Experiments on nuclear levels in 35Cl

CONTENTS

Chapter 1.Introduction. 1.1.Nuclear models. 1.2. The 2s-1d shell. 1.3.The nucleus 35Cl.

1.4.0utline of the experiments. 1.5.Composition of this thesis. References (chapter 1).

Chapter 2.Gamma ray transitions, part I. 2.1.Introduction •.

2.2.Apparatus and experimental procedure. 2.3.Yield curves and resonance strengths. 2.4.Investigation of the gamma ray spectra. 2.5.Properties of the levels in 35Cl. 2.6.Discussion.

References (chapter 2).

Chapter 3.Gamma ray transitions, part Il. 3.1.Introduction.

3.2.Experimental procedure and results. 3 .3.D iscussion.

References (chapter 3).

Chapter 4.Construction, performance and application of a three scintillator pair spectrometer.

4. 1. Introduction.

4.2.Construction of the spectrometer and electronic circuitry.

4.3.Performance of the spectrometer.

4.4.The decay of the level at 7.545 MeV in 35Cl. References (chapter 4). 7 7 8 9 9 10

11

13 13 14 17 19 36

40

44 45 45 45 46 46 47 47 48 50 51 54

Chapter 5. The determination of the spin of some excited states. 55

5.l.Introduction. 55

5.2.Theoretical background. 55

5.3.Apparatus and experimental procedures. 56

5.4.Analysis. 57

5.5.Measurements and results. 57

5.6.Determination of allo wed spins and mixing ratios. 64

5.7.Spinassignments and discussion. 65

References (chapter 5). 66

Chapter 6. The determination of parity and spin of twoexcited states. 67

6.l.Introduction. 67

6.2.Angular distributions and angular correlations. 68

6.3.Polarization measurements. 73

References (chapter 6). 82

Chapter 7.Elastic proton scattering experimen tso 7.1. Introduction.

7.2. Theoretical background. 7.3.Experimental arrangemen tso 7 .4.Experimen tso

7.5.Results and discussion. References (chapter 7). Chapter 8.Conclusion.

8.1.Introduction.

8.2.Summary of the results.

8.3.Predictions of nuclear models. References (chapter 8). Summary. Samenvatting. 83 83 83 86

89

91 95 97 97 97

99

102 103 105

Chapter 1 INTRODUCTION 1.1. Nuclear modeIs.

The experiments described in this thesis lead to an increased knowledge of the properties of several excited states in 35C1. It is important to have a detailed knowledge of the energy levels of many nuclei in order to allow a compatison with the predictions of several nuclear models, giving insight in their validity and in their limitations. Though it is beyond the scope of this work to judge completely the applicability of nucleor models to the nucleus under study, a short description of some nuclear models is given below in order to elucidate some of the discussions in this thesis.

The nuclear shell model, proposed by Ma y e rand H a x e 1, Jen sen and Suess in 19491)2), is based on the assumption that the nucleons move independently in a spherically symmetrie potential well, which repre-sents the average influence of the other nucleons. The shape of this po-tential weIl may be ápproximated by a har monie oscillator potential; the possible exdted states have then equal spacings f!wo(see fig. 1.1.a.). A somewhat more realistic potential shape is somewhere between a har-monie oscillator potential and a square weU potential. In that case a level order is obtained as indicated in fig. 1.l.b. The levels with orbital angular momentum l = 0,1,2,3, •.... are called s,p,d,f, •.•..• orbits; the lowest s,p,d, ••••• orbit is the ls,lp,ld, •.. orbit, 'the next higher the 2s, 2p,2d, ••. or bit. The total spin of the nucleon in an orbit is given by

J

= l±

112; the influence of a spin-orbit interaction places the level J = l + 112 at a lower excitation energy than the J = l-1I2 level. The level order thus

obtained is shown in fig. l.l.c. Here the j value is used to distinguish the J = l± 1/2 levels.

It is assumed that both the neutrons and the protons in the same level couple to pairs with spin zero, since this is energetically favourable. Thus, the ground states of even-even nuclei should have spin zero. When one uncoupled particle is present, excited states of the nucleus are deter-mirèd by the possible excited states of that particle. This sa called single particle description is clemly too simple in many cases. Sometimes more particles are present outside a nuclear "core"; then a complex level scheme may arise due to the various possible couplings between these nucleons. A quite different pieture of the nucleus is provided by the collective models, where collective motions of nucleons are assumed instead of in-dependent motions. In these modeIs, proposed around 1959 by B 0 hr and

Mottelson3)4), the nucleus is assumed to be generally not spherieaUy symmetrie but it has for instanee on ellipsoidal shape3)4)5)6). The nu-cleus may perform small oscillations around its average shape or it may

change from some shape into an other. These effects give rise to vibratio-nal levels. The nuclear matter mayalso perform rotations, giving rise to rotational level bands.

Some models have been proposed between these two extremes. Starting from the shell model point of view, Ni 1 s s 0 n 7) calculated the single particle states in an ellipsoidally deformed potential with an axis of sym-rretry; these states are the lowest states of rotational bands. This work has been extended by Ne w ton 8 ) to a more genera 1 ellipsoidal potential shape. Starting from the collective model point of view, the motions of single nucleons may be coupled to vibrating or rotating 11 cores", as has been described by B ohr and Motte 1 s on4) and by 0 av yd ovg).

Other models than those quoted above have not yet been applied very succesfully to the explanation of the properties of low-lying excited states. Some recent books containing reviews of the theory of nuclear models are given under referenceIO ).

2P% ₂ 2p ,

-

lf%

... -----

< ...

₆ " 2 po/z ,

-

I f

_,

.... "'>:::. ... _, 4 , 11 ~ '-8 25 ld

%

4 .::' ... ---- _... 2s ~ " - ld ...

---

₂ '. _{" , ld %} 6 lp~ 2 1 P

--

_.-lp ~ c. __ ... 4 IS Is ~ 2 o b c

Fig.I.!. Shell model level schemes: a: harmonie oscillator potential, b: poten-tial between harmonie oscillator and square weIl, c: spin-orbit interaction included. At right the maximum number of neutrons or protons in each shell is given.

1.2. The 2s-ld shell.

We shall now confine our attention to the 2s-1d shell (see fig. 1. l.c., from 160 to 40Ca). In the past decade the knowledge of the level schemes of these nuclei has been greatly extended, as can be seen by comparing the 1952 edition with the 1959 edition of the review paper of A j zen b erg

and Lauritsen 11 )12) and the review paper of Endt and Kluyver (1954)13) with the one by Endt and Van der Leun (1962)14). Areason'

for this interest may be the interpretation of level schemes by sever~l of the models discussed above; a review paper on this subject has been gi ven by Go ve 15).

The experimental investigations were first di rected towards the experi-mentally or theoretically more manageable cases. In the later years the investigations were extended, partly to more complex nuclei (for instance this work), partly to more systematic research (for instance the work on even-even nuclei at Chalk River 16»).

At the present time the experimental results are so extended, that it is tempting to try to perform nuclear model calculations with the purpose to fit the levels of all nuclei in this region. Some attempts were made: 1. G 1 a ude m an s performed shell model calculations between 28Si and 40Ca I7 ). The improvements may be seen by comparing this work with similor work by Arima l8 ) in 1956.

2. Nilsson model calculations by B hatt I9 ).

3. Studies by C hi and Dav id s on using a non-axially deformed core rotor modePO).

Most of these models give more or less good agreement at some weIl known nuclei and predict position and spin of several levels in other nuclei. 1.3. The nucleus 35Cl.

The nucleus 35Cl may be described in shell model terms as a 32S "core" (all proton and neutron levels filled up to the 2s 112 orbit) with added two ld 3/2 neutrons and one ld 3/2 proton. Some low-lying shell model levels are expected to arise from:

1. Excitation of the d3/2 proton to higher (e.g. f7/2) levels.

2. Excitation of a "core" proton (e.g. a s 1/2 or d5/2 proton) to the d3/2 shell.

3. Excitations which break the pairing of the two d 3/2 neutrons. Also vibrationalor rotational levels may be present.

A detailed knowledge of the position, spin, parity and decay properties of the excited states is necessary to be ab Ie to assign them to one of the possibilities given above.

1.4. Outline of the experiments.

The experiments were performed at the Laboratorium voor Technische Natuurkunde, Technische Hogeschool Delft, using the Van de Graaff genera-tor at the high voltage laboragenera-tory of the Afdeling voor Elektrotechniek. Af ter the work on the reaction 28Si(p,y)29p by Van Oostrum 21 )22)23) had been finished it was decided to study the nucleus 35CI. At the time

this work was started (auturnn 1959) the knowledge of this nucleus was limited to the excitation energies of several levels and to the parity of the first two ones (detailed references are given in sect.2.1.).

We studied first the reaction 34S(p,y)35Cl, in which resonance levels are formed in 35Cl at about 7 MeV excitation energy; they decay to several lower levels. Experimentally this gives very complicatedgamma ray spectra, ,

in contrast with the reaction 28Si(Piy)29p. The problem of their irlterpreta-tion was attacked by greatly extending the use of coincidence measurements and by the application: of accurate methods of analysis.

The gamma ray decay of nine resonance levels was studied in this way.

Among the excited lower-lying levels were two unknown ones. Then 12 resonm'lces were investigated somewhat more superficially with the purpose to see. whetherstrong transitions to low-lying levels were present, but these were not observ,ed.

The difficulty of the interpretation of the gamma ray spectra lead to the development of a three scintillator pair spectrometer, which was applied in this work.

Spins of several levels were found from the investigation of the angular distributions and correlations of the gamma rays. The parities of two levels appeared to be of importance; they could be determined by studying the lin~ar polarization of two gamma rays.

The experiments were concluded with a study of elastic scattering of protons from the 34S nucleus. The absence of resonances in the scattered proton yield gives an upper value for the widths of the resonance levels in 35Cl.

Finally the experimental evidence was compared with the predictions of some models.

l.s.

Composition of this thesis.

The experimental work discussed in the preceding section is described in 6 chapters. Each chapter is devoted to the application at one or more resonances of some types of experimemtal techniques and methods of ana-lysis, which are therefore described in a part of that chapter. References are given at the end of each chapter. This procedure gives the most com-prehensive compilation of the experimental work. The most' important re-sults are summarized in a level scheme at the beg inning of the last chapter,

which diminishes th~ inconvenience that the results of experiments at a

References (Chapter 1).

1) Mayer,M.G., Phys.Rev. 75 (1949) 1969.

2) Haxel,O., Jensen,J .H.D. and Suess,H.E., Phys.Rev. 75 (1949) 1766. 3) Bohr,A., K. Danske Vidensk. Selsk. mat.-fys. Medd. 26 (1952) no. 14. 4) Bohr,A. and Mottelson,B.R., K.Danske Vidensk. Selsk. mat.-fys. Medd.

27 (1953) no. 16.

5) Davydov,A.S. and Filippov,G.F., Nucl. Phys. 8 (1958) 237. 6) Davydov,A.S. and Chaban,A.A., Nucl. Phys. 20 (1960) 499.

7) Nilsson,S.G., K.Danske Vidensk. Selsk. mat.-fys. Medd. 29 (1955) no.16.

8) Newton,T.D., C.R.T.886, Chalk River Report (1960).

9) Davydov,A.S., Nucl. Phys. 16 (1960) 597.

10) Ajzenberg-Selove,F. (ed), Nuclear Spectroscopy, part B. New York

(1960). Endt,P.M. and Demeur,M. (eds), Nuclear Reactions. part J.

Amstercfam 195-9. Endt,P.M. and Smifh.PB. (eds), Nuclear Reactions,

part H. Amsterdam 1962.FHrgge .S. (ed). Handbuch d8r Phvsik, vol. 34.

BerlJn 1957. Preston,M.A., Physics of the nucleus. Reading 1962. 11) Ajzenberg,F. and Lauritsen,T., Revs. modo Phys. 24. (1952) 321.

12) Ajzenberg-Selove,F. and Lauritsen,T., Nucl. Phys. 11 (1959) 1.

13) Endt,P .M. and Kluyver,J .C., Revs. modo Phys. 26 (1954) 95.

14) Endt,P.M. and Van der Leun,C., Nucl. Phys. 34 (1962) 1.

15) Gove,H.E. in Bromley, O.A. and Vogt, E.W.(eds), Conference on Nu

-clear Structure, Kingston. Toronto 1960. 16) Gove,H.E., Nucl. Instr. Meth. 11 (1961) 63.

17) Glaudemans,P.W .M., Wiechers,G. and Brussaard,P.J., submitted to Nuclear Physics.

18) Arima,A., Progr. theor.Phys. 19 (1958) 421. 19) Bhatt,K.H., Nucl. Phys. 39 (1962) 375.

20) Chi,B.E. and Davidson,J.P., Phys. Rev. 115 (1963) 366.

21) Van Oostrum,K.J., Alster,J., Hazewindus,N. and Wapstra,A.H., Physica 24 (1958) 1051 •.

22) Van Oostrum,K.J.; Thesis. Delft 1959.

23) Van Oostrum,.K.J., Hazewindus,N., Wapstra,A.H., Olness,J.W. and Parker,J.L., Nucl. Phys. 25 (1961) 409.

Chapter 2

GAMMA RA Y TRANSITIONS,

PART

I

2.1. Introduction.

The knowledge of the nucleus 35Cl waS rather limited at the time the work described in this thesis was started (autumn 1959). Excitation ener-gies of levels in 35Cl were measured by End t e.a. 1) from inelastic proton scattering experiments. Kis t n e r e.a. 2), in a study of the

f3+

-decay of 35 A to 35Cl, found a positive parity for the ground state and first excited state. Some resonances observed in bombardments of sulphur with protons were assigned to the 34S(p,y) 35Cl reaction 3)4)5). During the course of our work, Antuv'jev e.a.6 )7) found 44 resonances in the 34S(p,y)35Cl yield curve bet ween 0.717 and 1. 904 Me V proton energy. They reported that the decay of several resonance levels, investigated with a gamma scintillation spectrometer, shows only transitions to the ground state and the first exci-ted state, except at one resonance (proton energy Ep = 1.214 MeV) where astrong transition through the 3.16 MeV level takes place. Spins of some resonance states and of the 3.16 Me V level (7/2) are deduced from angular distribution studies. St 0 r e y e.a.8) measured the angular <:iistributions of the 1. 22 and of the 1.76 Me V gamma rays following the inelastic scaUering of protons on 35Cl and found that spins

1/2

and

5/2 for the first and second

excited state respectively explained their results; Oleksiuk e.a.9 ) pu-blished, among others, a spin assignement 3/2 to the 3.16 MeV level.

t

w -(1/n ~1.214 ul.057 til l020

----r;p%

00.928

~~:

""":12

el)

:2 0.848 ~(2 til 0.838 .Ö 0.756 .366MeV

..

7/2~ %)

(%t

3;+

10000

~-

_T

t

. ~ ~~ Eo (MeV) 7.545 7.393 7.357 7.267 7.230 7.222 7·189 7.180 7.100 4.174 i~i~ 4.113 3.006 2.645 1.762 1.220 0

Recently the results were published of (p,y) work performed by Dub 0 i s 10). Lifetimes of some levels in 35Cl have been measured by B ooth e.a.l l ) from nuclear resonance scattering of .bremsstrahlung. The information on the level scheme of 35Cl is summarized in fig. 2.1. (adapted from 12)). The results of the work by Dubois 10) are not included in this figure; they are discussed in sect.2.6.

The first purpose of our experiments was to assign some resonances in the gamma ray yield curve, which is obtained by proton bombardment of a 34S target, to the reaction 34S(p,y)35Cl. A careful study of the gamma rays at each resonance with scintillation spectrometers may then give information on the position and decay properties of the excited states in 35Cl.

In this .chapter such measurements at nine resonances between proton energy Ep = 0.756 mld 1.214 MeV aredescribed.

2.2. Apparatus and experirnental procedure.

Protons were accelerated with the Delft 2.5 Me V Van de Graaff genera-tor*\ This apparatus has been built under guidance of Prof.Dr.Ir. F.A. Heyn; its main conshuctional details are described in a !Xlper by L e y ze r s V i s and Mak kink 13 ).

The proton beam emerging from the machine was deflected over 900 _and passed a slit system from which a signa1 was taken for the stabilization of the high tension. After passing through a diaphragme the beam was a1-lowed to impinge. on the target, where its energy spread amounted about

.1

keV at 1 MeV. This target, which consisted of a tantalum disc on which some target material was -deposited, was fixed at 350 _{to the beam in a target} holder. The gamma radiation emitted by the target under proton bombard-ment could be detected in two scintillation counters; they were mounted on an angular distribution arrangement, which permitted their rotation around the centre of the target. For the work described in this chapter the counters were manually placed in the desired positions.

The . arrangement as was described above has been discussed at some length by Va n 00 sir um 14 ), mainly in sections 1.2, m.l and III.3; its performance has been treated in ref. 14) and 15).

Targets of about 50llg/cm2 enriched Cd 34S(46 % 34S) evaporated unto 0.1 mm thick tantalum backings were obtained from A.E.R.E.Harwell. The current at the target, as was measured with a current integrator, had to be limited to about 20 Il A in order to avoid excessive heating. Pulse spectra from the two scintillation spectrometers (cylindrical Harshaw NaI(Tl) crystals, 7.5 cm long and 7.5 cm diam. on EMI 9531 photomultipliers;

solution*)9.4% at 0.66 MeV) were recorded with a 256 charmel (RCL model 20611) pulse height analyser. For some selected low energy measurements a third spectrometer (Quartz et Silice, 5 cm long, 4.4 cm diameter crystal on EMI 6097 AF photomultiplier, resolution 7.4% at 0.66 MeV) in combi-nation with a 512 charmei (Nuclear Data series 130) pulse height analyser was used; it will be referred to as the "smaIl spectrometer."

The further electronic circuitry consisted of a (Franklin model 348) single channel analyser and a fast-slow coincidence arrangement 16) with a time resolution of r = 4 x 10-8 sec. The output pulses of the single chan-nel analyser, together with those of the fast coincidence circuit, were both fed into a slow coincidence circuit which was used to open the gate of the RCL pulse height analyser. A triple coincidence measurement was per-formed using a Nuclear Data model ND 500 Dual Amplifier; the coinci-dences between its two channel outputs and a third detector were obtained fr om a slow circuit (r = 4 x 10- 6 sec.) which operated upon the gate of the

pulse height analyser. In sum coincidence measurements 17) this fast-slow arrangement was completed with an adding circuit; the coincidences were selected with the fast circuit and the single channel analyser was used to choose the desired sum energy. These three coincidencE arrangements are shown schematically in fig. 2.2.

The experimental procedure was generally as follows: Resonances in the gamma ray yield curve were assigned to 34S on the basis of a compari-son with the results obtained with a sulphur target with an other isotopic constitution. Gamma ray spectra at these resOnances were then measured with a scintillation spectrometer; the crystal was placed with its face di-rectly against the target holder at an angle of 550

to the proton beam. A background spectrum was taken at a proton energy slightly below the re-sonance with the same number of protons hitting the target. Afterwards it was subtracted from the spectrum obtained at the resonance, making a correction for a slight gain shift if necessary.

The complicated single spectra thus obtained were analysed in a marmer similar as described by Nor dh a gen 18); the gamma rays used for the calibration spectra are given in table 2.1. The necessity of such a rather complicated analysis will be clear from the discussions of the spectra presénted in this chapter; see also discussions in sect. 4.1.

An energy calibration of the spectrometers was performed before and af ter each measurement with 13 7Cs, 22Na, 88y, PoSe and sometimes 24Na and the 6.14 MeV gamma radiation of the 19F(p, ay} 160 reaction 12 )19)20). Using the efficiency calibration of the spectrometers as measured by Van

*) The resolution is defined as the energy spread of a peak at half maximum devided by its energy.

FAST SLOW COINCIDENCE

SUM COINCIDENCE

TRIPLE COINCIDENCE

Fig.2.2. Coincidence arrangements; from top to bottom: fast coincidence arrangement, sum coincidence arrangement, triple coincidence arrangement. T: target, I, 11, 111: scintillation counters, S.C .A.: amplifier-single channel analyser, C.F.: fa st coincidence circuit, C.S.: slow coincidence circuit, P.H.A.: pulse height analyser, E: equalizing circuit, A: adding circuit. Some cathode followers and delay lines, used for practical reasons, are not shown.

00 st rum and Me ij e r21 ) the intensities of the gamma rays could be

computed.

Tentative decay schemes were then constructed and coincidence

mea-surements were made to decide bet ween them. The scintillation crystals

were placed at angles of + 550 _{and - 110}0 _{with respect to the proton beam}

Table 2.1. Gamma rays used for the shape calibration of the spectrometers.

E y (MeV) Source Ey (MeV) Reaction

0.51 22Na 2.37 12C(p,y) 13N, _{Ep = 0.46 MeV} 0.66 137Cs 3.51 12C(p,y) 13N, Ep = 1.70 MeV 1.28 22Na 6.14 19F(p,ay) 160, Ep = 0.34 MeV 1.84 BBy 7.89 30Si(p,y) 31p, E = 0.62 MeV

24Na 13C(p,y) 14N, p

2.75 8.06 Ep = 0.55 MeV

4.43 PoBe

Where necessary, sum coincidence spectra 17) were taken in order to check points that were difficult to investigate otherwise. As can be seen from fig. 2.2. the signals from one counter are only registered when at the same time a signal is detected by a second counter; the additional require-ment for registration is that the total amount of energy dissipated in both crystals equals a certain value. By choosing this value equal to the energy difference between two levels, the two-step transitions existing between them will be detected. This method has some advantages over normal coincidence work such as a good resolution and a simple spectrum shape. Yet, we did not use this method very often for the following reasons. When the resonance to be investigated is weak (which is mostly the case in this work) the equalizing and setting procedures are time consuming. Rather heavy demands are imposed on the stability of both spectrometers and on that of the sum channel when the time required for the measurements is long, since the results of the measurements depend critically on the accurate equalizing of the both spectra and on the setting of the sum channel. More-over, the possibility of scattering of gamma rays from one crystal into the other complicates the interpretation, especially when both weak and strong cascades are present 22 ).

2.3. Yield curves and resonance strengths.

The yield curve of gamma rays with energies above 1.3 MeV in the proton energy range 0.750-1.250 MeV is shown in fig. 2.3.

The absolute gamma ray yield was obtained for all resonances using a target with a thickness several times larger than equivalent with the spread in the proton energy and taking the gamma ray spectrum at a proton energy in the flat top of the resonance. The absolute number y of the gamma quanta emitted by the resonance level per proton hitting the target can then be computed from the known decay scheme. The resonance strength wy23)

1.000 0 ~ w > >

...

500

'"

...

~ ~ ... C> 15.500 ~ w ;:: 1.000 >

...

'"

"S (p ,r I"CI

GAMMA RAY YIELD CURVE (E_r>1.3MeVI

0.889

I

0.928

I

0.848 0.756

j

0.881 0.838

I

j

MAGNET SETTING

lr

MAGNET SETTING

Fig.2.3. Yield curve (in arbitrary units) for the 34S (p,'Y) 35 C1 reaction between

E = 0.750 and 1.220 MeV.The low bump in the curve between the resonances

arE

=

0.848 and 0.881 MeV is due to a 19F target contamination.

p

(defined as cuy = ~(2J + 1) r r I(r +r )in which rand rare the proton width and 'the radiative widthPofYthe levll respectivefy and lthe spin of the resonance level) is derived from y using equation (31) in ref.23 ) and a value of 26.7 x 10-15 _{eV cm}2 _{per atom for the stopping power of 1 MeV}

protons in the target, taken from ref.24 ). The results of the determination of the resonance strengths are compiled in table 2.II. The following quanti-ties are listed for each resonance:

b. The excitation energy Eo of the resonance level computed from the proton energy and a Q-value of 6.366 MeV2.5).

c. The resonance strength eûy as derived above; its error amounts to about 30'~ mainly due to errors in the computation from the gamma ray

spectrum of the number of gamma quanta emitted by the resonance level detected by the spectrometer and to the absolute efficiency calibration of the latter.

Table2.II. The resonance strength wy for 9 resonances in the reaction

34 S(p,y) 35 Cl between E

=

0.756 and 1.214 MeVj the excitation energy of

the excited level in 35 Cl

i~

EO'

Ep (MeV) Eo (MeV) eûy (eV) Ep (MeV) Eo(MeV) eûy (eV)

0.756 7.100 0.040 0.928 7.267 0.31

0.838 7.180 0.056 1.020 7.357 · 0.48

0.848 7.189 0.17 1.057 7.393 0.067

0.881 7.222 0.073 1.214 7.545 2.0

0.889 7.230 0.23

2.4. Investigation of the gamma ray spectra. 2.4.1. Introduction.

In each tollowing subsection the resuIts of our measurements of the gamma ray spectra at a resonance are summarized in a table with columns: a. Emrgies of the observed gamma rays; the values given are averages

of this resonance.

b. The levels between which the transitions are assumed to take place. c. The branching ratios, normalized in such way that the sum over all gamma rays starting from one level equals 100. These values are averages of those obtained from the measurements of both single spectra and coincidence spectra.

d. The spectra in which the gamma rays were observed; S means single spectrum, A, Band C are coincidence spectra. Parenthesis around A or B mean that the gamma ray itself was not observed in measure-ment A or B but that the measuremeasure-ment indicated its existence in an indirect way.

The gamma ray energies given in the figures are computed from the known 12) energies of the levels bet ween which the transition is assumed

to take place in order to facilitate a quick comparison between the different spéctra. Brackets around an energy value indicate the same uncertain occurrence as in d above.

The order of the discussion of the resonances is chosen with a view on the ease of the discus sion of their results.

2.4.2. The resonance at Ep

=

1.0.20 MeV (Eo

=

7.357 MeV).

In the high energy part of the gamma ray spectrum (fig. 2.4.) the trans i-tions to the ground state and the first two excited ones are evident. In the lower part this can also be said for peaks due to the decay of the 1.76 and 1.22 MeV levels to the ground state and a peak due to the 0.51 MeV an-nihilation radiation originating from pair effect absorption of high energy

Table 2.II!. Gamma rays observed·at the resonance at Ep = 1.020 MeV.

Ey (MeV) Transition Branching Observed in 7.36 iO.lO 7.36 - 0.00 18 i 4 S 6.20 i 0.10 7.36 - 1.22 67 i 14 S 5.60 ±0.20 7.36 - 1. 76 10 ± 3 S 3.98 ±0.08 4.06.,... 0.00 24 ± 16 SC 3.35 ±0.05 7.36 - 4.06 5 ± 2 SBC 2.81 ±0.04 4.06 - 1.22 59 ± 18 SC 2.30 iO.lO 4.06 - 1. 76 17 i 12 S 1. 76 i 0.04 1.76 - 0.00 100 SABC 1.22 ±0.04 1.22 - 0.00 100 SABC

gamma rays in the materials surrounding the scintillation counter. The low energy part bf the spectrum has beencarefully investigated with the small spectrometer in order to detect possibly present low lines, for instance originating from a l. 76-1.22 MeV transition. No new lines have been found with energies between 0.3 and l.5 MeV; the shapes of the 0.51 Me V an-nihilation peak and the 1.22 MeV peak agree very weIl with the line shapes of a 22Na calibration source (see e.g. the insert in fig. 2.4.).

The 1.22 and 1. 76 MeV transitions are in coincidence with high energy ones as required by the proposed decay scheme (fig. 2.4): the coincidence spectrum with a channel 5.0-6.7 MeV (A in fig. 2.4) is shown in fig. 2.5. (A); the 1.76 MeV peak disappears when the channel is set above 6.0 MeV. Results of coincidence measurements with channels set on 3.6-4.1 MeV

0.511 r~N. ~ lQ.OOO

A

1.22 z

g

_j

'" w "-iOOO

T

Ep.'.020M.V SINGLE SPECT~UH t - -I!-A---I

ij

' j

! \

_{· 1} I i .i i (\ ... j

~

7.3' 4.06 1.76 o n i! ij 1_I I' .1 I i i i ... :'\ i .! \ .... I i Ij I' '1 I. j I .... '. " " I

·

w

,

."""

/1 I ,I! \j

I

12.301

r

r

J:

,

~

!~

_

f\ ... \ .. ~-:: ...

-::.

_

\

"":~~.: ... \ ~-4.06

l.

u J 2 3

•

PULSE HEIGHl fM.V) 1.22

Fig.2.4. Single spectrum at the resonance at Ep = 1.020 MeV. The shapes

of the single components are given in broken lines. Inserts show a careful

study of the annihilation peak and the decay scheme of the resonance level.

Channels used in coincidence measurements are indicated by A, 8 and C.

Ep_t:020M.V

COINCIDENCE SPECTRA A ,BoC

300 1.76

~

250 ;;j z z ~ ~ 200 z 1.22

..

:x:

l

u

..

~ I

..

1.22 :x: u

~

ffi 200

_:

..

,

:!? , z

:

"

_i

0 _u ISO on A ~ : 2.B4 z "

:::

100

.

,

~

-.,. : Mi: 100

V

1.

,

76

PULSE HEIGHT (M.V) PULSE HEIGHT (M.V)

Fig.2.5. Coincidence spectra at the resonance at Ep

=

1.020 MeV, obtained

Peaks at 4.1 and 2.8 MeV appear in the last spectrum; they are interpreted as transitions from the level at 4.06 MeV to the ground and first excited state, fed by the 3.3 MeV transition. The smal! bump at 3.3 MeV is due to the 3.3 MeV gamma ray in coincidence with the part of the Compton distri-bution of the 4.06 MeV gamma ray in the coincidence channel. Statistics were irisufficient to decide whether a transition from the 4.

i

MeV level to the 1.76 MeV one is pres,ent (see however sect.2.4.3).

2.4.3. The resonance at Ep = 0.838 MeV (Eo

= 7.180 MeV).

The high energy part of the single gamma ray spectrum (fig. 2.6) at this weak resonance shows transitions to the ground state and to the first excited one. The 5.42 MeV gamma ray leading to the 1.76 MeV level has been established with a coincidence measurement with channel setting

Table 2.IV. Gamma rays observed at th'e resonance at Ep

=

0.838 MeV.

E (MeV)

y Transition Branching Observed in 7.20 iO.1O 7.18 - 0.00 46 i 10 S 6.00 iO.lO 7.18 - 1.22 16 i 7 S 5.42 7.18 - 1. 76 4 i 3 (A) 4.50 iO.05 7.18 - 2.69*) 2 i 1 S 7.18 - 2.64*) 6 i 2 S 4.00 i 0.10 4.06 - 0.00 19 i 7 S 3.14 iO.05 7.18 - 4.06 26 i 6 S BC 2.81 iO.05 4.06 - 1.22 66 i 19 S C 2.69 i 0.08 2.69 - 0.00*) S B 2.64 - 0.00*) 2.30 iO.07 4.06 - 1. 76 15 ilO S C 1. 76 iO.04 1.76 - 0.00 100 SABC 1.48 iO.04 2.69 - 1.22 S B 1.22 iO.04 1.22 - 0.00 100 SABC

*) See discussion }:>resented in this section.

5.0-6.6 MeV (A in fig. 2.6), yielding results similar to those obtained at the resonance at 1.020 MeV. A coincidence measurement with the 4.0 and 4.5 MeV peaks in the channel (3.4-5.0 MeV, B in fig. 2.6) shows peaks at 2.69 and 1.48 MeV (fig. 2.7 (B)), indicating that the 4.5 MeV transition feeds at least one of the two 2.6 MeV levels. It is shown later that the 2.64 MeV level decays only to the ground state, (sect. 2.4.9.) and that the 2.69 MeV level decays for 41% to the ground state and for 590/0 to the 1.22 MeV one

Ep.D.UI "'eV SINGLE SPECTRUM

T

Tir

100

,

_4.06

·

"

!M2.64 1.76 1.22 o ---+---A~ 2.000 1.1'

t

1.000 2 5 . PUlSE HEIGHT(M.V)

Fig.2.6. Single spectrum at the resonance at Ep

of fig. 2.4.

0.838 MeV. See also text

~ ... z z

g

250 '" ...

..

'" 200 z ~ 8 150 100 50 0.51

t

xl 2

rT

I

)

-\-

_!:::~:

~

::.-<

/

3 PUlSE HEIGHT ,,,,..'1) Ep.O.' . M.V COINCIDENCE SPECTRA. B,C ... ~50

..

Q ei ~100 z :> 8 50

Fig.2.7. Coincidence spectra at the resonance at Ep with channels Band C indicated in fig. 2.6.

T T

(see sect. 2.4.4. and 2.4.7.). Since the ground state transition measured here is four times stronger than that to the first excited state, it is assumed that we observe here two transitions starting from the resonance level: 31% to the 2.69 MeV leveland 69% to the 2.64 MeV level. See, however, also the discussion in section 2.5.5.

The relatively strong decay of the resonance level to the 4.06 MeV level has been used to establish the presence of a 2.3 MeV transition bet ween the last level and that at 1.76 MeV. A coincidence spectrum ob-tained with channel 3.0-3.4 MeV (C in fig. 2.6) is shown in fig. 2.7 (C).

The peak at 2.3 MeV is obviously much too high to be only a pair peak of the 2.84 MeV gamma ray. The 2.69 MeV and 1.48 MeV peaks, due to coinci-dences with the Compton counts of the 4.5 MeV gamma ray in this channel,

are expected toOO undetectable with the present statistical precision and are indeed not seen.

In the analysis of the spectra remains a small burnp at about 2.1 MeV, possibly due to a decay to one of the levels at 5 MeV (see sect.2.4.7.). This

has not been investigated further.

2.4.4. The resonance at

Ep =

0.848 MeV

(Eo

= 7.189 MeV).

The gamma ray spectrum at this resonance is presented in fig. 2.8. A coincidence measurement with channel

>

4.6 MeV (A in fig. 2.8) shows the existence of a transition to the 1.76 MeV level in addition to that to the 1.22 Me V one already evident from the single spectrum. In the middle

Table 2.V. Gamma rays observed at the resonance at E = 0.848 MeV. _{. p}

Ey (MeV) Transition Branching Observed in 7.16 iO.l0 7.19 - 0.00 8 i 2 S 5.96 iO.lO 7.19 - 1.22 69 i 14 S 5.43 iO.15 7.19 - 1.76 5 i2 S(A) 4.17 iO.05 4.17 - 0.00 62 i 16 S B 3.92 iO.lO 4.06 - 0.00 39 i25 S 3.22 ±0.08 7.19- 4.06 6 ± 3 S 2.97 ±0.04 7.19 - 4.17*) 12 i 3 S B 4.17 - 1.22*) 38 ± 13 S B 2.76 iO.lO 4.06 - 1.22 61 ±34 S 1. 74 ±0.04 1.76 - 0.00 100 SA 1.22 ±0.04 1.22 - 0.00 100 SAB

E .... ".M.V Slf8LE SPECTRUM

~.---~

- 7.11 4.17 4.06 _ __'.71 - - 1.22 - - 0

!\

J

.

:1'[

..

".

J

(

c,,<Af

c

'~:!\

~

.

J

/

\~

I

2. PUlSE HEIGHT l"hV)

Fig.2.8. Single spectrum at the resonance at Ep

=

0.848 MeV. See also text of fig. 2.4.

part of the spectrum prominent peaks at 4.2 and 3.0 MeV are present. A

coincidence measurement with a channel set on the 3.0 MeV peak (B in

fig. 2.8) shows peaks at 4.2, 3.0 and 1.22 MeV (fig. 2.9 (B)). The most probable interpretation of this result is assuming a 3.0 MeV transition from

250 3

,

.0 Ep.O.8UMeV COINCIDENCE SPECTRUM B ~

...

1.22 z 200

~

z

..

:r u a:

_...

"- lSD V> r z ::> 0 u 100 50

Fig.2.9. Coincidence spectrum at the resonance at Ep

with channel B indicated in fig. 2.8.

the resonance level to the known level at 4.17 MeV which then partly decays

tO' the ground state and partly, with a second 3.0 MeV gamma ray, to the

first excited state. A peak at 3.12 Me V shown in fig. 2.8 can be

interpre-tedas due to a transition from the resonance level to the 4.06 MeV level.

The most intense gamma ray starting from the latter level is a 2.8 MeV one

(e.g. sect. 2.4.5.). which therefore has to be also present in the single

gamma ray spectrum (fig. 2.8). lndeed, the number of counts in the valley

between the 3.0 MeV peak and the pair peak at 2.5 MeV is obviously too

high. A careful analysis using a 24Na source yields a 2.8 MeV gamma ray

with an intensity in agreement with the proposed interpretation.

2.4.5. The resonance at Ep

=

0.881 MeV (Eo

=

7.222 MeV).

The 6 MeV peak in the single spectrum (fig. 2.10) is much stronger than

would be expected from the 1.22 MeV one; only 6% appears to be coincident

with the latter one, as a coincidence measurement (channel

>

5.1 MeV, A in

fig. 2.10) shows (fig. 2.11 (A)). This discrepancy may be caused by an

incorrect subtraction of the 6.14 MeV radiation originating from the

reso-nance at Ep

=

0.873 MeV in the reaction 19F(p,ay)160 19l.

A second coincidence measurement (fig. 2.11 (B)) has been performed

with a channel

>

4.4 MeV (B in fig. 2.10). The small bump at 2.69 MeV and

the peak at 1.48 Me V indicate a transition from the resonance level to that

at 2.69 MeV, which then decays for 34% to the ground state and for 66% to the first excited state. This branching ratio can he computed onlv assuming

Table 2.VI. Gamma rays observed at the resonance at Ep

=

0.881 MeV.

Ey(MeV) Transition Branching Observed in

7.15 iO.lO 7.22 - 0.00 68 i14 S 6.00 7.22 - 1.22 2 i 1 (S) (A) 5.46 7.22 - 1. 76 8 i 3 (A) 5.20 iO.1O 5.22 - 0.00 100 C 4.55 iO.l0 7.22 - 2.69 8 ± 3 S C 4.00 ±0.20 4.06 - 0.00 22 ± 22 S 3.08 ±0.10 7.22 - 4.06 7 ± 2 S 2.72 ±0.10 4.06 - 1.22 63 i24 S 2.66 ±O.OB 2.69 - 0.00 34 i 13 S B 2.30 i 0.10 4.06 - 1. 76 15 ± 15 S 2.00 ±0.04 7.22 - 5.22 7 i 2 S B 1. 76 ±0.04 1.76 - 0.00 100 SAB 1.46 ±0.04 2.69 - 1.22 66 ± 18 S B 1.22 ±0.04 1.22 - 0.00 100 SAB

.

~ .... z z ;! 7.0lI0 6J) u 5.000 0: ....

..

l!! z :> uoo o u 1000 2.000 1.000

.

_.

r-C--I 2." 2.691

1

tr

1.76 1.48

I

n 1 11 t ~200

i! ,

!i

I

t' '1 I' :! i:

n

(\230 : i

I: : :

i:

I \ f

\!

:1 \' /\ /'

.~: 'I' V ';"x/1-.\' \ Ep _ 0.881 M.V SINGlE SPECTRUM f B - ~A--• I '.J; ~, ..... A-"(I • ~

\ --.

--

.

-:

.J

\

\

\

6 PULSE HEIGHT I M.V) 7.22 5.22 4.06 2.69 1.76 1.22 0

Fig.2.1 O. Single spectrum at the resonance at Ep of fig. 2.4.

0.881 MeV. See also text

Ep.O.8S1 H.V

COINCIOENCE SPECTRA A,B,C

T

200 1.22 200

I

100 1.48 0: 150

Ir

ol

..

~ ~ 0 100 u 50 PULSE HEIGHT (H.V)

Fig.2.11. Coincidence measurements at the resonance at Ep obtained with channels A, 8 and C shown in fig. 2.10.

that there is no transition to the level at 2.64 Me V; see, however, also the discussion in sect. 2.5.5. The 2.00 MeV peak in th is coincidence spectrum indicates the existence of an unknown coincident gamma ray in the region above 4.4 MeV, probablya 5.2 MeV one. In the single spectrum no definite peak can be found, due to the complexity of the high energy part. Therefore a coincidence channel has been set on the peak at 2.00 MeV, taking care that it did not contain the 1.76 MeV peak (1.84-2.50 MeV, C in fig. 2.10): a coincident 5.2 MeV gamma ray should then show up as the highest peak in the coincidence spectrum. As is shown in fig. 2.11 (C) this is indeed the case. This result is not due to unwanted scaUering of gamma rays from one crystal into the other, as is shown in a measurement with the same coincidence channel at the resonance at Ep = 0.889 MeV which has a simi-lar decay exceptfor the 2.0 MeV gamma ray (sect. 2.4.6). One must therefore conclude that a level is present in 35Cl at 2.00 ± 0.04 or at 5.22 ± 0.04 MeV. The reasons for assuming the latter are given in sect. 2.5.3.

2.4.6. The resonance at Ep

=

0.889 MeV (Eo

=

7.230 MeV).

The gamma ray spectrum shows mainly a strong ground state transition (fig. 2.12). Further a coincidence measurement with channel

>

4.4 MeV (A in fig. 2.12) shows only peaks at 2.64, 1.76 and 1.22 MeV (fig. 2.13 (A)). Since there is no peak at 1.48 MeV in this coincidence spectrum the level

at 2.64 MeV must be involved here. The high energy part of the spectrum

obtained in a coincidence measurement with channel B (1.84-2.50 MeV) shows no 5.0 MeV peak of appreciabie intensity (fig. 2.13 (B)). This peak would certainl y be present if the results of the coincidence measurement C described in the preceding section were due to scattering.

Table 2.VII. Gamma rays observed at the resonance at Ep = 0.889 MeV.

Ey (MeV) Transition Branching Observed in

7.20 ±0.1O 7.23 - 0.00 92 ± 5 S 6.00 ± 0.20 7.23 - 1.22 2 ± 1 S 5.47 7.23 - 1.76 1 ± 0.5 (A) 4.64 ±0.10 7.23 - 2.64 5 ± 2 S B 2.66 ±0.06 2.64 - 0.00 100 S 1.76 ±0.04 1.76 - 0.00 100 SA 1.22 ± 0.04 1.22 - 0.00 100 SA

Ep:: 0.899 MeV SINGLE SPECTRUM ~---A--

________

_

or w

..

'" lOOO ~ 82.000 1.000 I - - B - i

\

n

j'\ V' ,I \ /'\.. i: \ --d\---- \ j \.

~

7.2J 2.64 1.22,.76 o PUL SE HEIGHT (M.V)

Fig.2.12.Single spectrum at the resonance at Ep 0.889 MeV. See also text of fig. 2.4.

Ep.O.889 HeY

COINCIDENCE SPECTRA A,S

lSO 1.22 4.59 ~

I

100

I

w z z _I

..

~ 200

t'

or 50 w

..

~ lSO z :> 8 1110 1.76

I

PUlSE HEIGHT (H.V)

Fig.2.13. Coinc.idence spectra at the resonance at Ep with channels A and B indicated in fig. 2.12.

2.4.7. The resonance at E_p

=

0.928 MeV (Eo

=

7.267 MeV).

The lower part of the single spectrum (fig. 2.14.) has been investigated with the small spectrometer, as in the case of the 1.020 MeV resonance. No new lines were found; in the insert of fig. 2.14 the 0.51 MeV annihilation peak is shown together with its 22Na calibration. The new peak at 2.24 i 0.05 MeV appears a1so in a coincidence spectrum (fig. 2.15 (A)) with channel

>

4.4 MeV (A in fig. 2.14), thus suggesting a 5.0-2.2 cascade. A

Table 2.VIII. Gamma rays observed at the resonance at Ep

=

0.928 MeV.

Ey (MeV) Transit ion Branching Observed in 7.30 iO.lO 7.27 - 0.00 72 ±14 S 6.10 i 0.10 7.27 - 1.22 20 ± 5 S C 5.50 ± 0.10 7.27 - 1. 76 1 i 0.3 (A) C 5.04 iO.15 5.03 - 0.00 100 (A)BC 4.65 iO.20 7.27 - 2.69 4 i 0.6 SABC 4.00 iO.l0 4.06 - 0.00 C 3.12 ±0.08 7.27 - 4.06 1 ± 0.3 S C 2.82 ±0.08 4.06 - 1.22 S 2.70 iO.08 2.69 - 0.00 35 ± 12 SA C 2.24 iO.05 7.27 - 5.03 2 i 0.5 SA C 1. 76 iO.04 1.76 - 0.00 100 SA C 1.50 iO.04 2.69 -1.22 65 i22 SA 1.22 iO.04 1.22 - 0.00 100 S,A C

coincidence channel was set from 1.84 to 2.50 MeV (B in fig. 2.14) in' the same way as at the resonances at E _p

=

0.881 and 0.889 MeV; the spectrum obtained (fig. 2.15 (B)) shows indeed a peak at 5.04 i 0.15 MeV. A sum coincidence measurement with the sum channel set on 7.27 MeV has been performed; it shows (fig. 2.16) that the 5.0-2.2 MeV transition is the strong-est twofüld cascade except the6.0..,...1.2 MeV one. Therefore we must conclude that a level is present at 5.03 i 0.05 MeV or at 2.24 ± 0.05 MeV; the rea-sons for choosing the first possibility are given in sect. 2.5.3. From the single spectrum and coincidence spectrum A the branching ratio of the 2.69 MeV level was found to be the same as was observed at the resonance at E _p = 0.881 MeV; see bowever also sect. 2.5.5.

7.000 ~ .noo ... ~ ~ u '" 5.000 ...

..

4.000 3.000 2.000 1.000 0.51 M.V -1 .. 22NA 1.22 \. X~ 5 Ep_O.928 M.V

SINGLE SPECTRUM

1 - - - -A - -- - . PUlSE HEIGHT I M.V)

1i

75.03 . 27 4.1)7 2.69 1.76 1.22 o

Fig.2.14. Single spectrum at the resonance at Ep 0.928 MeV •. Insert shows a carefu1 study of the 0.51 MeV annihilation peak. See a1so text of fig. 2.4.

~so z z C % U Ep.0.928M.V 150

COINCIDENCE SPECTRA A,B

1.48 ~

...

z ~ 100 iS ffi

..

~ 50 z :> o u 4

,

.57 B 4 5 •

r~

_{2.24 PULSE HEIGHT IM.V).}

ffilOO

..

i!? z :> o u

~

A 2 3 PULSE HEIGHT I M. V)

Fig. 2.15. Coincidence spectra at the resonance at Ep with channe1s A and B indicated in fig. 2.14.

200

iil Ep = 0.928M.V

l:E SUM COiNCIOENCE SPECTRUM

«

a

150 '" UI Cl. INSTR. CUT OFF " 1.22 100 ) 50 6.05

I..

Fig.2.16 •. Sum coincidence spectrum 'at Ep

=

0.928 MeV. Sum channel set on

7.27 MeV. The 1.22 MeV peak, which should be as intense as the 6.05MeV

one, has been cut oH by accident.

2.4.8. The resonance at Ep = 0.756 MeV (Eo = 7.100 MeV).

The gamma ray spectrum obtained at this resonance is similar to th at

at the prec"eding resonances. The results derived from this measurement

are summarized in tab1e 2.IX.

Table 2.IX. Gamma rays observed at the resonance at Ep = 0.756 MeV.

(Ey(MeV) Transition. Branching Observed in

7.09 iO.lO 7.10 - 0.00 19 i 4 S 5.90 i 0.10 7.10 - 1.22 62 i 10 S 5.44 7.10 - 1.76 7 i 2 (S) 4.43 iO.05 7.10 - 2.69*) 5 i 2 S 7.10 - 2.64*) 5 i 1 3.00 iO.06 7.10 - 4.06 2 i 1 S 2.84 4.06 - 1.22 (S) 2.70 iO.05 2.69 - 0.00*) S 2.64 - 0.00*) 1. 76 iO.04 1.76 - 0.00 100 S 1.48 iO.04 2.69 - 1.22 S 1.22 iO.04 1.22 - 0.00 100 S *) see sect.2.5.3.

2.4.9. The resonance at Ep = 1.214 MeV (Eo = 7.545 MeV).

This is by far the strongest resonance below

Ep

=

2 MeV. The gamma ray spectrum (fig. 2.17) exhibits mainly two strong gamma rays with energies

4.38 and 3.18 MeV, clearly due to a cascade through a 4.38 MeV level or

the known 3.16 MeV level. This spectrum has been compared with that of the resonance at 1.057 MeV (sect. 2.4.10.), which shows a similar decay, taken at the same run. There appears to be no energy shift between the 3.16 MeV

Table 2.X. Gamma rays observed at the resonance at Ep = 1.214 MeV.

Ey (MeV) 5.78 5.22 4.90 4.34 iO.07 3.18 iO.06 2.65 tO.05 2.33 tO.lO 1. 76 t 0.08 0.52 tO.Ol 30 20.000 10.000

Transition Branching Observed in

7.54 - 1.76 2 t 15 (B) 5.22 - 0.00 (S) (B) 7.54 - 2.64 3 i 2- (A) 7.54 - 3.16 93 i 5 S 3.16 - 0.00 84 i 7 S B 2.64 - 0.00 100 S A B 7.54 - 5.22 2 t 1.5 S B 1.76 - 0.00 100 S B 3.16 - 2.64 16 t 3 S (B)

~

5::

31.

Ep_1.214MtV SINGLE SPECTRUM

.

3

.

_{1. 2}

.

• , 2]"'

I

1.76 , 1 9 I A -o SUM PUl5ES

/

3 , PUlSE HEIGHT (MtV) C

Fig.2.17.Sing1e spectrum at the resonance at Ep = 1.214 MeV. See also text of fig. 2.4.

peaks, but the 4.4 MeV peaks show an energy difference of more than 0.1 MeV. This proves that the 3.16 MeV level is excited and not a 4.38 MeV one. The ground state transition is notably absent « 0.2%) as was checked by a series of measurements at several distances between target and scin-tillation counter in order to avbid piling up of gamma quanta in the spectro-meter. The analysis presented in fig. 2.17 shows that the peak at 2.65 MeV is much too high to be only a pair peak from the 3.16 MeV gamma ray; this indicates the existence of a gamma ray at about 2.65 MeV. In a coincidence measurement with channel setting > 4.6 MeV (A in fig. 2.17) the relative intensity of this peak is even a factor 3 stronger. However, the required intensity of a 4.9 MeV gamma ray to feed the 2.6 MeV level alone is more than a factor 3 higher than measured in the spectrum. The intensity of the 2.6 MeV gamltJa ray may be explained if a 2.6 MeV level is partly fed from the 3.16 MeV one. Indeed, a coincidence measurement with channel > 3.6 MeV (8 in fig. 2.17) shows (fig. 2.18 (8)) about the same intensity ratio between the 3.16 MeV and 2.6 MeV peaks as in the single spectrum. In order to detect a possible transition between the 3.16 MeV level and one of the 2.6 MeV levels, the 0.51 MeV annihilation peak was carefully measured with the small spectrometer. As shown in fig.

2.19,

the 0.5 MeV peak is broadened about one channel (corresponding with 0.006 MeV) to the high energy side with respect to the 22Na annihilation line. The same behaviour is found at the resonance at Ep

= 1.057 MeV, but at the previously discussed 0.928 and

1.020 MeV resonances no difference can be measured between the two an-nihilation peaks. Since these four measurements were performed during the same run, all with 22Na calibration before and af ter the meas ure ment , we must conclude that thereexists a gamma ray with an energy slightly higher

~ lol Z Z

..

::z: u ct: lol

..

f!! 2000 z 1000

B

Ep_'.214M.V COINCIDENCE SPECTRUM 9 2.64

I

PULSE HEIGHT tM.V)

Fig.2.18. Coinc1dence spectrum at the resonance at Ep = 1.214 MeV obtained with channe1 B indicated in fig. 2.17.

4000 ..J 111 z z « % IJ ::i 3.000 Q. Cl) !Ë ::;, 0 IJ 2.000 1.000 0.5M.V O.OO'Me\'

-H-I , I ~

*

"0

o 000 Ep.l.214M.V 000 SINGLE SPECTRUM ••• 22Na PULSE HEIGHT

Fig.2.19. Annihilation peak at the resonance at E _p

=

1.214 MeV with 22 Na calibration.

than 0.511 MeV. Therefore the 2.64 MeV level and not the 2.69 MeV one is

excited. The former decays only tothegro_undstate;an unobserved transition

to the 1.22 MeV level is less than 5% from the ground state decay. More

evidence for this 4.3-0.5-2.64 cascade has been obtained in a triple

coinci-dence experiment (C) with one channel set on the 4.3 MeV peak and an other

on the 0.5 MeV one. In order to prevent errors due to scattering processes (such as 0.5 MeV quanta escaping from the counter connected to the pul se height ana1yser into the one set on the 0.5 MeV peak), the crystàls had to be

well shie1ded from each other. The counting rate was very 10w. The results

(fig. 2.20) show on1y a peak at 2.6 MeV; it shou1d have been more similar

to fig. 2.18 if the 0.5 MeV channel did not contain a 0.52 MeV gamma ray •

. ~ Ep=l. 214 MeV

z TRIPLE COINCIDENCE SPECTRUM « Q 30

~

20

T'

::;, 8'0 o 2 3 PULSE HEIGHT (M.V)

Small bumps at 2.34 and 1.76 MeV present both in the single spectrum

and in the coincidence spectrum B indicate low intensity transitions from

the resonance level to the 5.22 and

1.

76 MeV levels.

2.4.10. The resonance at Ep

=

1.057 MeV (Eo = 7.393 MeV).

The single gamma ray spectrum at this resonance shows approximately

the same behaviour as that of the 1.114 MeV resonanee. The cascade through

the 3.16 MeV level is the strongest decay mode of the resonance level, but

competing transitions to the levels at 5.22, 2.64 and 1.76 MeV are also

present. The information obt~ined from the analysed single spectrum is

presented in table 2.XI.

Tab1e2.XI. Gamma rays observed at the resonance at Ep = 1.057 MeV.

Ey(MeV) Transition 5.63 7.39 - 1. 76 5.20 ±0.07 5.22 - 0.00 4.74 ±0.06 7.39 - 2.64 4.23 ±0.05 7.39 - 3.16 3.16 ±0.O5 3.16 - 0.00 2.64 ±0.05 2.64 - 0.00 2.18 ±0.05 7.39 - 5.22 1.76 ±Q.04 1.76 - 0.00 0.52 ±0.01 3.16 - 2.64 2.5. Properties of levels in 35Cl. 2.5.1. Introduction. Branching Observed in 7± 3 (S) S 22 ± 5 S 59

±

19 S S S 12

±

3 S 100 S S

This section contains the energy values and branching ratios ot all

levels excited in this investigatibn. They were obtained by averaging the

results of section 2.4., using the appropriate statistical weight factors.

The information in th is section is summarized in table 2.XII, where also

estimated maximum values are given for several possible transitions that

were not found in our investigation. The branching ratios (with their errors)

assigned to the transitions starting from a level may be interpreted as

per-centages of the total decay of that level. In a recent publication27 ) the

branching ratios with their errors were obtained by multiplication of the

measured intensities and their errors with a factor which made the sum of

"*'I~;

11

1

m! ,

ip',

J

~,li

11

tjl

1

flJill

liJ

I

lIJ

1

111["'"

N ' " N !!!

_'"

_{'" ~}

i

N "'I. N ' " ~ ~ ~ ....

'7

N N ~It

'"

I N _'" N . . 5.0 3522 34174 i

1

11

11

1

~a 4.11 4.u"," 1163 3.006 2.64 S 1.76 1.220 0

Fig.2.2L Proposed level scheme of 35 Cl.

~:!:~~ (%.WI I -I , .. ~:8 (g M

,

. ..- (5121+ I ê (l/2)+

lr

3//

as percentages. This is the reason why some errors in ref. 27 ) are larger than those given in table 2.XII.

Fig. 2.21. shows the proposed level schemeof 35Cl. 2.5.2. The resonance levels.

The energies of the resonance levels computed from our gamma ray energies agree within the errors with the values computed from the proton energy and

Q

value (6.366 MeV 25»). The branching ratios of the levels can beobtained from the tables of section.2.4.

2.5.3. The levels at 5.22 and 5.03 MeV.

The two previously unknown levels at 5.22 ±0.04 or 2.00 ±0.04 and at 5.03 ±0.05 or 2.24 ± 0.05 MeV are excited only as intermediate levels in cascades from the resonance level to the ground state. We feel that indeed two different levels are involved, considering the enetgy difference of the low energy members of the cascades. The positions of the levels could not be specified further from our measurements; we are inclined to believe the first alternatives for the following reason. The levels at 1. 22, 1. 76, 2.64 and 2.69 MeV were found from high inelastic scaUered proton groups of approximately equal intensity leading to these states 1); any proton group leading to a 2.0 or 2.2 MeV level is at least a factor 25 weaker than the others, which can hardly he expected.

2.5.4. The levels at 4.17 and 4.06 MeV.

The average energy values of the two levels excited at about 4 MeV as obtained from the measurements are 4.20 ± 0.03 and 4.03 ± 0.02 MeV res-pectively. They are interpreted as the outer two members of the known 4.06 - 4;11 - 4.17 MeV triplet.

2.5.5. The levels at 3.16,2.69 and 2.64 MeV.

The existence of the 3.16 Me V level in 35Cl, which was not certain from the investigations by End t e.a. 1) could be proved by our meas urements

at the two resonances at Ep = 1.057 and 1.214 MeV (sect. 2.4.9.and 2.4.10.; also ref. 7) and9 ). The measured excitation energy is 3.17 ± 0.03 MeV.

Transitions to and from the two levels at 2.64 and 2.69 MeV could not be separated in the gamma ray spectra, since the resolution of the scintil-lation counter was insufficient. Fortunately the analysis of the resonance at 1.214 MeV shows that the 3.16 MeV level decays to that at 2.64 MeV and the last one to the ground state only. The branching ratio of the 2.69 MeV one is somewhat uncertain since a 2.6 MeV gamma ray in a spectrum can partially originate from the 2.64 MeV level. However, when the 2.69 MeV

~

5.22 <1 <0.5 5.03 <1 <0.5 4.174 <2 <5 4.058'1 2± 1 26± 6 3.163 <0.2 <3 3.006 <2 <3 2.695*) 5± 2 2± 1 2.645*) 5± 1 6± 2 1.762 7±2 4± 3 1.220 62± 4 16± 7 0.000 19± 2 46± 7 <5 7± 2 <1.5 <0.5 <2 12± 2 2±1 -<5 <1.5 <1.5 2.0±0.5 <2 <8 <3 <23 -12±3 <2 <0.5 <0.3 <2 <15 <1 <60 <25 -6± 3 7± 2 <0.5 1.0±0.3 5± 2 < 6 <5 <10 <10 <20 -<0.6 <2 <0.5 <0.2 <I 59±7 93±3 <IS <50 <20 <6 -a) <2 <1.5 <1 <1 <11 <5 <25 <20 d) <3 <10 -<4 8± 3 <2*) 4.0±0.6 <2 < 5 <1 <15 <25 <15 <6 < 2 -<6 b) 5±2 c) <2 22± 5 3±2 <15 <25 <15 <6 16±3 e) 5± 2 8± 3 1.0±0;5 1.0±0.3 10± 3 7± 3 2±1 <25 <20 <20 18±5 < 1 <20 69±5 2± 1 2±1 20± 5 67± 7 <15 <0.5 <40 <22 38±1O 64±5 < 1 66± 8 8± 2 68±6 92±3 72± 5 18± 4 <10 <0.2 100 100 62±1O 18±4 84±3 34± 8

Table 2.XII. Branching ratios of levels in 35 CI •

-+) The intensity of a possible transition to the level at 4.113 MeV is included in that to the 4.058 MeV level.

*) See discussion section 2.5.5.

No branchings are given of the transitions marked a) • • • • e):

a) If present, masked by astrong 3.0 MeV gamma ray originating from an other transition.

bl cl See discussion section 2.5.5.

d) If present, masked by strong 3.0 and 1.2 MeV peaks in the spectrum. e) Very 1ow-energy transitlons not investigated.

-<20 . < 5 100 -<20 100 -100 W lD

level shows a transition to the

J =

1/2 level. at 1. 22 Me V it is rath.er pro-bable that a transition to the

J

= 3/2 ground state is also present. Since the maximum observed ratio of the intensities of the 1.48 and 2.69 MeV gamma rays in two resonances (2.4.5.and 2.4.7.) is the same, this value i3 ten-tatively used as the branching ratio. The measured average excitation energies of these levels are 2.69

±

0.02 and 2.65

±

0.03 MeV.

2.5.6. The levels at 1.76 and 1.22 MeV.

Fo! both levels only a ground state decay was observed. A transition between the 1.76 and 1.22 MeV level was not detected. The excitation energies of these levels agree with our values obtained from the gamma ray

spectra.

2.6. Discussion.

A comparison of our work with that of Antuv'jev e.a.6 ) shows that the

gamma ray yieldcurves are very similor; their intensities of the decay of the resonance levels at 7.189, 7.230 and 7.267 MeV to the ground state and first excited state agree reasonably weU with our values. At the resonance at E

=

1.214 MeV we obtained cuy

=

2.0 eV which value dlffers strongly from PAntuv'jev's 7)

cuy

= 0.4 eV*); consequently our values at the other

resonances differ from those given by Endt and Van der Leun12 ) which are computed from ref. I).

The gamma ray decay of several levels can be compared with the results of 0 ub 0 i s which were published recentlylO). He investigated the

34S(p,y)35Cl reaction in the reqion bet ween 0.8 and 1.4 MeV proton energy.ln addition to the resonances at Ep = 0.848, 0.889, 0.928, 1.020 and 1.214

MeV also studied in this chapter, he measured at the resonances at Ep =

1.267, 1.341 and 1.345 MeV which will be mentioned in chapter 3.

GeneraUy his results on the gamma ray decay of the resonance levels agree reasonably with our work. More interesting, also from a theoretical

point of view, is the gamma ray decay of the lower excited states, where some striking differences are present. Therefore, we compare below 0 u b 0 is'

results for the lower levels with ours; in fig. 2.22 the two decay schemes

are given.

*)It may be pointed out that the values given by A nt u v'j e v e.a. 7 ) (ZnS

1 2 -1

target with natura isotopic constitution, stopping power 150 keV cm mg at

1.214 MeV, thick target yield 2.56 x 10-9 decay per proton) inserted in the for-mula described in seet. 2.3, yield "''Y

=

400 eV instead of the value 0.4 eV given by the authors.

The previously unknown level at 5.03 MeV was also found by D ubois; he did not report the level at 5.22 MeV, which we discovered in the decay of the resonance at E _p

=

0.881 MeV. The desintegration of the level at

-Jg

~~;: 1~o g g ....

,

.

-r

t

é

b c>

+

t

t

5.22 5.03 4.17 4.06 3 16 z. 3.01 ·2.b' 1.76 1.22 0

.!.!. ..

M'" ex>

I

t

J

~ c>

1 T 1

...!.:.

"'M !!!~i

t

, 1

C> C> C> C>

-

-Fig.2.22. Gamma ray decay of the lower levels of 35 C1 according to Dubois (left) and to our work (right).

4.17 MeVas quoted by Dub oi s does not quite agree with our work. He finds a transition to the level at 1.76 MeV with an intensity of 11%; in our

work the evidence for such a transition was not convincing and only an

upper limit of 200;6 of the total decay was set for the intensity of the

transi-tion.

But according to Dub 0 i s, the transition from the level at 4.17 MeV

to the ground state is a factor of four weaker than that to the first excited state, in contradiction with our results. An examination of Dub ois' spectra

and his elucidations leads to thefollowingcomments.Insect.4.9.of his paper

Dub 0 is compares the single spectrum (his fig. 14) with a sum coincidence

spectrum (fig. 15). He remarks that the peak at 3.51 MeV in the latter spec-trum is relatively low which he ascribes to the low percentage of ground state transitions from the level at 4.17 Me V. We note however that also the peak at 4.17 MeV is relatively low in fig. 15, which cannot he explained in a similar way. Consequently, the analysis of the spectrum

i.n

fig. 14 does not agree with the results of fig. 15 .This may be caused by the pfesence of a transition to the level at 4.06 MeV, which would account for a part of the intensity of the 3.51 MeV peak; it is also possible that the height of the Peok at 4.67 MeV is underestimated.

In fig. 16b Dub ois shows a coincidence spectrum obtained with a

coincidence channel set at the peak at 3.51 MeV; this channel contains also a part of the Compton distribution of the 4.67 MeV gamma- line. Coincident with the 3.51 MeV gamma rays are those of 4.l7MeV and those of 2.95 MeV; coincident with the 4.67 MeV gamma rays are only those of 3.01 MeV.