The excited levels of 79Br

i

.E.I: [ 1.E.4

[

Nuclear Physics 79 (1966) 145--158; (~) North-Holland Publishing Co., Amsterdam Not to be reproduced by photoprlnt or microfilm w i t h o u t written permission from the publisher

T H E E X C I T E D L E V E L S O F 79Br H. LANGHOFF, L. FREVERT, W. SCHOTT and A. FLAMMERSFELD Zweites Physikalisehes Institut der Universitiit Gb'ttingen Received 29 October 1965 Abstract: The resonance fluorescence technique has been applied to study the excited levels of 7~Br. Using gaseous sources of 7SKrfour well-known levels at 834 keV, 606 keV, 398 keV and 307 keV have been excited. A fifth level excited at 526 keV was newly established. The cross sections for resonance scattering yield the following mean lifetimes: 834 keV level z = (0.17:k0.05) psec, 606 keV level T = 3.7=k0.4 psee, 526 keV level T = 3.5~1.0 psec, 398 keV level ~ ~ 45=/=15 psec, and 307 keV level T = 18 :k 10 psec. The results have been confirmed by a self-absorption experiment. The angular distribution of the resonantly scattered radiation gives further information about the spins of the excited levels. The decay scheme of 7~Kr was investigated using a Li,drifted germanium counter and a two-dimensionN coincidence spectrometer. In addition to the results of former decay scheme studies y-transitions of 526 keV, 617 keV, 811 keV, 858 keV and very likely 1165 keV have been identified. Coincidence experiments revealed that 79Kr decays by emission of positons to the ground state (7.6 %), to the 261 keV level (0.13 %), and to the 398 keV level (0.015 %). NUCLEAR REACTIONS 79Br(7, 7% E~ : 307, 398, 526, 606, 834 keV; measured a(E; 0). Deduced levels, J, T~. Natural target. RADIOACTIVITY 7~Kr [from 7SBr(d, 2n)]; measured I~, 77-coin. Deduced aK, d(E2/M1 )

1. Introduction Several excited levels o f 79Br are p o p u l a t e d in the d e c a y o f 79Kr. The r a t h e r c o m p l i c a t e d d e c a y scheme has been investigated very intensely b y Thulin et al. l, 2) a n d B o n a c a l z a 3) using /~-spectrometers a n d N a I coincidence spectrometers. A d e c a y scheme b a s e d o n 15 o b s e r v e d ?-transitions was established b y Bonacalza. The scheme has been r e p r o d u c e d in fig. 1. T h e o b t a i n e d c o n v e r s i o n coefficients indicate t h a t these 7-transitions have M1 a n d / o r E2 character. Investigations o f the level scheme o f 79Br b y C o u l o m b excitation 4 - 6 ) have also been r e p o r t e d . Levels at 220, 260, 310, 400, 530, 610 a n d 765 keV have been excited. Some o f these levels exhibited large p r o b a b i l i t i e s for an E2 t r a n s i t i o n to the g r o u n d state. I n f o r m a t i o n a b o u t the 79Br nucleus can also be o b t a i n e d b y r e s o n a n c e fluorescence. B o o t h et al. 7) tried to excite levels o f 79Br b y b t c m s s t r a h l u n g ; however, no r e s o n a n c e fluorescence was observed. This article will describe r e s o n a n c e fluorescence experiments which were p e r f o r m e d using gaseous 79Kr sources as exciting r a d i a t i o n . The m e a s u r e m e n t s yielded n o t only i n f o r m a t i o n a b o u t the lifetimes o f the excited levels in 79Br b u t also o t h e r spee145

K. LANGItOFI~et al.

146

troscopic data. In addition, an investigation of the decay scheme of 79Kr with the aid of a Li-drifted germanium detector and a two-dimensional coincidence spectrometer will be reported.

1336 Z ~ - (I3%EC) logft=~Z

; s/z-if/s4 /936

z/e- (s, ;. )

lO;Z

93B IL2B

/l[sl

(13%EC] Iogft =$.Z

I i 836 526

,-)

096 " /

J t 606 s~g sz6 ; ~o

(Sis_Z/H- -)

- ] - F

606 , / -

.....

L

-

,

t

,

-

T

-

,

'1~ 101 /3,6 e!; ; 31Z-J

s/z-

s/e

I

525

217 261 307"398

',

/

(ll%fg) logftoS.7

es%[cuogrt.eo

,~ (<0.16V ~loqft 390 /,,/:;(o.~oM.;

>03 387 ~±(-O.25%E£]to#lt.-eg79 ~EC)Iogtt o6.3

o / z (ze %n÷,e~%Ec/Iogtt°SS

35eff Fig. 1. Decay scheme of 7~Kr according to Bonacalza 8).

2. Resonance Fluorescence Experiments In the decay of a 79Kr atom to an excited level of 79Br the nucleus {akes over a recoil from the emitted neutrino and subsequently the frequency of the following v-quantum will be changed due to the Doppler effect. Therefore, the energy losses the 7-quantum experiences during emission and absorption may be compensated and resonance absorption becomes possible. The energy available in the decay of 79Kr amounts to 3) 1635 +_5 k e y and is sufficient to re-establish the resonance condition for levels up to 810 keV. A partial overlap of emission and absorption line caused by thermal broadening shifts this limit to somewhat higher energies. Hence in the present experiment resonance scattering up to the 834 keV radiation may be expected. In general, the use of gaseous sources for resonance fluorescence investigations implies assumptions about the influence of the chemical binding on the shape of the primary 7,spectrum. In the present case, however, no uncertainties arise since krypton as a noble gas is monoatomic. The nucleus 79Kr was produced by the 79Br(d, 2n)79Kr reaction with 28 MeV deuterons. KBr in copper capsules was irradiated using the internal beam of the G6ttingen cyclotron. For each source a total of 25/~Ah was dissipated in the target. The capsules were transferred into a quartz oven, some inactive Kr was added and

'ZgBr E X C I T E D

i

source

abs°rbe-L~.~

lead shield

147

LEVELS

kl

tI

sca/__tterer

Na]- crystal

1

scattering angle h

•, photomu/tiplier Fig. 2. Experimental arrangement for the resonance scattering measurements. The broken lines indicate the position of the absorber in the self-absorption experiment.

6

~u

I I

398

channel

006

A •

!

834

•

Zoo ToO\:.EI

o .

r

300

,.~G

500

60g

700

800 E~ [keV]

900

Fig. 3. Spectrum o f t h e y-radiation resonantly scattered by ~gBr. Curve 1 was obtained with a gaseous ~gKr source, curve 2 with a cooled 79Kr source adsorbed on charcoal.The background, determined with the comparison scatterer, has already been subtracted. The average scattering angle was 123 °. The difference in the two spectra is only explainable by the assumption that the 5 T-lines at 834 keV, 606 keV, 526 keV, 398 keV and 307 keV are caused by resonance fluorescence.

148

H. LANGHOFF et al.

the KBr was heated up to 600 °. After purification in a calcium oven the krypton was distilled into a glass ampoule. The obtained sources were very pure and had a strength of about 5 mCur. The gas pressure in the ampoule amounted to several mm Hg. The time required to slow down the recoiling nuclei in these sources is large compared with the lifetimes of the decaying levels. The experimental arrangement for the resonance fluorescence experiment is shown in fig. 2. The scatterers were ring-shaped lucite containers one filled with NaBr and the other - for comparison - with a mixture of Zn and SrCO3. The two scatterers were matched to scatter ~-quanta of 600 keV equally. The scattered radiation was registered using 5 cm 2~ x 5 cm or 7.5 cm N x 7.5 cm NaI-detectors and then recorded in a 256 channel analyser. In order to avoid pile-up effects the 7-quanta of low energy were suppressed by shielding the detector with 1-2 g/cm 2 of lead. Fig. 3, curve 1, shows the spectrum of the radiation scattered by NaBr. The background, determined with the comparison scatterer, has already been subtracted. The 5 7-lines at 830, 610, 530, 400 and 310 keV are only explainable by resonance fluorescence. To confirm this assumption and to check the matching of the scatterers, the 79Kr source was adsorbed on charcoal and cooled to the temperature of liquid nitrogen. The experiment was repeated under these conditions and yielded fig. 3, curve 2. As expected, no appreciable resonance effect appeared. Therefore the lifetimes of the levels excited resonantly are larger than the time required to slow down the recoiling nuclei in solid krypton or charcoal. Curve 2 reveals the mismatching of the scatterers at low energies. For the analysis of the scattering results this effect has been taken into account. The 7-lines at 830 keV and 610 keV in the scattered radiation are due to direct excitation of the 834 keV and the 606 keV levels. The 606 keV level decays partly by 77-cascades through the 398 keV and the 307 keV levels. However, the branching ratios are small, therefore the observed lines at 400 k e y and 310 keV cannot be caused by the excitation of the 606 keV level. This means that the levels at 398 k e y and 307 k e y must have been excited directly. An indication of a 526 keV radiation in the decay of 79Kr has been reported by Thulin et al. 1). From the investigation of the decay scheme (sect. 3) it may be concluded that the decay of the 834 keV level by emission of a 530 keV radiation is rather rare. Therefore, the 530 keV radiation in the resonance spectrum originates predominantly by the excitation of a 526 k e g level. Measurements 6) of Coulomb excitation in 79Br also yielded a strong 530 keV radiation indicating the existence of a level at 530 keV with a large E2 transition probability to the ground state. The resonance fluorescence experiments demonstrated that this level is also populated in the decay of 7~Kr. In order to investigate the angular distribution of the resonantly scattered radiation, spectra have been taken at the four scattering angles of 102 °, 123 °, 138 ° and 153 °. According to the assignments of Bonacalza 3) only the spins ½ and ~ are possible for the 834 keV, the 606 keV and the 398 keV levels. Therefore the angular distribu-

79Br EXCITED LEVELS

149

tions of the corresponding transitions were analysed in terms of the function W(~9) = I+A2Pa (P2 = 2nd Legendre polynomial). After correction for the finite angular resolution of the apparatus A2 = 0.075-t-0.04 for the 606 keV transition has been obtained. The angular distributions of the 834 keV and the 398 keV radiations exhibited isotropy within the uncertainties for A 2 of +__0.10and -I-0.13, respectively. These results are compatible with spin assignments proposed by Bonacalza 3); however, more rigorous statements about the spins are not possible because of the experimental errors. The angular distribution of the 526 keV radiation indicates a strong A, term. Of 7 spin assignment ~ to the 526 keV level is favoured by the two possible spins ~ and ~this result. For the evaluation of the absolute cross sections, the source strength has to be determined. After having decayed sufficiently, the source was placed at the position of the scatterer, and a spectrum was taken. By comparing this spectrum with that of the resonantly scattered y-quanta the average cross sections for resonance scattering become nearly independent of the size and the efficiency of the detector. Some of the y-lines in the primary radiation are not resolved completely by the NaI-detector. In these cases the corresponding intensity ratios determined with the germanium counter (sect. 3) have been used. A further correction on the intensities of the 526 keV, the 398 keV, and the 307 keV lines in the resonance spectrum becomes necessary, because these lines originate partly by the excitation of the upper levels. The resulting average cross sections for the individual levels of 79Br are summarized in table 1, column 2. The investigation of the decay scheme could not clarify whether the 526 keV level is populated only by the decay of the 1336 keV level (a) or in addition by the decay of the 834 keV level (b). From that, two cross sections lesult for the 526 keV level. As long as self absorption in the scatterer is negligible, the average cross section for resonance scattering asc per 79Br.atom is given by 8) al

~(A1,A2) =

2

NfE)exp ~ F~h2cz 2 #oF4n E;, (AI+A 2) 3-~o

(E--E,) 2

dE.

(1)

A~+A~

In this formula gl and go are the statistical factors of excited and ground state; E~ is the energy of the absorption line, Fo is the partial width for the ground state 7transition and F the total width; A1 and A2 are the Doppler widths of the emission and absorption lines, respectively, caused by thermal motion of the atoms in the source and the scatterer. N(E) is the energy distribution of the emission line, which results from the neutrino recoil neglecting the thermal motion of the 79Kr atoms. For the evaluation of g~F~/goF from the experimental cross sections assumptions about N(E) have to be made. In general, when using the gaseous source technique, N(E) is a slowly varying function in the vicinity of E~. In this case the knowledge of N(E~) is sufficient. However, the condition is not fulfilled for the resonance scattering from the 834 keV level. In fig. 4 the energy distribution of the emission line is described

150

H. LANGFIOFF e t

a[.

b y the rectangle, whereas the n a r r o w line represents the thermally b r o a d e n e d a b s o r p t i o n line which is shifted from the center of the emission line by E 2 / M c 2 ( M = mass o f a 79Br atom). The integral (1) depends sensitively o n the n e u t r i n o energy. I n the calculation of the cross section for resonance scattering the average of varicus experi-

kiiE) I%1 .0

ernlsSlon /l))e

line / - ahsorptto;,, -

d

_

I

-20

-I5

~rid

L

-so

-5

o

c-E ,ieVl

Fig. 4. Energy distribution of the 834 keV radiation. The emission line has the shape of a rectangle due to the broadening by the neutrino recoil. The absorption line is shifted by an energy of Er2/Mc ~ from the centre of the emission line. N(E) has been normalized so that S+~ N(E) dE = 1. TABLE 1 Results of resonance scattering and absorption experiments E~ (keV) 834 606 526 a b 398 307

%° (mb) 70 4-10 604-4 330±90 1154-30 234-4 2404-110

go 1, (meV) 1.25 4-0.28 (0.954-0.07)' 10-1 (2.8 ±0.7) • 10-1 (1.0 4-0.3) • 10-1 (1.234-0.22)" 10-2 (5.4 ±2.5) " 10-2

1,o __ 1,

e

go

(meV)

0.80 0.73

(18.1±6.7) ~ (6.0 4-1.7) ~

1.6 4-0.6 0.154-0.04

1.00

(20.74-8.8) ~

0.284-0.12

0.92 0.99

The average cross sections for scattering %e yield the values for g11,o2/goF given in column 3. e represents the self absorption in the NaBr absorber and allows the calculation of glFo/go. I"o/I' have been obtained from the measurements described in sect. 3. m e n t a l results for the t r a n s i t i o n energy (sect. 3), QEC = 1630 keV, has been used. T h e influence of X-rays a n d A u g e r electrons emitted after a K - c a p t u r e of 79Kr was neglected. However, a small correction results f r o m L-capture, which occurs in 9 o f the 79Kr decays. F o r the e v a l u a t i o n of the r e m a i n i n g m e a s u r e m e n t s the knowledge of N(E~) is sufficient. N(E~) d e p e n d s o n the t r a n s i t i o n energy a n d o n the way the c o r r e s p o n d i n g

79Br EXCITEDLEVELS

151

level is populated in the decay of 79Kr. This information was supplied by the investigations of the decay scheme reported in sect. 3. The resulting values for 91F2/9oF are listed in table 1, column 3. A small correction for the resonant self-absorption in the scatterer has been included. Since ~c for the 834 keV radiation is strongly dependent on N(E), a self-absorption experiment has been performed. The cross section for self-absorption O'abs is independent of N(E) as long as N(E) is a slowly varying function. In the present case this condition is not fulfilled, however, O-ab~is less dependent than o-soon the assumptions about N(E). The arrangement used in the scattering experiment was supplemented by a NaBr absorber (broken lines in fig. 2), which could be exchanged for a comparison absorber of Zn and SrCO 3 with the same electronic absorption. In order to match the absorbers a detector was placed in the position of the scatterer and the thickness of the Zn absorber was varied until the transmission of the T-radiation in the energy range of interest was the same as that of the NaBr absorber. In the actual self-absorption experiment the number of counts C,s (a = absorber, s = scatterer) has been determined for the four possible absorber-scatterer combinations. The self-absorption e is obtained from 8

(CZnBr- Cznzn) -- (CBrBr- CBrZn) CznBr-

Cznzn

The results for the 834 keV, 606 keV and 526 keV radiations are compiled in table 1, column 5. For the remaining two lines the experimental accuracy was not sufficient. Theoretically z is given in a first order approximation by = dN79Br glFoh2c2as¢(A1,A2/~/2) go4(2rc)~A2asc(A1, A2) ' where d is the average thickness of the absorber and N79Br the number of 79Br atoms per cm 3. The comparison of the theoretical and experimental values for e yields 91Fo/9o. The consideration of the second order approximation for e leads to a small correction on these values. Fig. 5 shows the result of the absorption experiment for the 834 keV radiation as function of the transition energy. For comparison, the result of the scattering experiment divided by the ratio Fo/F = 0.80 has been added. The value Fo/F was obtained from the relative T-intensities reported in sect. 3. The intersection point of the two curves yields the most probable value of the transition energy QEc = 1630_ 30 keV, which is in good agreement with the values obtained by other methods (sect. 3). The results of the absorption measurement are summarized in table 1, column 6. The value o f the 606 keV radiation is in good agreement with the result of the scattering experiment, if the branching ratio Fo/F = 0.73 is taken into account. The value of (g l/"0/9"0)526 key is only compatible with one of the two possible results of the scatter-

H. LANGHOFF e t al.

152

ing experiment (a). Both measurements combined allow the conclusion that the 526 keV level is populated only by the decay of the 1336 keV level (sect. 3).

-~F #z i1o4e

s ca tterm~ exper/ment

/

3 absorption exper/ment

I I I

0

1600

1700 OEC[/
Fig. 5. gll~o/gofor the 834 keY transition determined by the scattering and by the absorption experiments is plotted as a function of the transition energy QEC.

3. Investigations of the 79Kr Decay Scheme In order to analyse the resonance fluorescence measurements a detailed knowledge of the 79Kr decay scheme is necessary: Particular attention was focussed on the observation of the 526 keV radiation in the decay of 79Kr, and on the question of where a 526 keV transition can be located in the decay scheme. Firstly, the 7-spectrum was investigated by means of a 2 m m thick Li-drifted germanium counter. A spectrum recorded with a 1024-channel analyser is reproduced in fig. 6. All 7-transitions observed in former measurements have been confirmed. Because of the good resolution of the germanium counter it was possible to separate the double lines at 210 keV, 310 keV, and 400 keV as well as the quadruple line at 1000 keV. Therefore, more accurate values for the intensities of these transitions have been obtained. The missing 526 keV transition was observed in the edge of the line due to annihilation radiation. Further 7-transitions u n k n o w n u p to the present appeared at 811 keV, 858 keV and very likely at 617 keV and 1165 keV. The assignment of these lines to 79Kr is based on the decay time which was observed over a period of 5 half-lives. Coincidence measurements have been performed by means of two 7.5 cm ~ × 7.5 cm N a I detectors. The coincidences have been recorded two-dimensionally with a

79Br EXCITED LEVELS

153

16 x 64 channel analyser. In general, the results reported by Bonacalza 3) have been confirmed. No coincidences appeared between the 44 keV and the 261 keV transitions. The strong resonance scattering from the 307 keV level indicates a very short lifetime for this level (table 4). Therefore a competing 44 keV transition to the 261 keV level is very unlikely. 106

39B

107~

6O6

1

7336

(7765)

]j

/ ~xzo-eX

/Oh -

6/7

/

83h

/03 20g

4aO

6gO

BOO

lOOg

7200

f,~ [keg]

14ag

Fig. 6. y-spectrum o f r~Kr observed with a 2 m m Li-drifted. germanium detector.

Coincidences between the 44 keV and the 217 keV radiation have been observed directly. A 84 keV radiation has not been detected. A so far unknown coincidence between a 617 keV and a 217 keV transition may easily be identified as an additional decay mode of the 834 keV level through the 217 keV level. In addition, coincidences between 811 keV and 526 keV have been found. These two ?-transitions, also observed in the single ?-spectrum, arise from the decay of the 1336 keV level through the 526 keV level. The observation of coincidences between the 526 keV radiation and the other 7-transitions is rather difficult because of the intense annihilation radiation of the positons. For that reason a 10 cm ~ x 10 cm NaI detector placed opposite to the detector recording the 526 keV radiation supplied a veto pulse when an annihilation

H. LANGHOFF et al.

154

of a positon occurred. Coincidences between these two counters and a third detector arranged at 90 ° to them were recorded in the two-dimensional analyser. The spectrum showed apart from the 511 keV-261 keV coincidence, strongly suppressed by the anticoincidence detector, a further coincidence line at 530 keV-310 keV. This 7YTABLE 2 Comparison of the experimental K / f i + ratios with theoretical values ~) calculated for the transition energy QEc = 1635 keV quoted, by Bonacalza 3) and QEc = 1610 keV quoted by Thulin 1).

Excited level

N/~+

(K/fl+)

(K/fi+)

Elevel


QEC = 1635 keV ~)

0 261 398

7.6 % 0.13 ~ 0.015 ~

a) Ref. a).

7.6 e) 57zE10 430±100

b) Ref. 1).

e) Ref.

QEc = 1620 keV b)

7.0 50 360

7.0 55 500

1o).

Nfl + represents the number of positons per ~gKr decay.

cascade presumably originates from the decay of the 834 keV level. The measurements, however, do not allow to distinguish between an intermediate level at 526 keV or 307 keV. The resonance scattering and the self absorption experiment are in agree-

s/e-/~/e.... -/I

-

38Kr~ (3~.92h) / O.9%fC / 1336 /

-

•

(~5%EC)

/

1165 /

....

o76

~/e-( s,~)

ION

936

, 1119 1020

I

1165 (

I t I I gll I 858

// 834 /

I I

/ II.0%EE T.=(3.7eO~),104~ec 606/~,z o(3.s-.1.oj.lOo-12sec

617 526

/

I

312-(~/2)

Sl;-

I

' I 219'

h83h

;89

3/2-~d-

J

co?

35

i J,I,, | 'C' I

15% Eg Z=(1.7*-O¢),lO43sec

526 ~ / 12015%fl'*Z3~ECE=lZ5-'ZS]'lO-llsec ,

;co

/,, -c,as;l.oj.lo-~ ,,,alS%y. 8.eo/occ

13718,307 ~/ I1261,"

I

1

!

806

|

at

217 / ~ 6 % p 0

÷. 63./,% fC

/

Fig. 7. Decay scheme of 79Kr supplemented by the results of the present investigation.

merit only, if a transition from the 1336 keV level through the 526 keV level is assumed. Hence, the 834 key level must decay through the 307 keV level. In a further measurement the two facing detectors were used in coincidence in order to record the annihilation radiation of the positons from the decay of 79Kr. The third detector observed coincidences between the annihilation radiation and the

7aBr EXCITED LEVELS

155

r e m a i n i n g ~-quanta. This experiment d e m o n s t r a t e d that the 261 keV level as well as the 398 keV level are excited n o t only by E C a n d y-decay b u t also by p o s i t o n decay. "Ihe c o m p a r i s o n of the n u m b e r of triple coincidences with the total n u m b e r of the recorded a n n i h i l a t i o n q u a n t a yields the p o s i t o n b r a n c h i n g ratios, while the comparis o n with the y - q u a n t a of 398 keV a n d 261 keV registered by the third detector allows the evaluation of the K / f i + ratios for the c o r r e s p o n d i n g transitions. As a correction it was t a k e n into a c c o u n t that these levels are populated, too, by y-decay. I n table 2 the e x p e r i m e n t a l K / f i + ratios are c o m p a r e d with values calculated by Zweifel 9). The theoretical values have been o b t a i n e d using Bonacalza's a) results for the t r a n s i t i o n e n e r g y ( c o l u m n 4) a n d also the values o b t a i n e d by T h u l i n 1) ( c o l u m n 5). The exp e r i m e n t a l K / f i + ratios are in good agreement with the theoretical values a n d confirm these t r a n s i t i o n energies. The results of the investigation of the decay scheme are compiled in table 3 a n d fig. 7. C o l u m n 3 contains the relative y-intensities resulting from the direct a n d the coincidence m e a s u r e m e n t s n o r m a l i z e d with respect to the 261 keV radiation. The TABLE 3 Energies of the ~,-quanta E~, relative intensites of 7-quanta and conversion electrons, I v and le, and the K-conversion coefficients c~K in the decay ~gKr -~ ~gBr. agleve1 (keV)

Ee (keV)

Is

Ie" 10~

~K" 10~ exp

C~M1" 10~ theory

1336

1336 1119 1074 1028 936 811 (1165) (858) 834 617 526 606 389 345 299 208 526 398 181 137 307 261 44 217

3.3 2.7 0.6 1.3 1.1 1.5 (0.8) (0.7) 12.6 0.6 2.5 91 15 0.9 12 6.2 1.5 85 0.7 5.9 19 100 1.6 17.7

9.2 11 3.1 4.6 3.8

2.8 4.1 5.2 3.5 3.5

2.1 3.0 3.3 3.5 4.3

2.2 3.2 3.5 3.8 4.8

MI+E2 MI+E2 MI+E2 MI+E2 M1 -{-E2

62

4.9

5.5

6.3

M1 +E2

46 1100 460 31 770 920

a) 12 31 35 64 150 a) 28 210 390 74 79 9600 150

15 10.5 31 40 56 140 15 29.5 220 460 53 78 6000 130

(1165) 834

606

526±2 398

307 26l 217

2400 150 2300 1400 7900 15400 2600

C~E2" 10~ multipole theory order

22.4 14.3 60 84 135 490 22.4 56 860 2500 125 218 80000 440

MI-bE2 M1 M1 M1 -]-E2 MI M1 M1 MI M1 -b 132 M1 MI -k (E2) MI

~a) a conversion coefficient cannot be evaluated, since the two transitions of 526 keV have not been :separated.

156

H. LANGHOFFet al.

values are in good agreement with former investigations 1-3). In column 4 the relative intensities of the K-conversion electrons reported by Thulin et al. 1) and Bonacalza 3) are reproduced. These intensities are normalized so that the K-conversion coefficients for the 606 keV and the 398 keV transitions are in accordance with the corresponding theoretical values for M1 transitions. The predominant M1 character of these transitions has been demonstrated by combining the results of resonance fluorescence and Coulomb excitation measurements (table 4). The K-conversion coefficients of all y-transitions can now be given more accurately (column 5). A c o m parison with the theoretical values of Sliv and Band 11) reveals that all observed transitions have M1 and E2 character. 4. Results and Discussion The most likely spin assignments based on the experiments of Bonacalza a) have: been accepted for a further discussion of the present results. Using the experimentat values 91F2o/#o F and the branching ratios F J F (F i corresponds to the partial width for the ith decay mode of the level) the partial y-transition probabilities have beert calculated (table 4, column 5).

Fig. 8. Angular distribution of the 606 keV radiation. The coefficient AS expected for a ,~-.~-z cascade is plotted versus the mixing ratio 6 (solid curve). The horizontal hatched strip represents theresult of the angular distribution measurement, while the vertical hatched strip indicates the value for lal obtained from the combination of Coulomb excitation and resonance fluorescence measurements. The results of the Coulomb excitation experiments 6) provide the y-transition probabilities for the E2 part of the transition (table 4, column 6). Combining the two experiments the mixing ratios lal = ~/T(E2)/T(M1) are obtained independent of assumptions about the spins of the levels involved in the transition. Since for the 526 keV radiation as well as for the 307 keV radiation lal < 1, the radiation character must be M1 + E2; therefore, from the spin assignments suggested

157

79Br EXCITED LEVELS

by Bonacalza 3) only spin ~ is compatible with the present results. This conclusion is confirmed by the observed anisotropy of the resonantly scattered 526 keV radiation (sect. 2). The spin ~ of the 307 keV level implies that the 834 keV level with spin ½ decays into this level by an E2 transition. In fig. 8, A2 in the angular distribution of the resonantly scattered 606 keV radiation is plotted versus the mixing ratio 6. The comparison of the theoretical curve with the result of the angular distribution measurement (horizontal hatched strip) and the value of 161 deduced from the Coulomb excitation and resonance fluorescence experiments (vertical hatched strips) yields - 0 . 5 0 < 6 < - 0 . 2 5 . The accuracy of the measurement, however, was not sufficient to exclude spin ½ completely. TABLE 4

?-transition probabilities in ~gBr Spins

]'i

TM 1 + E2

F

[101° sec-1]

Elevel

E~,

(keV)

(keV)

834

834 617 526

(½)-+~ ~ ~

606

606 389 345 299 208

~-+~33 ) ~ ~ ~

0.73 0.12 0.00072 0.077 0.040

20 3.2 0.2 2.1 1.1

2.1

0.32

21 35 400 25 15

10

526

526

~---~

1.00

28

3.1

0.33

10

30

398

398 181 137

~-+~ ) )

0.92 0.008 0.065

2.0 0.018 0.14

0.05

0.16

61 610 34

2

307

307

~---~

0.99

5.6

0.05

0.10

10

7

0.80 0.04 0.16

TE2

[O[

[101°sec-1]

485 20 100

TM 1

Tsp

T~,p

TE2

2 (90) (1000)

TMI+E2 represents the partial transition probability determined in the resonance fluorescence investigation. The E2 transition probabilities TE2 obtained from Coulomb excitation experiments yield together with TM1+E2 the mixing ratios [6[. The results are compared with predictions of the single-particle model in column 8 and 9.

In table 4, columns 8 and 9, the experimental transition probabilities are compared with those calculated from the Weisskopf formula 22). It was assumed that according to the obtained conversion coefficients the multipole orders of the transitions are predominantly M1. According to the shell model the proton in the ground state of 79Br is in a p~ state. As excited levels the p~_, f~, and f} states may be expected. M1 transitions between f- and p-levels are/-forbidden. Since in 79Br, having 35 protons and 44 neutrons, the shells are half filled, the single,particle states have strong phonon admixtures, so that the/-selection rules are not exactly fulfilled ~3). Beside the 346 keV and the 181 keV

158

H. LANGHOFF e t al.

transitions the results reveal no considerable hindrance. The fast 834 keV M1 transit i o n might be interpreted as a p~-p~ spin flip transition. However, the large t r a n s i t i o n probabilities for the 617 keV a n d the 526 keV t r a n s i t i o n are n o t quite c o m p a t i b l e with spin a s s i g n m e n t ½ to the 834 keV level. W e t h a n k Dr. W. R i t t n e r for his assistance in the chemical p r e p a r a t i o n of the 79Kr sources. This work was supported by the B u n d e s m i n i s t e r i u m ftir wissenschaftliche F o r s c h u n g a n d the K u l t u s m i n i s t e r i u m des Landes Niedersachsen.

References 1) 2) 3) 4) 5) 6) 7) 8) 9) 10) 11) 12) 13)

S. Thulin, J. Moreau and H. Atterburg, Ark. Fys. 8 (1954) 229 S. Thulin, Ark. Fys. 9 (1955) 135 E. C. O. Bonacalza, Ark. Fys. 26 (1964) 141 E. A. Wolicki, L. N. Fagg and E. H. Geer, Phys. Rev. 105 (1957) 238 R. E. Holland, F. J. Lynch and E. N. Shipley, Bull. Am. Phys. Soc. 5 (1960) 424 R. L. Robinson, F. K. McCowan and P. H. Stelson, Bull. Am. Phys. Soc. 8 (1963) 60 E. C. Booth and H. H. Wilson, Bull. Am. Phys. Soc. 10 (1965) 119 F. R. Metzger in Progress in nuclear physics, ed. by O. R. Frisch (Pergamon Press Inc., New York, 1959) vol. 7 P. F. Zweifel, Phys. Rev. 107 (1957) 329 Nuclear Data Sheets, compiled by K. Way et al., (Printing and Publishing Office Nat. Acad. of Sci., Washington D.C., 1958 and 1962) L. A. Sliv and I. M. Band in K. Siegbahn, Alpha-, beta- and gamma-spectroscopy (NorthHolland Publ. Co., Amsterdam 1965) J. M. Blatt and V. F. Weisskopf, Theoretical nuclear physics (John Wiley & Sons, Inc., New York, 1952) chap. 12 R. A. Sorensen, Phys. Rev. 132 (1963) 2270