Gamma ray transitions in 57Fe

I I. E. 1: I 3. A

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Nuclear Physics 74 (1965) 177--183; ( ~ North-Holland Publishino Co., Amsterdam Not to

be reproduced by

photoprint or microfilm without written permission from tho publisher

GAMMA RAY TRANSITIONS IN STFe G. D. S P R O U S E t and S. S. H A N N A

Stanford University, Stanford, California tt Received 2 July 1965 Abstract: The gamma ray transitions among the low-lying levels of bTFe have been studied with a high resolution Ge(Li) detector. All possible transitions among the first five states are observed in the decay of 67Co. In the Coulomb excitation of bTFe by alpha particles all possible gamma ray transitions from the 367 keV level are obtained. Energies and intensities of the gamma rays were measured. L o g f t values were determined for the decay of 5¢Co. The observations strengthen previous assignments of ~- and [ - to the levels at 367 and 707 keV, respectively. E [

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RADIOACTIVITY 6'Co; measured E~, I v. Deduced log ft. b',ssCo; measured E~. NUCLEAR. REACTION 5:Fe(g, ct'y), E = 3.3 MeV; measured Er,I r. Enriched target.

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1. Introduction Although the first five levels of STFe have been well established 1), only a few of the radiative transitions between these levels have been observed and studied with NaI detectors. In particular, it has not been possible to resolve transitions from the higher levels to the closely spaced ground state and first excited state at 14.4 keV. The high resolution now available with lithium drifted germanium detectors makes it possible 2-4) to detect and study all the transitions among the five levels and to establish the decay scheme of these levels in 57Fe.

2. Decay of S~Co In the well-known decay of STCo the levels in 57Fe at 367 and 707 keV are very weakly populated 5). Only the three gamma rays resulting from the principal decay to the level at 136.4 keV have been studied extensively. These gamma rays E r = 136.4, 122.0 and 14.4 keV represent the cross-over and the cascade through the 14.4 keV level to the ground state. In the present investigation the gamma radiations from a 57Co source ( ~ 0.4 mCur) were observed with two Ge(Li) detectors, 1.5 and 3.0 mm deep. The detectors were maintained in vacuum at the temperature of liquid nitrogen. The pulses from t National Science Foundation Predoctoral Fellow. tt Work supported in part by the U.S. Army Research Of Sce, Durham, the National Science Foundation and the Office of Naval Research. 177

178

G.D.

SPROUSE AND S. S. HANNA

I

I

I

122 keV

03 I--

Fe57

Co57(EC)

Z,o

7-RAYS

0 C, LL 0 (/3 r',, Z

2.4

keY

(/3

~5 k-

136.4 keY

220

260 CHANNEL

300

Fig. 1. The 122.0 and 136.4 keV gamma rays from the decay oPTCo observed with a Ge(Li) detector 3 mm deep.

i

200

'

I

i

L" ".

\h,j.

(./3

I00 Z~ 0

o o3 Q z --, I0 0 -rF-

ENERGY (keY) 400 500 600 I -i -I-

300

/

"'.~,~//

7 230 keV \34o key

700

800

I

i

'

69;

keV

i J

x I~0

f/ ~

~" 511 keY

coS?(EC)Fe57 7' -RAYS

lO0

200

" ~

CHANNEL

300

Fe58 t 812 keY56 J

LI

ji' v 400

Fig. 2. Complete gamma ray spectrum above 130 keV from the decay of 6tCo observed with a Ge(Li) detector 3 nun deep. The letters indicate structure identified in table 1. The region of peaks formed by backscattered gamma rays is indicated by bs.

179

~-RAYTRANSI~ONS

a detector were amplified in a charge sensitive preamplifier and linear amplifier and stored in a multi-channel analyser. During the course of the investigation the resolution in different runs varied between 2.4 and 4 keV, depending on the detector I

I

I

5 4 0 keY

~ 40

"." :552 keV

o

:

(~

N.,°° , o0 367 keV

0 "11--

50

-

"A

.

Co 57 (EC) Fe 57

_i

"":" • o

y - RAYS

" eeee

__

1 t60

tSO CHANNEL

e

200-----

Fig. 3. G a m m a rays in the region of 350 keV in the decay o f 67Co observed with a Oe(Li) detector 3 m m deep.

270d

7•2-

keY 706.5 -

I t,-

O~

:566.8-

'~ ~_

°L

156.4

14.4 X, 0

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/ ~.

i

/5/2-

j

y'

.

Fe

57

l

I/2

Fig. 4. Decay scheme of *~Co. The g a m m a ray branching from each state is indicated.

180

O. D. SPROUS£ AND S. S. HANNA

used, the input tube of the preamplifier, the coupling between detector and preamplifier a n d t h e m i c r o p h o n i c s in t h e s y s t e m . T h e 57Fe l i n e s a t 136.4 a n d 122.0 k e V a r e s h o w n i n fig. 1 w i t h a b o u t 2.4 k e V r e s o l u t i o n . TABLE 1 Location of possible structure in the gamma ray spectrum of a ~vCo source Parent

Daughter

8°Co e°Co ~6Co 6sCo 67Co 67Co 5~Co ~eCo, 6aCo bTCo 6~Co 6~Co 67Co 6,Co Pb ~7Co Pb

e°Ni 6°Ni 56Fe bSFe STFe S~Fe ~¢Fe (fl+) b~Fe bTFe 6~Fe 57Fe s~Fe (K=) 6~Fe (K#)

Photopeak

Compton edge

Backscatter peak

One escape peak

Two escape peak

Sum peak

(keV)

(keV)

(keY)

(keV)

(keV)

(keV)

1332 1173 849 812 706.5 692.1 570.1 510.9 366.8 352.4 339.7 230.4 122.0 74.0 ] 122.0 | 86.3

1118 963 653 (a) 618 (b) 518.9(c) 505.5(d) 393.7(e) 340.6(t") 216.2 204.3(g) 193.9(h) 109.3

214 210 196 194 187.6 186.6 176.4 170.3 150.6 148.1 146.8 121.1

821 662

310(i) 151

196.00) 208.3(k)

The letters in parentheses indicate structure identified in fig. 2. TABLE 2 Energies and intensities of transitions in the decay of s~Co Level in 6'Fe (keV)

Ev measured (keV)

Er adopted (keV)

G a m m a ray intensity e) (per 10) 6'Co decays)

EC energy a) (keV)

log ft

706.5

706.8 5:0.4 692.1 +0.3 570.35:0.4 339.7Jc0.5 366.75:0.5 352.45:0.5 230.65:0.6

706.5 692.1 570.1 339.7 366.8 352.4 230.4 136.4 a) 122.0 a) 14.37 b)

6.7 159 14.4 4.8 0.7 3.7 0.5 10 740 85 310 8 370 e)

129

7.6

366.8

136.4 14.37 ") D c) a) e)

469

> I 1.4

700

6.4

822

Ref. ~s). Ref. xs). For internal conversion coefficient of 9.2 :k 0.4 (average of six recent determinations, refs. s, 14,x6)). For ~7Co: 67Fe mass difference of 836 keV, refs. x,~). For internal conversion coefficients of 0.142 and 0.0268 for the 136.4 and 122.0 keV transitions, respectively, ref. ~6).

~/-RAY TRANSITIONS

181

The spectrum of higher energy g a m m a rays was obtained with a suitable absorber placed between the source and the detector to eliminate pulse pileup from the intense low-energy g a m m a rays. A spectrum obtained with the 3 m m deep detector is shown in fig. 2. The structure observed in this spectrum can be identified with the aid of table 1. The region around 350 keV is shown on a linear scale in fig. 3. The energy of the 692 keV g a m m a ray was measured by calibrating it separately with the 834.8 keV radiation from a 54Mn source, the 661.6 keV radiation from a 137Cs source and the 510.9 keV radiation from a 22Na source ~). The result obtained was 692.1 +0.3 keV. The complete electronic system was calibrated with a pulser having an accuracy of better than 0.1%. This calibration was then converted to an energy calibration by means of the 57Fe lines at 692.1 and 136.4 keV. The energies of the lines in fig. 2, as obtained from this calibration, are listed in table 2. In addition, the energies of the lines in 56Fe and 5SFe (from 56Co and 5SCo impurities) were determined as 849-t- 1 and 812+ 1 keV, respectively. The g a m m a rays observed in the decay of 57Co can be fitted into the known level scheme of 57Fe as shown in fig. 4. The energies of the g a m m a rays which are consistent with this decay scheme are listed as adopted values in table 2. The only possible transition in this scheme not clearly seen in the g a m m a ray spectrum of the 57Co decay is the 230.4 keV line. In order to establish this transition, the g a m m a rays resulting from the Coulomb excitation of STFe were investigated. 3. R e a c t i o n

STFe(~, ~'3,)STFe

A foil of iron metal 1.9 mg/cm 2 thick and containing 93 % 5~Fe was bombarded with a beam of alpha particles of energy 3.3 MeV from a Van de Graaff accelerator. I

I

I0-

I ~

I

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I

:352 keY

_

Fe 57 ((2, cl I y ) Fe 57

,,

),'-- RAYS

% 2.30 keY o~p~. 5--

eql,o Iio 3 6 7 keV

ot

,

120

,

,'7 200

,

24(

CHANNEL

Fig. 5. Gamma rays observed in the Coulomb excitation of 87Fe by alpha particles. Note that the gamma ray at 340 keV is missing in this spectrum (cf. fig. 3).

182

G.

D. SPROUSE AND

S. S. H A N N A

Spectra of gamma rays were observed with the 3 mm deep Ge(Li) detector placed at angles of 0 °, 45 ° and 90 ° with respect to the alpha particle beam. None of the gamma rays associated with the 706.5 keV level (see fig. 4) was observed in these spectra. On the other hand, all the possible transitions from the 366.8 keV level were seen, as shown in the composite spectrum in fig. 5. The Coulomb excitation of the 366.8 keV level has been studied previously 6), but the gamma ray transitions were not resolved. The observation of gamma rays connected with the 366.8 keV level but not with the 706.5 keV level confirms the decay scheme in fig. 4.

4. Intensity Measurements The relative efficiency curve for the 3 mm deep detector was obtained by measuring the intensities of the 93.3, 215.2, 332.5 and 443.8 keV gamma rays from a ta°mHf source 7) and the 511 and 1274 keV gamma rays from a 22Na source t). The shape o f the curve derived from these measurements agreed satisfactorily with the efficiency measurements of Camp and Armantrout s). The intensities of gamma rays observed in the decay of 57Co were corrected for the amount of absorption between the source and the detector and for the relative efficiency of the detector. The results are expressed as branching ratios in fig. 4 and as numbers of gamma rays emitted per distinegration of 57Co in table 2. The relative intensity of the 230.4 keV gamma ray was obtained from the observations on the reaction 57Fe(0q ~'?)57Fe. The gamma ray intensities were then used to obtain the branching ratios to the states in 57Fe in the decay of 57Co as given in fig. 4. In accord with previous work, the branching to the two lowest levels of 57Fe is considered to be negligible. The ft values for the transitions were computed for a half-life 1) of 270 d for STCo and a mass difference 1, 9) for 57Co and 57Fe of 836___3 keV. The l o g f t values are given in table 2. 5. Discussion The energies of the gamma rays listed in table 2 are in very good agreement with those given by Kistner and Sunyar 3) in their recent high resolution study of the gamma rays in the 57Co decay. However, the intensities of the gamma rays above 300 keV in table 2 are on the average about 15 ~ higher than the values given by Kistner and Sunyar. The spins and parities of the three lowest states of 57Fe have already been established l). Assignments of ½- and ~ - have also been made for the states at 366.8 and 706.5 keV (refs. 1,1 o)). These latter assignments may be considered in the light of the present evidence. For the 366.8 keV state the large log ft value indicates I ~ ~ - or I < ~}. The strength of the Coulomb excitation selects I" = ½-. This assignment is also consistent with the observed branching ratios for transitions to the lower states. For the 706.5 keV state the log ft value would indicate ~ < 1 < 9.

)'-RAY TRANSmONS

183

T h e strength o f the g a m m a ray t r a n s i t i o n to the ½- level at 14.4 keV relative to t h a t to the I - level suggests a spin o f ~ for the 706.5 keV state. This choice is s u p p o r t e d by the strength o f the t r a n s i t i o n to the ½- g r o u n d state. This latter transition also indicates negative p a r i t y so t h a t one o b t a i n s I" = I - . These assignments were also o b t a i n e d by K i s t n e r a n d S u n y a r 3) f r o m similar considerations. F o r an assignment o f I - to the 706.5 keV state the electron c a p t u r e represents a n allowed transition for which the l o g f t value o f 7.6 is a b n o r m a l l y high. It should also be n o t e d t h a t the l o g f t value o f 6.4 is large for the allowed transition to the t - level at 136.4 keV. These large l o g f t values m a y reflect the fact that the states in STFe c a n n o t be c h a r a c t e r i z e d as g o o d single particle levels 1~). In this case, however, it s h o u l d be r e m a r k e d t h a t assignments based on relative transition p r o b a b i l i t i e s are n o t as s o u n d as otherwise a n d s h o u l d be c o n f i r m e d by a d d i t i o n a l evidence. W e wish to t h a n k Dr. Peter Paul for v a l u a b l e discussions a n d assistance in this investigation. References

1) Nuclear data sheets, compiled by K. Way et al. (National Academy of Sciences, National Research Council, Washington, D.C.) 2) G. D. Sprouse and. S. S. Hanna, Bull. Am. Phys. Soc. 9 (1964) 717 3) O. C. Kistner and A. W. Sunyar, Phys. Rev. 139 (1965) B295 4) J. M. Mathiesen and J. P. Hurley, Bull. Am. Phys. Soc. 10 (1965) 424 5) J. M. Ferguson, Nuclear Physcis 10 (1959) 405 6) G. F. Pieper and N. P. Heydenburg, Phys. Rev. 108 (1957) 760 7) W. F. Edwards and F. Boehm, Phys. Rev. 121 (1961) 1499 8) D. C. Camp and G. A. Armantrout, UCRI_,--12245 (1965) 9) C. H. Johnson and A. Galonsky, Bull. Am. Phys. Soc. 5 (1960) 443 10) L. V. Groshev, A. M. Demidov, G. A. Kotelnikov and V. N. Lutsenko, Nuclear Physics 58 (1964) 465 11) I. Hamamoto and A. Arima, Nuclear Physics 37 (1962) 457 12) E. L. Chupp et aL, Phys. Rev. 109 (1958) 2036 13) J. B. Bellicard and A. Moussa, J. Phys. Rad. 18 (1957) 115 14) R. H. Nussbaum and R. M. Housely, Nuclear Physics 68 (1965) 145 15) S. S. Hanna and R. S. Preston, Phys. Rev. 139 (1965) A722 16) R. G. Albridge and D. C. Hall, Bull. Am. Phys. Soc. 10 (1965) 244