The negaton decay of 122Sb

.E

Nuclear Physics A117 (1968) 686--696 ; © North-Holland Publishing Co., Amsterdam Not to be reproduced by photoprint or microfilm without written permission from the publisher

T

AT N DECAY

122

b

S . T . HSUE t, M . U . KIM, L. M . LANGER, W . F. PIEL Jr, and E. H . SPEJEWSKI tt

Physics Department ttt, Indiana University, Bloomington, Indiana, U.S.A ., 47401 Received 17 June 1968 1s2Sb was investigated using a 180" shaped magneticAbstract : The negaton spectum in the decay of The energy signature technique was employed in order to discriminate field spectrometer . background and instrumentally scattered electrons . The spectrum shape factor for the against 2 --2+, 1414 keV beta transition of 1$9Sb is essentially statistical within the experimental errors . The result of this shape measurement makes it possible to limit the number of solutions for the six nuclear matrix elements which enter into this once-forbidden non-unique transition . Only three beta groups were observed, and their end-point energies (keN'), intensities and loglo ft values are : 723 (4 .0±0 .5 Y, 7.7), 1414±3 (69 .0±0 .2 Y, 7.6) and 1980±3 (27 .0±0 .2 %, 8 .6) .

E

RADIOACTIVITY 1112Sb ; measured Ep, IR, #-spectrum shapes ; deduced Qe, nuclear matrix elements .

.

Introduction

The 122i Sb nucleus is known 1,6) to decay to 122 50 Sn by electron capture with an intensity of 3 .1 % and to 1 22Te by negaton emission with an intensity of 96.9 j. The negaton spectrum of 12 2 Sb has been investigated by a number of authors 1-6), and at least three beta groups have been reported . The level scheme of 122 Te proposed by Jha 36) and Arutyunyan et al. ') suggest a few additional excited states in 122 Te. The ground state of 1 2 iSb has a proton-neutron configuration (gj. , h * ) according to the shell model, and a spin and parity of 2 - have been suggested by measurements of the once-forbidden unique spectrum shape for the 122 Sb- 122 Te ground-state transition 2-6 ). This spin and parity assignment is consistent with a comparison of the experimental and theoretical K-capture ratios for the 122Sb-122Sn ground-state transition 8) and with the results of a nuclear orientation experiment 9). The spin and parity assignments of2 + for the 564 and 1257 keV levels in 122Te have been made from a study of the -conversion coefficient 2,5-6 ) and the beta-gamma 10) and gammagamma angular correlations 2,5 -6,11-14) . t Present address : University of Northern Iowa, Cedar Falls, Iowa . Present address : Princeton University, Princeton, New Jersey. ttt This work was supported by the U .S . Office of Naval Research under Contract Nonr-1705(02) . tt

686

NEGATOIV DECAY of

12 ISb

687

The spectrum shape of a once-forbidden non-unique transition from the 2ground state of 122Sb to the 2' first excited state of 122Te has been reported as being either statistical 1,1 ®6) or non-statistical 3). The beta-gamma directional correlation 1'). The (refs. 15-16)) agrees well with the predictions of the ~-approximation logI of value for this beta transition is 7.6, which is reasonable for a once-forbidden non-unique transition . Using data from experiments of spectrum shape s-6), betagamma angular correlation 1 s ), beta circularly polarized gamma angular corre22 ) lation "a), nuclear orientation 19-2 °) and nuclear resonance 21 ), Pipkin et al. were able to determine nine possible sets of values for the six nuclear matrix elements jiy s , j a - r, j ie, jr, j idxr and j Bi; that can contribute to this 2- -2' beta transition 23) t . In particular, their data indicate that the contribution of the fBi; matrix element is not negligible as demanded by the ~-approximation . From the ana122Sb decay is lysis of their data, Pipkin et al. concluded that the character of the probably the result of a cancellation effect rather than a selection rule effect. 2') and the The experiments on the beta-gamma circular polarization correlation beta-neutrino angular correlation 2 s) were carried out in order to reduce the number of possible solutions proposed by Pipkin et al. 22) for the matrix elements. 122Te proposed by Jha 36) and Arutyunyan In consideration of the level schemes of 122 Sb was undertaken not only et al. 7 ), the present work on the negaton decay of to determine possible beta transitions from the ground state to the 1181 and 1357 kepi levels in 122Te but also to measure more precisely the 1414 keV beta-spectrum shape so as to determine whether or not the spectrum shape of thib once-forbidden non-unique transition exhibits any deviation from that of an allowed transition . 1,.periment

proceaure

2.1 . SOURCE

The source material of "2Sb was obtained from Oak Ridge National Laboratory in the form of SbC13 in a 4.7 N HCl solution. Because SbC13 is deliquescent, it was converted to Sb2S3 . This was done by bubbling H2S through the SbC1 3 solution which was first diluted with water. A yellowish red precipitate of Sb2S3 was centrifuged and then washed with water several times. Finally, a liquid suspension of the precipitate was used to make the source . It was prepared by depositing the activity onto an N 20 jig/cm2 zapon film which was s ipported by a 0.58 mg/cm2 aluminized mylar backing. Insulin was used to define the source area of 4 x 25 mm. 2 . The source was then dried and covered by another thin zapon film. The activity and average thickness of the source prepared in this way `were approximately 2.5 mCi and 76 Pg/ cm2, respectively . The source material was found to be contaminated with a small amount of the 60 d 124Sb isotope . The activity of 124Sb present at the time of this experiment was less than 4 % of that of 122Sb . This was determined by taking another run with the t See ref. ") for a comprehensive review of first-forbidden beta decay.

s. °r. HSUB et d7l.

4688

low 1 eV same source 30 d after the first run was completed. 'or the region 26) were u . the end-point energies and relative intensities report by Stelson The results of our previous subtracting the 124Sb contribution to the 122S d experiment 27 ) on 124Sb were used for the re on above 1 e . 2.2. BETA-RAY MEASUREMENT

e beta-spectrum, measurements were performed with the high-resolution 2 ). radius of curvature, 180* focussing, shaped magnetic-field spectrometer '8 The magnet current was furnished by a transistorized constant-current supply, and. the magnetic field measured with a Rawson precision rotating-coil gaussmeter was stable e to much better than 0.1 V during the time required to obtain any datum point. spectrometer was calibrated with the -conversion line of 1 -1 "Cs. With source and detector slit widths of 4 mm, the resolution of the spectrometer was .0 0.68 j.

g

é

Nscaler PHA

Ge 0.6 2

3

Fig. l . Backscattering correction curve obtained with a ' 4®Pr source . The quantity p represents the ratio of the number of electrons stopped within the detector to the total number of electrons entering the detector. It was obtained by comparing the number of counts within the spectrum k recorded with the 200-channel analyser to that detected with an integrally biased scaler for each magnetic field setting.

A lithium-drifted silicon detector with a depletion depth of 5 mm and a thin (< 0.12 mg/cm2) entrance window was employed as the detector . t was operated at liquid nitrogen temperature in order to reduce the intrinsic noise and thereby improve its energy resolution . The discrimination against counts arising from background and instrumental scattering is accomplished by analysing the pulse-heights of the signals from the semiconductor detector and accepting ordy those which have the proper energy signature corresponding to the momentum setting of the magnetic spectrometer. Since the great majority of the background events and the instrumentally scattered electrons have'energies different from this, the ability to discriminate against them is increased by a factor of about 10' .

N GATON DECAY or l22sb

689

au fraction of the incoming electrons scatter out of the detector before they re stop and since this fraction is a slowly varying function of energy, it is necessary to obtain a bac scattering correction curve 29) . This is a graph of the ratio f the number of counts within the spectrum peak recorded with the pulse-height nalyser to the number of counts detected with an integrally biased scaler plotted a inst the total electron energy . n order that such data may accurately represent the ackscattering of electrons from the detector, it is necessary for the counts caused by instrumental scattering to be a very small fraction of the total number of counts. other words, it is necessary that the source used for this determination be of high enough end-point energy so that the intensity available in the region of interest may yield good statistical accuracy. Also, there should be no strong inner beta groups or internal conversion lines which might contribute measurable numbers of scattered electrons. For the analysis of the negaton ay ®f "Sb, the backscattering correction curve (fig. 1) was obtain with a source of 144 r whose outermost beta spectrum has an end-point energy and an intensity ofabout 2.98 eV and 97.7 %, respectively .

The level scheme of 122 Te (fig. ) is the result of a nuclear reaction study by Bergtrbm et al. and jewski 3s ), and in table 1 data from investigations of the negaton decay of " 2 Sb are su riz f7

TABLE 1

r.d-point energies (keV) and intensities of the partial beta spectra of '$'Sb

Cork et al.') 450(8 %) Moreau') 720±20(12 %) Glaubman®) 730±20(4 .4%)

50(7 %)

1400±20(56 5/,)

2000±30(36 l)

1410±10(60 %)

1970±10(21 ,o)

142.3

1987±20(26 .5 %)

10(69 ~)

Farrelly et al. 4)

740±20(4.1 %)

1400±10(64.9 %)

1970± 5(31 %)

Present work

723(4.0±0.5 ió)

1414± 3(69 .0±0 .2 %)

1980± 3(27 .0±0 .2 %)

The ermi- urie plot of the total beta spectrum of 122 Sb is shown in fig. 3. A 2.7±0.1 d half-life of 122 Sb was used in the decay correction of the data. Only three beta branches are observed in the negaton decay of 122 Sb. This is in agreement with the results of Cork et al. 3), Glaubman s) and Farrelly et al. 6 ) . The experimental shape factor of a once-forbidden unique transition from the 2 ground state of 122Sb to the 0' ground state of 122Te was fitted with a correction factor of the form C{ W) = Lo q2. + 9L1 , where q is the neutrino momentum and L o and L1 the combinations of electron radial wave functions in the tables of Bhalla and Rose 30), where corrections for the finite nuclear size and the finite de Broglie

s.

690 1232cb

»sue et ai.

l

Fig. 2. Decay scheme of 1°'Só. The

0.3

T.

1940

1"Te

level scheme was determined by Spejewski a®) .

122Só

0.2

Fig. 3. Fermi-Kurie plot of the total beta spectrum of 1 "Sb.

tröm et al. and

MEGATON DECAY OF 122Sb

691

0.014 0.012 0.010

Fig. 4. Experimental shape factor of the 1414 :4- 3 keV beta group of 122Sb. The error flags are compounded from the statistical error in the data and the quoted end-point error. A theoretical correction factor of the form C(W) oc k(1 +aW+bl W+cW') was used to obtain the solid lines corresponding to the nine sets of nuclear matrix elements reported by Pipkin et al. 22).

0.2

Fig. S. Fermi-Kurie plot of the 1s2Sb beta spectrum after stripping the outermost beta group on the basis of a correction factor C( W) = Lo q'+9L1 .

692

L T. HSUE Ot 01 .

wave length are incorporated . The end-point energy, intensity and log, oft value for this outermost beta transition are found to be 1980 ± 3 keV, 27.0± 0.2 % and 8.6, respectively . The quoted end-point error resulted in a X2 -value of 12.5 and a confidence level of 0.33. The experimental shape factor for a beta transition from the 2 - ground state of 1225b to the 2' first excited state in "'Te was carefully analysed after the outermost beta group had been stripped. As shown in fig. 4, this once-forbidden non-unique shape factor has essentially a constant value over the region of W from 2.4 to 3.7, where W is the total electron energy in mc' units. It should be noted that the experimental shape factor for the 1414 keV beta trausition does not deviate from a

Fig. 6. Fermi-Kurle plot of the innermost beta group of 122Sb . The error flags correspond to counting statistics only .

constant value even when the outermost beta group of a once-forbidden unique transition is subtracted on the b"sis of a shape correction factor of the form Q W) = (Lo q'+A,Al+blW) with 0.3 -5 b < 0.4. That is, the fit is insensitive to the empirical correction factor which was used by Langer et al. "') in order to get the best fit to their data. Also shown in fig. 4 and normalized at W = 2.973 are nine curves for the spectrum shape factor. These curves were obtained from the nine seta of nuclear matrix elements given by Pipkin et al. "). Curves I and III as well as V1 and VII are indistinguishable on the scale of the drawing . Only four out of the nine sets are compatible with the present shape measurement . The four sets are summarized in

table 2, where the last set IX corresponds to the modified Bij approximation

31) .

From the FK plot of fig. 5, the end-point energy, intensity and log, oft value for the ®_2+ 2 beta transition to the 564 keV level are found to be 1414_h 3 keV, 69.0 ± 0.2 %

4!n1

1i k 5 1

XDECAY OF CD lasSb

15

NEGATON

693

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0 X

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694

T. WsuE

et al.

and 7 .6, respectively . The quoted uncertainty in the end-point energy represents tie range of values which yield essentially a constant value for the experimental shape factor . The FK plot in fig. 6 was obtained by subtracting the two outer beta groups. This innermost beta group corresponds to a once-forbidden non-unique transition from the 2 - ground state of ,22Sb to a 2+ excited stale (1257 keV level) ùn 122Te . From the linearity of the FK plot, an allowed shape is assumed for this transition, which has an end-point energy, an intensity and a logeoft value of 723 keV, 4 .0±0.5 % and 7.7, respectively. The end-point energy of 723 keV is just the difference in the energies of the outermost beta group and the gamma transition from the 1257 keV excited state to the ground state of ' 22 Te. Discussion Our measurement of a once-forbidden unique shape for the outermost beta group ® of 122Sb agrees with the results of the previous authors 2 6 ) . This shape is consistent with the spin and parity assignment of 2 - to the ground state of 122Sb. TABLE 3

Values of the coefficients in C(W) oc k(: +aW-Pb/W+cW 2 ) computed with the nuclear parameters corresponding to the four sets of nuclear matrix elements in table 2 k

a

b

c

Set V

62.76

-0.199

-0-085

0.031

Set VI

54.32

---0.036

--0 .173

0.006

Set VII

47.89

-0.013

0 .090

0.003

Set

50.48

---0.012

0.0

0.003

lx

A 1414 keV beta transition to the 2+ first excited state in 122'íe has, within the experimental errors, a statistical shape that most once-forbidden non-unique transitions exhibit . This shape measurement disagrees with the result of Cork et al. 3), who fit their data with a once-forbidden unique shape factor, but does not disagree with the findings of other authors 1,4-6 ) . An allowed shape for the 2_-2' 1414 keV beta transition is consistent with the ~-approximation . The experimental shape factor was fitted with a theoretical correction factor of the form C(W) oc k(1 + aW + bl W + c W2 ) using the nine sets of nuclear matrix elements obtained by Pipkin et al. 22) . Here k, a, b and c are certain combinations of nuclear parameters 32 ). They are independent of the total electron energy W in the Konopinski-Lihlenbeck approximation 33 ) . For the four sets of nuclear matrix elements in table 2 which give the best agreement with the shape factor measurement, the computed values of k, a, b and c are listed in table 3 as an aid to any future investigation of this shape factor.

NEGAT®N DECAY OF '"Sb

695

The considerable size of tr e Blj matrix elements in table 2 may suggest that a selection rule effect is operative in the decay of 122Sb . It is noteworthy that the character of the beta decay of 124Sb which has two more neutrons than 122Sb is known to be due to a selection rule effect 23). If I V1 or 1 YI are much larger than unity, where the parameters V and Y are the relative magnitudes of the contributions from matrices of rank zero and one 32) with respect to that of fBij, the shape correction factor in the modified Bij approximation reduces to that for the ~-approximation, i .e. C(6V) = I V12 + Y1 2 , which is independent of the electron energy . This means that the selection rule is less effective, and that the deviation from an allowed shape in the modified Bdj approximation is less prominent as indicated by the curve IX (V = - 7.0 and Y = -0.60) in fig. 4. The sets of nuclear matrix elements in table 2 are the four sets, V, VI, VII and IX given by Pipkin et al. 22). The three sets, I, II and III in ref. 22 ) are in rather poor agreement with the present shape factor data as seen in fig. 4. Grabowski et al. 24) used the sets I and I:K, which agreed with the beta-gamma circular polarization data, and Palathingal ZS) used the sets I and UI for the analysis of the beta-neutrino angular correlation experiment. e fourth and eighth sets of Pipkin et al. can be eliminated on the basis of the present shape factor measurement. The conserved vector current theory of the vector part of the beta interaction predicts a definite relationship between the f is and fr matrix elements 34-35) A -v -

ƒ

ia

r1R = 6aZ+(Wo-2 .5)R,

where 6V® and R = (a12)A 1 are the maximum electron energy and the nuclear radius, respectively, in units of h = c = m = 1 . For the 1414 keV beta transition of "2Sb, one gets = 0.46. The three sets V, VI and VII in table 2 yield A v = 0.48, Acvc

Av, = 0.49 and Av = 0.29, respectively . The first two values satisfy the above relationship well. The set IX implies A,x = ± oo, since the Bif approximation assumes f r ;:e, 0 a priori. A spin and parity assignment of (0+ ) for the 1357 keV level in "'Te is based on a gamma-gamma angular correlation study 36) of the 793-564 keV cascade following the positon decay of 1221. The logl(,ft value for a possible beta transition from the ground state of 122Sb to the 1357 keV level in 122Te is estimated to be 9.7 from an intensity measurement of the 793 keV gamma transition ', 36). Arutyunyan et al. ' ) suggested that this log, oft value could be smaller by one or two orders of magnitude if there are low-energy gamma transitions from the 1357 keV level . In this case, the logloft value would approach that for the 122Sb-122Te ground state to ground state transition. In the present experiment, an upper limit intensity of a few tenths of a percent for a beta group populating the 1357 keV level in 122Te was too weak to be measured. 122Te The FK plot (fig. 6) of the 723 keV beta branch to the 1257 keV level in is essentially linear within the experimental errors. However, the 723#-693y directional

696

s. T. HsuE et al.

correlation data 16) indicate strongly that the 723 keV beta transition does not follow the ~-approximation. There is also no evidence for a beta transition to the 1181 keV level in 12 'Te. As seen from table 1, the result of our present work is in the best agreement with that of Glaubman 5 ). References P. A. Macklin, L. I . Lidofsky and C. S . Wu, Phys. Rev . 82 (1951) 334 M . J. Glaubman and F. R. Metzger, Phys. Rev. 87 (1952) 203 J. M. Cork, M. K. Brice, G . D. Hickman and L. C. Schmid, Phys. Rev. 93 (1954) 1059 J. Moreau, Compt. Rend. 239 (1954) 1130 M . J. Glaubman, Phys. Rev . 98 (1955) 645 B. Farrelly, L . Koerts, N. Benczer, R. van Lieshout and C . S. Wu, Phys. Rev . 99 (1955) 1440 E. A. Arutyunyan, B. S. Dzhelepov, Yu. V. Khol'nov and G . .E. Shchukin, lzv . Acad . Nauk. 29 (1965) 1108 8) M. L. Perlman, J. P. Welker and M. Wolfsberg, Phys. Rev . 110 (1958) 381 9) F. M . Pipkin, Phys. Rev . 112 (1958) 935 10) 1. Shaknov, Phys. Rev . 92 (1951) 33 11) R. M. Steffen, Proc . 1954 Glasgow Conf. on nuclear and meson physics (Pergamon Press, London, 1955) p. 206 12) F. Lindquist and J. Marklund, Nucl . Phys. 4 (1957 189 13) C . F. Coleman, Nucl . Phys. 5 (1958) 495 14) 1. Asplund, L. G. Stromberg and T. Wiedling, Ark . Fys . 18 (1960) 65 15) R . M . Steffen, Phys. Rev. 123 (1961) 1787 16) R . S. Raghavan, Z. W. Grabowski and R . M . Steflèn, Phys. Rev . 139 (1965) 111 17) T . Kotani and M . Ross, Phys. Rev . 113 (1959) 622 18) J . P. Deutsch and R. Lipnik, J. de Phys. 21 (1960) 806 19) G. E. Bradley, F. M. Pipkin and It. E. Simpson, Phys. Rev . 123 (1961) 1824 20) B. N . Somoilov, V. V. Sklyarevakii and E. P. Stepanov, JETP (Sov . Phys.) 11 (1960) 261 21) E. Sloan, Harvard University thesis (1962) unpublished 22) F. M. Pipkin, J. Sanderson and W. Weyhmann, Phys. Rev . 129 (1962) ?626 23) H. A. Weidenmiiller, Revs. Mod . Phys. 33 (1961) 574 24) Z. W. Grabowski, R. S. Raghavan and R. M. Steffen, Nucl . Phys. 70 (1965) 170 25) J. C. Palathingal, Phys. Rev . Lett. 14 (1965) 983 26) P. H. Stelson, Phys. Rev . 157 (1967) 1107 27) S. T. Hsue, L. M. Langer, S. M. "Fang and D. A. Zollman, Nucl. Phys. 73 (1965) 379 28) L. M . Langer and C. S . Cook, Rev . Sci . Instr . 19 (1948) 257 29) S. T. Hsue, L. M. Langer and S. M. Tang, Nucl. Phys. 89 (1966) 47 30) C. P. Bhalla and M. E. Rose, Oak Ridge National Laboratory Report ORNL-3207 (Jan. 1962) 31) Z. Matumoto, M. Yamada, I. T. Wang and W. Morita, Bull. Kobayashi Inst. Phys. Res . 5 (1955) 210; Phys. Rev . 129 (1963) 1308 32) T. Kotani, Phys. Rev . 114 (1959) 795 33) E. J. Konopinski and G. E. Uhlenbeck, Phys. Rev . 60 (1941) 308 34) J. 1. Fujita, Phys. Rev . 126 (1962) 202; Progr. Theor . Phys. 28 (1962) 338 35) J. Eichler, Z. Phys. 171 (1963) 463 36) S. Jha, Phys. Rev . 132 (1963) 2639 37) L. M. Langer, E. H. Spejewski and D. W. Wortman, Phys. Rev . 135 (1964) B581 38) 1. Bergstrom, C. J. Herrlander, A. Korzk and A. Luuko, private communication ; E. H. Spejewski, private communication 1) 2) 3) 4) 5) 6) 7)