIly as weIl as vertically at some distance from the magnet. The exit slit*) has been placed here in order to make the proton current through the slit as big as possible. The slit width determines the energy spread in the beam and also the beam current to the target; it is therefore made variabie. The two plates defining the slit must be cooled and those parts, which can be hit by protons , must be covered by a heavy material to avoid background gamma rays. The last requirement is clearly illustrated by measurements of Hunt and co-workers (Hu58),

who bombarded thick targets of several materials 'with

protons and measured the gamma radiation produced as a function of the proton energy. In our case both the exposed slit plates as weIl as the backings of the targets were made of tantalum being the most suitable heavy material for these construction purposes . The target itself has been placed about 75 cm behind the exit slit, because just behind the slit the beam is too much concentrated causing excessive heating of the target.

The targets were made by evaporating the target material in vacuo unto a tantalum disc with a thickness of 0.1 mm and a diameter of 25 mm, tiIl the thickness required was reached. These discs were put in a target hol der , two of which are shown in figures 1. 2 and I. 3.

In the hol der of figure I. 2 a target can be placed with an angle of 35 degrees with respect to the proton beam, so that a counter can be put as close as possible to the target its axis making an angle of 55 degrees with respect to the proton beam. Taking a gamma ray spectrum in this position has some advantages as described in section V. 3 of this thesis. The target is pressed firmly on the backside of the holder by an inner cylinder for good heat conduction. For that purpose too the cooling oH is forced to flow just along that part of the holder , which is close to the target heated by the beam. All connections between the vacuum tube parts have been standarized in the way usual in the High Voltage Laboratory. The target holder shown in figure I. 3 allows several targets to be bombarded one af ter another without opening the target holder and interrupting the experiments. It consists

*) Constructed by W. Leijzers Vis. 10

~ ~

160cm

I. 1 Survey of the vacuum tube behind the magnet, side view and front view 1. Analyzlng magnet 6. vacuum valve

2. rails 7. flex1ble connecUon

3. llquld air trap 8. flange contaln1ng dlaphragm and

4. adjustable support repeller

SECTION A.B

I. 2 550 _{Target holder with lead shielding.}

1. cooling oil passage

2. partition leading the cooling oU along the heated target

3. inner cylinder

4. nut pressing the inner cylinder and the target against the back of the target holder

12

5. flange 6. clasp

7. lead shielding

8. ring supporUng the aluminium foil inserting the Na! crystal 9. Na! crystal

10. target

of a wheel with eight target positions. This wheel can be

rotated from the outside to turn the desired target in the proton beam. Number indications on the wheel visible through a perspex window (not drawn in figure I. 3) show which target is bombarded. The cooling oU circulates through a passage

in the wheel. This construction again allows placing the counter very close to the targ~t, which is very convenient

when weak resonances are investigated.

1. 3 Revolving target holder .

1. flange 2. O-seal

3. bombarded target

4. house in tbe shape of a truncated

cove witb a thin brass window allowing tbe counter to be placed close to tbe bombarded target.

5. cooling oH circulation (direction indicatedby arroWs). In tbe.wheel

tbe oH is prevented to flow directlyfrom one (dotted) channel

into tbe otber by a partition (not

drawn} in tbe passage in tbewheel. 6. knob to turn tbe wheel

7. oH and vacuum packings

8. revolvable wheel supporting tbe

targets .

In order to prevent the beam hitting the brass parts of

the target holder , whlch would cause considerable background

gamma radiation, these parts are shielded by a tantalum

plate witb a hole of 15 mm diameter. This diaphragm serves

at the same time as a repeller of the secondary electrons from the target, escape of which would cause a wrong value

for the proton current measured. The current unto the

diaphragm is used as an indication to see whether the proton

beam has the right direction. A carbon deposition on the

target from the silicon 011 high vacuum pump has to be 13

avoided. since otherwise the strong and broad resonanees at 450 and 1700 keV from 12C(p, y ) 13N and at 550 keV from

13C(p, y ) 14N disturb measurements near these proton

energies. Therefore, an extra liquid air trap bas been

placed just bebind the magnet to supplement the two traps before the magnet near the pumping system. These traps and the long distanee between the pump and the target kept the carbon contamination rather low indeed.

A valve has been placed in the middle of the vacuum tube in order to r.educe the time necessary for changing the target. Thus. only a small part of the tube has to be evacuated af ter inserting a fresh target and the experiments have to be stopped only for about 15 minutes.

The proton beam produces a considerabie quantity of heat on impact. Therefore, .cooling is provided at several points. The vertical target bombarded when no magnetic field is present is cooled with water. The exit slit and the target holder must be cooled by oH, since both the difference amplifier of the stabilization and the current integrator connected with the target have a high entrance impedanee. The oU itself is cooled with water in a little heat exchanger . This rather heavy equipment is kept in position by two supports fastened on two beams fixed in the floor. The position of the second part of the tube can be adjusted horizontally as wen as vertically to correct little deviations in the direction of the beam. A flexible connection was placed between the two parts of the tube; it is supported to prevent implosion when evacuated. An extra support has been constructed for the revolving target holder .

The beam current unto the target is limited only by the production of heat in the target. A heat production of 10 watt was found to be a safe limit; then there is no danger for evaporation of the target. Thus the current was varied between 33 !lA at 300 keV proton energy and 5 !lA at 2 MeV.

I. 3 CALIBRA TION OF THE PROTON ENERGY

The most usual way to calibrate the proton energy is by measuring the value of the magnetic field deflecting the beam. The proton energy can be derived from this quantity by the well-known equation:

(1.1)

where: B is the magnetic induction in W m 2 Ep is the proton energy in e V

p is the radius of curvature of the proton path in m

m is the pr?ton mass in kg

e is the proton charge in C

Formula (1. 1) has been tabulated (Wa59) and would give

a very accurate value of Ep, if only B could be measured

accurately enough. However, this is difficult, since the

magnetic field is not homogeneous and possesses spray

fields. Therefore, we calibrated the magnet with well-known

proton energies , viz. some (p, y) resonances. A coil

rotating in the field was used to obtain a rough indication of the magnitude of B, but the resistance R defining the current through the magnet was taken as a more accurate measure

for the magnetic field. The magnet was demagnetized

carefully each time before a run was taken in order to avoid the effe cts of hysteresis. The calibration was made

the revolving target holder of figure 1. 3 being used with the

(p, y) resonances given in table 1.1.

Formula (I. 1) shows the proton energy to be a square

function of B. The values of R measured at the (p, y)

resonances of table I. 1 indicated approximately a square

depen-dence of Ep on R also. We therefore define a calibration function

E

c(E )

=

-E

(1,2)

P R 2

The experimental calibration points given in figure I. 4 show c(Ep) to be about 3.0 on an average.

The curve drawn in figure I. 4 allows the proton energy to be calculated for a certain value of R. However, some deviations from this calculation may occur. Therefore, we use the same curve to interpolate an unknown proton energy between two well-knownenergies measured in the same run.

The last mentioned energies can be (p, y) resonancesfrom

accidental contaminations in the target under investigation or they can be obtained using the revolving target hol der . Differentiation of (I. 2) gives for the enel'gy difference between two closely spaced resonances measured at positions Rand

R + 6R in the yield curve:

(I. 3)

TABLE 1.1

(p, y) Resonanees used in calibrating the proton energy REACTION RESONANCE PROTON ENERGY REFERENCE

19F(p, .a:y) 160 341 keV Ku59

2981(p, Y )30p 415 keV Ku59

3 081(p , y_')31p 498 keV Ku59

3081(p,y)31p 620 keV Ku59

3081(p, y)31p 670 keV Ku59

29S1(p,y)30p 697 keV Ku59

2981(p, y )30p 738 keV Ku59

3~(p, y)31p 776 keV Ku59

30S1(p, y)31 p 840 keV Br56

29S1(p, y)30p 916 keV Br56

"29S1(p, y)30p 956 keV Br56

27Al(p, y)2881 992 keV Br47

27Al(p, y)2Ssi 1018 keV Br47

27 Al(p, y)28si 1112 keV Br47

23Na(p,a. y )20Ne 1166 keV St54

23Na(p,a. y )20Ne 1213 keV 8154

23Na(p,a. y )20Ne 1258 keV St54

23Na(p,a y )20Ne 1329 keV StM

27 Al(p, Y )2881 1355 keV Br47

27Al(p,y)28S1 1372 keV Br47

23Na(p,a.'y )20Ne 1398 keV St54

23Na(p,a y )20Ne 1419 keV St54

27Al(p,y)2881 1500 keV P140 27Al(p,y)2881 1570 keV P140 27Al(p,y)2881 1640 keV P140 27Al(p,y )2881 1659 keV P140 27Al(p,y)2881 1700 keV P140 27Al(p, Y )2881 1781 keV P140 27Al(p,y)2881 1890 keV P140 I 27 Al(p, y)2881 1940 keV" P140 27Al(p,y)2Ss1 2026 keV P140 16

3.2 ~ In R2 3.1 keV (kn)2 3.0

1

2.9 2·8 2.7 2.6 2.5 0 500 1000 1500 x 1'F(p,ocyl 1'0 ~2tSi(p'l)JOp o lOSi (p'l )]lp • 27Al(p,,) _2'Si o 23No (p,OI,) lONe 2000 - - - Ep in keV

I. 4. Callbration curve of the analyzing mapet.

Figure I. 4 and formula (I. 3) enable us to reach a better accuracy in the determination of the proton energy in such an interpolation. The relative error made can be as small as 0.5% at small proton energies .

CHAPTER II ENERGY SPREAD OF THE

BOM-BARDING PROTONS .

II. 1 INTRODUCTION

When we are working with a Van de Graaff generator on (p, y) reactions it is necessary to know the energy spread of the protons bombarding the target. This spread is deter-mined mainly by the quality of the analyzing magnet. A good measure for this quality is the resolution defined as the" selected proton energy divided by the total energy spread (as defined at the end of section 11.2) of the partieles passing through the exit slit of the magnet, if the incident partieles have an energy distribution, almost homogeneous over a range larger than that selected by the analyzer.

1bis energy spread will be larger than the spread at the target under working conditions, due to the action of the high voltage stabillzer controlled by the signal from the exit slit parts (see chapter I). The last energy spread is the most important for the interpretation of our measure-ments. Nevertheless, at the request of ir. J.H. Makkink, supervisor of the Van de Graaff generator, the first spread (and therefore the resolution of the magnet) was also determined.

II.2 SHAPE OF A RESONANCE IN THE GAMMA RAY YIELD CURVE

We consider first the gamma ray yield dN_y prosluced by a proton beam with an average proton energy Eb and an energy distribution function g(E-Eb), where g(E-Eb)dE is the number of protons with an energy between E and E + dE hitting the target per second. The target contains No target nuclei per cm3 ; its thickness dx is assumed to be so small, that protons passing through it do not l08e any energy. The (p, y ) reaction producing the gamma rays takes place with a cr08s-section cr over a proton energy range

r

much less than the tota! energy spread t.Ep in the beam. The gamma ray yield per cm2 target area and per second produced under these assumptions is:

A measurement of this thin target gamma ray yield as a function of Eb would give us the form of g(E-Eb).

However, the target thickness dx has. to be smaller than about 50 eV proton energy loss in order to give no considerabie contribution to the shape of the resonance peak in the yield curve. No and therefore Ny would be too small in that case to give a determination of g(E-E]) with ~nough accuracy. Moreover, some protons would still lose more than 50 eV

by the effect of energy straggling. ..

The use of a thick target is therefore preferred. We make the same assumptions about cross -section and beam energy distribution, but we assume now the target to absorb Ö E keV proton energy, where 0 E>flEp. A proton entering the target will be able to produce a (p, y) reaction, if its energy E satisfies:

Eres

<

E

<

Eres + Ö E (il.2)

The gamma ray yield per cm2 target area and per second, produced by protons with energies in a small range dE for which (il.2) is valid, is given by:

Integration over E gives:

E

=

Eres + öE

N

_y

Noa} (E-Eb) d (E-Eb)

E = Eres The maximum value of (IT. 4),

E._DO

Ny m';'

~

N o f (E-Eb) d (E-Eb)

E = 0 is only obUüned when

(il.3)

(IT. 4)

(IT. 5)

Eres + ÖE _lflEp > Eb> Eres +.1 flEp . (11.6)

2 2

If Eb is taken variabie in (Il.3) instead of E, the following expression can be derived from (TI. 3):

dN

__ Y_ = NoO' g (E~Eb) (TI. 7)

dEb

Formula (II. 7) enables us to derive the energy distribution function also from the thiek target yield curve by differentia-tion with respect to the beam energy.

Aresonanee peak measured with a thiek target is shown in figure TI. 1 (0059). The low energy slope AE is due to the beam energy spread and corresponds with (11.4).

0._{1 NmolC}. - -

-G

- R

II. 1 Predieted shape of a (p, y) resonanee measured wlth a thlek target;

BD Is the proton energy spread, C the resonanee energy, EF the regl.on of maximum yield and FG the high energy taU.

The maximum yield level EF eorresponds with (TI. 5). The high energy taU FG is due to energy straggling of the protons in the target. The energy differenee between points B and D at 0.1 and 0.9 of tqe maximum yield is taken as the total energy spread ~Ep of the analyzed proton beam.

TI.3 EXPEmMENTAL PROCEDURE

The resonanee used in this measurement is the 992 keV resonanee in the reaction 27 Al(P. Y )28Si beeause of lts smaU natural width

r

(about 100 eV. Be49) and lts large yield.

T~gets of evaporated aluminium were used with a thickness

The homogeneous energy distribution of the protons entering the magnet as mentioned in section 11. 1 is not ensured by the fluctuations of the accelerating voltage of the top terminal alone. Therefore, the insulated vacuum box in the magnet was put at 750 V a. c. voltage*>' It is clear that the exit slit cannot be used for stabilization of the beam; otherwise the energy spread of the incident protons would be partly compensated by the corona stabilization device. At the other hand, if the stabilization is shut off completely, the incident proton energy drifts so much that no measure-ments can be made at all. A second slit, 5 mm wide, was mounted in front of the energy defining exit slit and used to stabilize the beam energy enough to make measurements possible.

This arrangement is shown schematically in figure Il.2 Of course the target holder had to be put on the same 750 V

a. c. voltage; otherwise the protons have lost their energy spread when hitting the target.

insuLated magnet box to high voLtage stabi Lization device extra sLit b

9

:2!OV

II. 2 Experimental arrangement for measuring the resólution of the analyzing magnet.

n.4 RESULTS

The experimental arrangement described in the preceding sectl.on was used in a series of measurements of the energy spread 6. Ep with various values of the width d of the exit slit. The results are plotted in figure IT.3. The experi-mental curve is not a straight line through the origin. This is due to the finite diameter s of the beam spot at the place of the exit slit a. If s >d (fig. IL 4a) the totaI vertical deviation that the spot can make, if a part of it bas to pass the exit slit, ls equal to s and corresponds to a minimum energy spread L\Emin. If s <d (fig. n.4b) tbe straight line through the origin is showing up.

*) Tbis 1llethod was suggested by ir. J.H.Makkink.

AR inn 20

..

10

t~

-

t

-L

' (

.-

_"

"

"

2 3 - -___ din mm

n.3 Proton energy spread as a function of exit slit width d. The solid curve (drawn through the points) was measured with the arrangement as indicated in figure n.2, the dotted curve (drawn through crosses) in the· usua! arrangement, where the signa! to control the high voltage stabillzation is taken from slit a. (at d

=

1. 0 mm. the low energy slope of the gamma ray yield curve as drawn af ter the measurement showed a break indicating a shift in the magnetic field; shifts like this sometimes occur shortly af ter switchin, the magnet on. The pOint at d

= 1.

2 mm may be influenced by a similar shift).

fig.o fig. b

n.4 Two possible situations for the beam spot and the exit slit, a; as seen from the magnet box. b is the extra slit for controlling the high voltage stabillzation. The situation drawn in fig. 4a corresponds with the horizontal part of the curve in fig. n. 3. Fig. 4b with 'the other extreme part of the curve.

Some measurements made with the exit slit providing the control signal for the high voltage stabilization showed an about 30% lower minimum energy spread (dotted curve in figure IT.3). This clearly shows the reality of the effect described in section IT.3.

The minimum energy spread allowed by the magnet alone was established in this way to he 1. 6 keV. The spread allowed by the combination of analyzer and stabilization is 1 keV. This corresponds with a

reSOIUtiO~~Of

620 and 1000 respectively at about 1 MeV proton

energy~

which is quite sufficient for the present experimental work.

CHAPTER III GAMMA RAY DETECTION EQUIPMENT

111. 1. THE SCINTILLATION COUNTERS

Two scintillation counters are available to study the gamma radiation from the bombarded target. They consist each of a cylindrical Harshaw 3" x 3" Na! crystal mounted on a photomultiplier. A drawing of one of these counters is given in figure 111. 1. The other one is quite the same except

for the socket of the photomultiplier. The crystal is'

surroun-ded by 45 mm of lead for suppressing background r adiati on , and some shielding or a collimator can be placed in front with two clasps . The socket has been mounted on springs.

in such a way, that crystal and window of the photomultiplier

are kept firmly pressed together; there can be no danger for destruction of one of them, when the screws would be fastened too strongly. This construction has the additional advantage that the counter can be mounted and dismounted very easily for such a heavy thing because both front and back can be opened and closed with screws. The back of the counter contains a cathode follower and a high voltage divider for the photomultiplier both of the standard type used at this department (N058).

One of the crystals has been selected for homogenity in the factory. One of the photomultipliers was a Dumont type

(window diameter 110 mm) and the other an E. M. I. type

(window diameter 75mm), the latter selected for homogenity of the photosensitive layer. Four combinations could be made with these four elements. For each of them the half-width of the full-absorption peak was measured produced by the 0.66 MeV gamma radiation of 137 Cs as usual. The results are given in table 111.1. From these combinations the first and the last were chosen.

In most cases the best counter was used for single gamma ray spectra and the other one as gate counter in coincidence meàsurements and as monitor in angular distribution measure-ments. The great advantage of the use of selected crystals and small photomultiplier windows with good homogenity is obvious. The dependence of the half -width on the gamma ray energy as measured with the best counter in the usual arrangement of figure I. 2 is given in figure 111.2.

The background is due to various sources. The

t.:I

C1I

'Ocm

m.

1 Sclntillation counter

1. Exit plug

2. supply voltage plug 3. eathode follower box

4. conneetion wlth photomultiplier soeket

5. elastie mounting for the photo-multiplier socket

6. photomultiplier base

7. E. M. I. photomultipl1er 8. lea4 shieldlng

9. transparant eonneetion between the glass wlndowsof photomulti-pl1er and crystal

10. Harshaw 3" x 3" crystal 11. clasp

12. aluminium foil

TABLE m.1

Half-width of the 137Cs full-absorption peak measured with four combinations of crystals and photomultipliers.

~

multiplier

NaI window

crystal 75mm 1l0mm

Selected 8.8% 10.4%

Unselected 9.6% 10.6%

line from 40K the latter being the most ,troublesome. The use of a potassiumfree glass window (in the E. M. I. photo-multiplier) did not give considerable improvement over the Dumont photomultiplier; probably their use will be . more effective in laboratories built with potassiumfree concrete with a much lower background from the walls. In the first year of these experiments a 1.1 MeV background line was very troubles ome. It was identified as radiation from 6&Zn formed in brass parts of the vacuum tube during previous deuteron beam experiments (0058). Due to the decay (the

~ in 0/0

ElI

12 10 8 6 4 2 OL---~--~--~--~--~--~~--­ o 2 3 4 5 6 - El in MeV

m.2 Half-width of the full-absorption peak as a function or" gamma ray energy as measured with the arrangement shown in figure I. 3.

half-life being about 245 days) it caused no trouble during

the subsequent experiments. Of course, contaminations in

the target could give further background spectra, but they will be discussed later on.

In order to calibrate gamma ray energies a set of standard radioactive samples was collected (tabie ffi.2). The 65Zn source was produced in the 12 MeV cyclotron at the High Voltage Laboratory via a (p, n) reaction in a cupper target. Radioactive polonium was obtained from the Delft Radio-chemical Laboratory and the other radioactive samples were delivered by I. K. O. at Amsterdam.

TABLE In.2

Gamma ray sources used for energy calibration (Wa59)

Source Gamma ray Half-life

energy 198Au 0.412 MeV 67 h 137Cs 0.662 MeV 26.6 y 22Na 0.551 MeV _, 2.58 y 1. 28 MeV 88y 0.908 MeV 0.288 y 1. 85 MeV 65Zn 1.11 MeV 0.67 y PoBe 4.43 MeV 0.38 y

Figure 111.3 shows some standard spectra of monoener-getic gamma rays measured with the scintillation counter placed against the target holder . The radioactive samples used were mounted on the same tantalum bacldngs as the targets to obtain exactly the same arrangement as used to measure the spectra to be analyzed. The height of each spectrum has been normalized on the full absorption peak. 27

1.75 1.25 1.00 0.15 0.50 0.25 - -.... _ E, in HeV 2!Si(p",lOp Ep.12ikeV 5.IIHeV

III. 3 Standard spectra from monoenergetic gamma ray sources N y means the number of pulBes per kicksorter channel Nyo is the value of Ny in the top of the full-absorption peak

lIl. 2. ELECTRONIC EQUIPMENT

The cathode followers mounted in the scintillation counters are connected with the other electronic equipment via long low - capacitance cables terminated by a resistance equal to their own characteristic impedance (135 ohms). One pair of cables 20 m long, brings the signal to the control room, another pair of 100 m length to a kicksorter in the Nuclear Physics Department in another building. The signa! is not distorted by either of these cable~. In the control room the signa! is amplified in a linear amplifiër giving a pulse with a minimum risetime of 0.2 fl sec and'a maximum height of about 150 V. The spectrum can then be analyzed roughly

with two wide- channel discriminators and be counted in electronic counters. In this way gamma ray yield curves can be measured for two intervals of gamma ray energies. The amplifiers, discriminators , electronic counters and high voltage supplies for the photomultipliers are of the type used at the Nuclear Physics Departrnent and described elsewhere (N058). The protons are monitored by a current integrator (E149) giving a pulse af ter every O. 34 ~C proton charge. These pulses are registered in a counter which'

automatically stops itself and the gamma ray counters af ter a preset number of counts. A microswitch is used to reset and to' start all counters together. This equipment enables quick and accurate measurements to be made.

The pulse spectrum from the output of the linear ampli-fier can also be measured with a photographic method. The pulses are then stretched, made visible on a Tektronix oscilloscope screen and photographed through a vertical slit. This method was used before the ldcksorter became available.

A densitogram of such a photographed spectrum is shown

in figure

In. 4.

It is interesting to compare ft with the same spectrum measured with the ldcksorter afterwards (see fig. V. 4).

OENSITV

!

o 2 3

- El in MeV.

.

m

.

4

Densitogram of the gamma ray spectrum from 28Si(p.,y)29p

. at 370 keV proton energy

TheR. C. L. kicksorterhas 256 channels, which can be split ui)"

in four groups of 64. This

possibility has been used when angular distributions were meas-ured. The infotmation is stored in a magnetic memory, that can contain 216 puls es in each channel. The dead time is 84 ~ sec per pulse on anaveragel'heldcksorter corrects for this de~d time, if it is working for pres et time. However, a gamma ray spectrum from . a (p,y) reaction must be measured for a preset number of bornbarding protons , monitored by the current integrator, while the strength of the source is not constant at all in our case due to fluctuations in the intensity of the bornbarding proton beam. The automatic correction for dead time cannot be used in our case for this reason. The nurnber of pulses per second entering the ldcksorter must be made so 29

small, that no severe inaccuracy can be caused by this effect. . The percentage of time, the kicksorter is open, is indicated on a meter.

For coincidence measurements a Franklin (model 348) linear amplifier and discriminator has been used in combina-tion with the kicksorter . This apparatus required pulses with a 100 Il sec long tail as input, whereas all the other equipment requires shaped pulses with tails of a few micro-seconds. The coincidence scintillation counter was therefore equipped with a high resistance at the anode of its photo-multiplier.

ill.3 ANGULAR DI8TRIBUTION ARRANGEMENT

The arrangement shown in figure ill. 5 bas been constructed for measurements of angular distributions of gamma radia-tions with respect to the direction of the incident proton beam. The whole setup is placed on a 12 mm thick steel disc of 1. 20 m diameter. The plane through this table must be horizontal. This can be provided with three adjustable supports and an air level. The counters can be rotated around an axis, perpendicular to the table and going through the beam spot on the target.

The counters have been mounted on carriages, one of which is provided with four little wheels and a servomotor. This carriage can be moved by the motor acting on a bicycle chain around the side of the tabie. A third carriage can be connected to this one to measure spectra at two angles at the same time. This possibility has not yet been used since the selective storage of the kicksorter has not worked so far. In order to control the movable counter from the generator control room a circuitry has been constructed as shown in figure IU.6. The motor can be controlled from a panel under the table by switching 81 from the drawn position. 82 deter-mines the direction of the moving counter. This counter can be moved from the control room with S3, if S4 and 85 are closed and if 81 is in the drawn position. If 84 is opened the control is automatic with relay RIU, although it is still possible to stop the counter carriage by hand by opening 85. Microswitches Mi mounted on the table with intervals of 22io can be pressed down by the carriage. Figure UI. 6 shows I that just by closing Si at the control panel the counter is moving till the micro'switch Mi of the desired position is pressed down. Every time the counter carriage passes a microswitch the corresponding lamp Li on the control panel gives a flash. Two safety microswitches Ma and Mb placed at the extreme positions • which the counter must not pass, prevent the counter from crashing the vacuum tube or the fixed counter.

m.5

Angular distribution arrangement, top view and side view 1. bicycle chain 7. target holder

2. servomotor wUh toothed wheel 8. safety microswitches mounted 3. sledgeformovingcounter (counter on little arms

not drawn) 9. plate with slit to fix the safety 4. projection operating the micro- microswitches

switches 10. sledge for the fixed counter 5. microswitches 11. holder for the' counter

6. projection operating the safety 12. adjustable supports of the table microswitches 13. T-beams consolidating the table

14. air level

m.6 Electronic circuit for controlUng the angular distribution arrangement. The target holder of figure I. 2 is used both for the angular· distribution measurements and for the single gamma ray spectra. If the beam deviates x mm from its position hori-zontally the beam spot on the target shifts 1. 6 x mm causing the following relative error in the intensities measured:

6W(-&)

w

(-&)

= 2 x cos (-& + 350 ) L cos 550

(m.1)

where L is the distance from the target centre to the front of the counting crystal and -& the angl~ between beam direc-tion and crystal axis. At angles -& equal to 67io and 450 this error may be ignored compared with the other inaccuracies in the intensity measurements. L = 55 mm was only used at the weakest resonance. In this case the relative errors in the intensities at 67io, 450 _{~d 22io amount to 1.5%, - 1.1%}

and -5.4% respectively assuming x= 1 mmo The use of values of 100 mm and 200 mm for L at stronger resonances gave rise to much smaller errors. These considerations are of course valid only, if no lead collimator with a small entrance hole is used in front of the scintillation crystal.

There is evidence, that no bigger deviations than 1 mm of the bearn occur. At first, the position of the beam spot was studied between the various angular distribution measure-ments with a quartz target coated on a perspex· window. As the spot is about 5 mm wide a deviation of 1 mm from the marked centra! position would have been observed if present. Secondly, an angular distribution (mentioned in chapter V) which we found to be isotropic, was proved later on by others to be isotropic indeed. A deviation of the beam would certainly have been detected as an anisotropy in this angular distribution.

CHAPTERIV DETERMINATION OF THE CRYSTAL EFFICIENCY

IV. 1 INTRODUCTION

The determination of gamma ray energies is the first problem to be solved when a spectrum is analyzed. The intensity ratio of the various gamma rays is to be considered next. Af ter that we want to know the number of resonance levels produced per. proton hitting the target and therefore the absolute gamma ray intensities . The measurement first mentioned requires an energy calibration of the crysta! only. A relative efficiency curve must be obtained to measure intensity ratios of gamma rays with t different energies . The

absolute yield of a gamma ray in a reaction can be measured, if the relative efficiency curve mentioned above is combined with one or more measurements of standard sources of well-known strength.

We define the efficiency E ofthe experimental arrangement used as the number of pulses counted in the full-absorption peak in the gamma ray spectrum divided by the number of gamma quanta emitted by the source. The value of E depends on the geometry of the arrangement. An efficiency curve giving E as a function of the gamma ray energy has been determined for the arrangement given in figure I. 2. This curve has been obtained in a new purely experimental way described in sections 2 and 3 of this chapter. The more usual half-empirical determination is given for comparison in section 4.

IV.2 TWO-LINES METHOD

A two-lines method (As53) was used to obtain values for the efficiency s for gamma ray energies below 2.76 MeV. These determinations were carried out radio-active samples being used given in table IV.1. All these samples we re mounted on tantalum backings in the target holder shown in figure I. 2 in order to obtain an efficiency curve, that may be applied to the real (p,y) experiments. Spectra of 22Na,

88y and 24Na samples were measured in this way. The ratio of effiC1encies for the two energies of gamma rays emitted byeach source was obtained from these spectra the well-known intensity ratios given in table IV. 1 being used. Care was taken, that all positons emitted by the 22Na sample 33

TABLE IV.l

Gamma ray sources used in the efficiency determination

Source Half-life Gamma ray :Relative Reference energy intensity 198_Au 67 h 0.412 MeV Wa59 137 Cs 26.6 y 0.662 MeV Wa59 22 Na 2.58 y 0.5.11 MeV 1. 78 Wa59 to 1.276 MeV 1

8Sy _{0.288 Y 0.908 MeV 1 Wa59}

to 1. 853 MeV 1 24 Na 15 h 1.369 MeV 1 Wa59 to 2.756 MeV 1

29Si(p,y)30p 0.684 MeV 1. 09 Le58

to

at Ep=o.414 MeV 5.28 MeV 1

29Si(p,y)30p ( ~+)30Si ~:2.55 + min 1 Le58 to

at Ep =0.326 MeV y:5.88 MeV 0.86

28S1CP ,y)29p ( _,t)29Si ~:4+ . 6 sec 1 chapter V

to of this

at Ep=1.64 MeV y:4.35 MeV 0.88 thesis

annihilated near the souree so as to apply the intensity ratio glven for 22Na. A (relative) efficiency curve was obtained when these three efficiency ratios were used and this curve was assumed to be smooth. The absolute efficiencies at 412 and 662 keV were obtained by measurinr the spectra produced by 198Au and 137Cs standard sources of well-known strength. For calibration above 2.76 MeV use has been made of (p, y) reactions producing higher gamma ray energies . Resonance levels deexciting via a cascade of one high energy and one low energy gamma ray to the ground state are rather seldom (Aj55, En57); mostly, too many modes of decayare possible glving rise to a too complicated spectrum for the purpose of calibration. ünly the 414 keV resonance in the 29Si(p,y)30P reaction shows approximately the required features. Figure IV.1 shows its decay scheme. There is a small intensity

Ep 0 .• ,.

f

5.96

I I

5.57 0.326 _5.8·1

,

I , 29 5i + P 86 '4 I _, 6 90

•

I I

t

,

to to other other levels levels 0·684

o~

IV.I Decay scheme of two levels in30p exclted in the reaction 29S1.(p,y)30P. Data were derived from van der Leun's thesis (Le58). Enerpes are

glven in MeV.

difference between the 5.28 MeV and the 0.68 MeV lines, due to other modes of decay of the resonance level, which must be taken into account, when the ratio of the efficiencies for both lines is calculated. Furthermore, the weak ground state transition produces a background in the gamma ray spectrum, which has to be subtracted. Yet, this resonance gives rather an accurate measurement of the efficiency for a gamma ray energy, much higher than the limit of 2.76 MeV of 24Na.

IV. 3 APPLICATION OF ANNIlflLA TION RADIATION

No . more suitable· cascades of two gamma rays are available for the efficiency determination described in the preceding section. However, some more points are desired to . determine the efficiency curve for higher energies . We obtained them by the following method.

Resonances were selected, in which astrong ground state transition is followed by short-lived positon emission. The positons annihilate in the target backing or in the surrounding material of the target holder and produce two annihilation quanta of 0.511 MeV each. Thus, a weU determined intensity ratio of high and low energy is obtained permitting use of the two-lines method.

If all emitted positons annihiiate in the direct neighbourhood of the target, the target will emit two times as much 0.511 MeV quanta as gamma quanta from the ground state transi-tion; the effect of K-capture can be neglected for the low Z nuclei and large decay energies used in these experiments . Of course, all radio-active nuclei produced in this reaction must decay during the measurement of the spectrum. There-fore, this measurement must be continued af ter the proton bombardment for a time equal to several half-lives for positon emission of the produced isotope.

However, the emitted positons will not annihilate all near the target. We consider figure I. 2 again. Half of them will enter the backing material and will annihilate there or in the brass part of the target holder behind it. The other half will partly annihilate in the brass wall opposite the target and will partly disappear in the vacuum tube in a direction opposite to that of the beam. Thus, the ratio of 0.511 MeV quanta to the gamma quanta from the ground state transition will not be 2, but will be equal to a geometry factor g with a value between 1 and 2. This factor g will strongly depend on the arrangement used, but will be a constant for different reactions, if the same target hol der is used. Therefore, g can be determined with a reaction involving a gamma transi-tion with an energy, for which the efficiency is known already, and which is followed by positon emission. This can be done with three different reactions , given in tabel IV. 2. The decay schemes are given in figure IV. 2 and IV. 3. The reaction on 288i at Ep= 370 keV described in chapter V of this thesis and illustrated in figure VI. 1 was taken as another check on the determination of g. From these three reactions a value of g has been deri ved of 1. 42 ± O. 07.

In applying this calibration method the following effect must be taken into account. High energy gamma radiation will cause pair production in the surrounding material , especially

I

I

'l'ABLE IV.2

Determinations of the factor g

Reaction Reference Proton Gamma ray g energy energy

32S(p,y)33C1 ( ~+)33S Le59 0.580 MeV 2.87 MeV 1.43 t 0.10 12C(p,y)13N( ~+)13C Aj55 0.450 MeV 2.37 MeV 1.37*0.10 28Si(p, '()29p ( ~+)29Si chapter V 0.370 MeV 1.14 MeV 1.45 * 0.15

of this 1.38 MeV thesis 1. 72 MeV 1. 95 MeV

-05'01!p~2

.

8~5~~

__

~

0.450

I

t

P :;::2 . .:..37:....-....-_ _

~

32 S+p o

IV.2 Decay scheme of the 2.85 MeV level in 33C1 excited in the reaction 32S(p, y)33Cl. Data were derived !rom a paper by van der Leun and Endt (Le59).

12 C+ p

o

1

_2.221 nc

IV.3 Decay scheme of the 2.37 MeV level in 13N excited in the reaction 12C(p, y)13N. nata were derived from a review article by Ajzenberg and Lauritsen (Aj55).

in the lead around the crystal. The created positons produce annihilation radiation giving a contribution to the 0.511 MeV peak in the gamma ray spectrum. The quantity of annihiiation radiation was measuredfor some gamma rays not accompanied by any other annihilation radiation so that the 0.511 MeV peak is only due to this effect. The results shown in figure IV.4 give us a measure for this effect, which is depending again on the geometry of the arrangement. We use a symbol f defined as;

N

f = _an_n_ _{(IV. 1)}

X 3 19Ft p.Ot _{yl 160}

t

2 o 7 - - - - E ~ in M.V

IV." The factor f def1ued by e41UaUou (IV. 1) as a funcUon of gamma ray energy.

I

where: Nann is the number of counts in the full-absorption peak of the annihilation radiation from the surrounding material.

Ny is the number of counts in the full-absorption peak of the gamma ray considered.

Summarizing the considerations of this section we may write:

€ .

b 0.511

NO.511

=

g

y

-Ey

N_y + f N_y (IV. 2)

where by is the branching ratio of the resonance level for the gamma ray

y.

The value of E was determined for two other high gamma ray energies with this method. These cases are given in table IV.! also; the decay schemes are shown in figures IV.! and VI.!, the latter dealing with 28Si(p,y)29P.

A sufficient number of measurements is available now to determine the efficiency curve from 0.5 MeV up till 6 MeV gamma ray energy. This curve is shown in figure IV. 5 in a double logarithmic scale as usual for this quantity. The accuracy in the determination of E for the points measured with the positon emission method is worse than that for other measurements, since determinations of g, f and b

tin'" 20 0.\ 0.' 0.7 0.6 198 Au 2~

Uh

• 24_Na • 2. Si lp.y) 29 plp.) 29Si 29SilP.') JOp • Silp.,) JOplp.) 30Si o.5~---+'0.2~~o.J~~~M~O~6~U~1~---+--~J~4~5~6~7~'~1·0

---IV.5 Efficiency curve for the gamma ray detecting equipment used. Values

obtalned by the semi-empirical method are indicated as triangles·.

also have errors. However, these points are measured relatively with respect to 0.511 MeV, whereas the other points found with the first two-lines method may accumulate errors in the slope of the curve.

IV.4 DETERMINATION VIA A SEMI-EMPIRICAL METHOD

We define the total intrinsic efficiency E i of a crystal

as the detected number of gamma quanta divided by the number of quanta entering the front face of the crystal. This detection can take place via pair production, compton scattering or photoelectric effect.

The efficiency E defined in section 1 of this chapter can

be calculated from € i with the following expression:

E

=

g i (1 - a) - .Q R (IV. 3)

where: a is the fraction of gamma rays absorbed in the material between source and crystal.

Q is the soUd angle of the crystal front face with

respect to the source.

R is the ratio of the number 'of counts in the

full-absorption peak to that in the whole gamma ray spectrum.

Graphs of the calculated values of E i as a function of the gamma ray energy are given in a review article· by. Mott and Sutton (M058) for various distances between source and crystal and for various crysta! sizes.

Values of R were determined from the standard spectra shown in figure lIl. 3, while it was assumed, that the compton distribution of each gamma ray was constant down to zero energy. This assumption.had to be made, since some gamma radiation of lower energy is always present accompanying a high energy gamma ray. The Rvalues for various standard gamma rays are gi ven in table IV. 3.

TABLE IV.3

Determinations of the ratios R of full-absorption peak to tota! spectrum

Gamma ray Gamma ray R

energy source 0.412 MeV 198Au 0.60 0.662 MeV 137Cs 0.44 1.11 MeV 65Zn 0.33 2.37 MeV 12C(p,y)13N 0.20 at Ep=0.450 MeV "-3.51 MeV 12C(p,y)13N 0.13 at Ep=1. 70 MeV 4.43 MeV PoBe 0.12 5.88 MeV 29S1(p, y)30p _0.073 at Ep=0.326 MeV

The determinations of E obtained in this way are shown as triangular points in figure IV. 5. Both independent methods give approximately the same result. However, we prefer the exclusively experimental way for the following reason: The lead shield of the crystal may cause a contribution to the low energy part of the standard spectrum by scattering gamma rays into the crystal. However, the intrinsic efficiency

Ei was calculated for unshielded crystals. Therefore, errors may be caused, which cannot be estimated easily. The extrapolation of the compton distribution in the standard spectrum is only hypothetical. On the other hand, the experimental method extended with the application of (p

,r)

reactions can be adjusted to every expedmental arrangement. Moreover , the number of (P,r) resonances suited for this method may be expected to increase in the near future. 40

CHAPTER V. THE REACTION 2'SSi(P,y)29P. V.1 INTRODUCTION

An investigation of the reaction 2SSi(p,y)29p has been made with the apparatus described in the preceding chapters for proton energies between 300 keV and 2300 keV. This particular reaction was chosen, because little information was available about the levels of the 29p nucleus when the present investigation was started. That information is summarized in the review article of Endt and Braams (En57) and is shown in fig. V.l. Neutron spectroscopy

9.0 4.88 1.18 / 4.7'1 4.31,

,

: 3-12r--~--~2 I ,

,

~5 2.9 2.724 28Si+_p 2.5 1.9 1.3

V.l Experimental data of the low levels of 29P. Energles . are glven in MeV .

. "

measurements with the reaction 2SSi(d, n)29p gave rather inaccurate values of the excitation energies of the five lowest excited levels only, while no definite spin assignment could be made for them (Ca57, Ca56 and Gr55). The other excited levels were known from elastic and inelastic scattering of protons on 28Si reported in short communications on meetings of the American Physical Society (G055, Wi56 and V057), while the gamma ray deexcitation of the 4.32 MeV and the 4.74 MeV levels and the spin of the former were known from a private communication to the authors of the review article mentioned above.

Several groups made already investigations on (p,y)

, reactions in silicon targets of natura 1 isotopic composition, 92.2S%2S

sf

;

4.67%29Si and 3.05%30Si (H041, Se55, Ta46, 41

Se57, Ts57 and Gr57). Some of the resonances which were due to 29Si(p,y)30p and to 30Si(p,y)3lp have been investigated already extensively (Le58 and H058). However, no resonance below 1. 6 MeV proton energy has been attributed definitely to 28Si(p,y)29p in any of the reports mentioned above. There-fore, the experiment described in this chapter has been carried out targets of natural composition being used as weIl as 28Si targets enriched electromagnetically by A. E. R. E. , Harwell. The data about 28Si(p,y)29P given in fig. V. 1 indicate that resonances would occur at about 200 and 800 keV proton energy.

V.2 GAMMA RAY YIELD CURVES

The gamma ray yield curves were measured as a runc-tion of the bombarding proton energy in order to find resonances in the reaction considered. The current through the analyzing magnet was varied by steps, corresponding with proton energy steps of about 1 keV. The whole range of the )rield curve from 300 up till 2300 keV is too large to be given in a few figures; the interesting parts of it are given in fig. V.2 and 3.

The targets of natural composition used were obtained by evaporating SiO in vacuo from a tantalum strip heated electrically unto a tantalum backing. It is assumed, that the target is oxydized in air to Si02. The enriched silicon targets were prepared in the same way in HarweIl using electromagnetically enriched SiO.

Although an exact value of the target thickness 0 E is not required in these measurements, an estimate is desired to predict the shape of aresonanee (see chapter II section 2). Two closely spaeed resonances cannot be resolved if 0 E is taken too large, whereas aresonanee cannot be measured with enough accuracy, if 0 E is taken smaller than several times the proton energy steps mentioned above. Values of about 4 to 8 keV were chosen thereforé in the measurements of the gamma ray yield curves.

The target backings were weighed before and af ter thè vacuum evaporation of the target material. If G grams of it are evaporated homogeneously unto the target backing with 0 cm2 area, the target thickness is given by:

oE

= __

G _ _ Op

dE dx

(V.l) where: 0 E is expressed in MeV

p is the target density in g/cm3

dE is the proton energy loss per cm path in the target <IX

Values of _1_

~

, necessary for the application of

, p ux

(V. 1), are derived from experimenta! data summarized by Gove (0059) the following expression being used:

1 dE _ N dE

PdX -

AN_o dx (V.2)

where: N is Avogadro's constant

A is the atomic weight of the elementunderconsideration No1s the number of stopping atoms per cm3 target material

2-

~

is expressed in MeV /atom/cm2 and is given as a No ux

function of the proton energy for various elements in the review article mentioned above.

If the target material has a chemica! composition with a weight ratio of al: a2 : a3 etc., the following formula holds for the proton energy loss in it:

L

1 dE

1 dE i ai (

P

<IX')i

<IX'

= (V.3)

P

t

ai

Ö E can be determined from G using formulae (V, l),(V, 2) 1 dE

and (V.3). The value of

p

--ax- '

valid for Si02 at 1 MeV proton ~nergy is 0.18 keV /flg/cm2 •

The first interesting part of the yield curve is shown in flgure V. 2. Curves a and b were measured in bombarding a natura! silicon target, curve c with an enriched 28Si target. Three resonances appear in this proton energy regionFrom the difference between curves a and b which were measured incounting gamma rays of different energies, it can be derived already, that the resonances at 341 and 415 keV cannot be due 10 28Si, because the gamma .rays produced mainly have energies above 4 MeV, whereas the possible excitation energy in 29p at this proton energy is only about 3 MeV. However, the 370 keV resonance produces gamma rays with an energy mainly below 2 MeV and may be due to 28S1.

A measurement in the same region was made with a 28Si target in order to prove this statement (fig. V. 2c). The 415 keV resonance disappeared completely, but the 370 keV resonance was still present. The other two peaks are found to be due 10 19F(p,a.y)160 at Ep= 341 keV (Ku59) producing 6.14 MeV gamma radiation, and to 29Si(p,y)30P at Ep = 415 keV (Ku59) producing . gamma radiation with an enerlY up to 5.9 MeV (Le58). The background rising from 43

gamma 19

F 28Si 29Si

ray _{341 keV 370 keV 415keV} yield

l

El>2Mt

I

t

a

350 400 .. Ep in keV gamma ray yitld 1.3<Ey< 4 MeV b

t

350 400 Ep In keV

...

gamma ray yitld 0.7<EV<3 C MeV

1

! 350 400

..

_{Ep in keV}

V.2 Gamma ray yield curves obtained wUb Si02 targets. a. natural isotopic composition

about 400 keV is due to a broad resonance in 12C(p,y)13N its maximum lying at Ep = 450 keV and producing gamma r adiati on of 2.37 MeV. The 370 keV resonance studied here was measured also by Tangen (Ta46), but he assumed it to be due to 30Si(H041); it is not mentioned in the review article of Endt and Braams (En57) and was assigned an energy of 367 ± 2 keV by the present author (0058). More careful interpolation between the 341 and 415 keV peaks now yields a value of 370 ± 2 keV. In a recent article (Ku59) Kuperus and co-workers report the energy to be 368.9

± 0.7

keV; this accurate value was obtained with a proton resonance method.

In . the gamma ray yield curves obtained with natural silicon targets many resonances were measured up till 2300 keV proton energy. They could be attributed all to 29Si and 30Si by the use of two channels for gamma radia-tion, as was described above. The search for 28Si resonances, was moreover hindered very much by resonances due to various contaminations :

A contamination of fluorine is present in every target causing resonances in the reaction 19F(p,o::y)160. Especially the resonance at 873 keV was very troublesome.

A carbon contamination due to oil deposition on the target from the pump system··· caused the 450 keV resonance in 12C(p,y)13N producing a gamma ray in every spectrum taken in this region. The energy of this gamma ray depends on the proton energy and is equal to 2.37 MeV at Ep

=

450 keV. The resonance in 12C (p,y)13N at Ep

=

1700 keV, which is weaker than the former, did not cause much trouble, neither did the 550 keV resonance in 13C(p,y)14N.

Especially the enriched 28Si targets were contaminated by sodium giving rise to many strong resonances in the reactions '23Na(p,y)24Mg and 23Na(p,cxy)20Ne, the latter with only one gamma ray of L 6 Me V.

The first enriched 28Si targets used in these experiments had been contaminated with aluminium causing many strong resonances with gamma ray energies up till 12 MeV. The cause of this contamination was the use of aluminium weighing pans according to a communication from the Electromagnetic Separation Group at Harweil. When these were replaced, the aluminium contamination was five times lower in the next target delivered. Only the strongest resonances survived. No resonances could be attributed to 28Si in the region between Ep

=

400 keV and 1600 keV. The resonance at about 800 keV proton energy éxpected according to section 1 of this chapter, was not discovered. This result is discussed in a comparison with the mirror nucleus 29Si in chapter VI of this thesis.

The . interesting parts of the gamma ray yield curve for proton energies above 1500 keV are given in figure V.3. These curves were measured with a natural silicon target. The broad' resonance at 1640 keV was quite smooth, wh en . it was measured with an enriched 288i target. Therefore, all peaks appearing on the slopes of this broad resonance were attributed to 298i or 3081. The resonance at 2090 keV was measured too more clearly with a 288i target; evidently a part of the background in measurements with natural silicon is due to 298i and 3081.

gamma ray yield

1.5<El<5!

MeV 28_Si 1640 keV

1

1550 1600 1650 1700 28 Si 2090 keV.

•

I , I I I 2000 2050 2100 2150 2200

---1,,_

Ep in keV

V. 3 Gamma ray yield curves obtained with 8i02 targets of natural isotopiç composition.

The resonance energies were measured with the revolving target holder the aluminium resonances (see chapter I) being used as calibration points. They are in good agreement with measurements described in references mentioned above. The half -widths of the resonances were derived from the smooth curves obtained with a 28Si target. They are given in section 6 of this chapter, where their values are also discussed.

V.3 GAMMA RAY SPECTRA

The gamma ray spectra measured in the three resonances mentioned in the preceding section are shown in figures V. 4, 5 and 6. Measured points were omitted in the figures, since the spectra were measured with a 256 channel kicksorter and the statistics was quite sufficient to establish the spectra wen enough. Measured points will only be indicated in doubtful cases.

The measured intensities of the various gamma rays should be corrected for angular distributions described in 46