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Electron-gamma and gamma-electron directional correlation experiments on the 496 keV-124 keV cascade in 131Cs
1
2.B
1
Nuclear Not
Physics A96
(1967) 42-48;
@ North-Holland
Publishing
Co., Amsrerdam
to be reproduced by photoprint or microfilm without written permission from the publisher
ELECTRON-GAMMA AND GAMMA-ELECTRON DIRECTIONAL CORRELATION EXPERIMENTS ON THE 496 keV-124 keV CASCADE IN 131Cs S. K. SEN and D. A. DOHAN r Department
of Physics,
University
of Manitoba,
Received 2 January
Winnipeg,
Canada
1967
Abstract: Careful studies were made of the directional correlations between the 496 keV K-conversion electron and the 124 keV gamma ray and between the 496 keV gamma ray and the 124 keV Kconversion electron in the 620-124-o keV cascade of iJICs formed by the electron capture decay of islBa. The coefficients ASS = -0.113~0.031 and A,, = +0.140&0.036 obtained from the K electron-gamma angular correlation measurements and the coefficients & = -0.012~0.012 and A,( = +0.014&0.014 obtained from the gamma-K electron angular correlation measurements together with the previously measured’) mixing ratio for the 124 keV transition give the spin sequence ++(Ml +E2)Q+(E2 + Ml)*+ for the cascade. The experiments were performed with a system, capable of electron resolution of 5 keV FWHM at 1.O MeV, allowing the K conversion lines to be unambiguously separated from the L lines. Evidence is presented for the presence of electric monopole EO electric transitions in direct competition with Ml and E2 internal conversion transitions from the 124 keV level. The experiments also confirm that the 496 keV transition is predominantly Ml but establishes that it has a 23&7 % E2 admixture. The new measurements together with previous measurements of the gamma-gamma angular correlation and the internal conversion coefficients give a more complete picture of the cascade. E
RADIOACTIVITY
islBa [from lsOBa(n, r)]; measured EO/E2. Enriched target.
yce(0). i*lCs deduced J, n,
1. Introduction Several authors 2-4) measured the y-y angular correlations of the 496 keV-124 keV cascade but their spin assignments for the cascade are not in agreement. Lindqvist (97 ‘4 Ml + 3 ‘AE2)3+. The measureand Karlsson 2, gave a spin sequence J+(E2)$+ 2 ments of Haskins 3, are consistent with the assignment ++(Ml)$+(E2+Ml)t+ or 3+(Ml)-t+(E2)3+, while Bodenstedt et al. 4, indicated the 496 keV-124 keV cascade to be isotropic indicating the spin of 124 keV level to be 4’. We have performed the angular correlation measurements involving conversion electrons using a high-resolution, solid-state electron detector. 2. Apparatus and the source preparation The electron detector was a gold-silicon, surface-barrier diode obtained from ORTEC. It has a circular area of 50 mm2 and the maximum depth obtained at a t Present address: Department of Physics, McMaster University, Hamilton, Ontario, Canada. 42
43
e-Y AND y-e CORRELATIONS
bias of 475 V was 1500 #m which is the range of 980 keV electrons in silicon. The pulses from the solid state detector were amplified by the Nuclear Enterprises 52315230 low-noise, amplifying system. The silicon detector was cooled to liquid nitrogen temperature to give a full width at half maximum ( F W H M ) of about 5 keV at 1.0 MeV. At room temperature (25°C), the same detector was capable of an energy resolution of 10 keV FWI-IM at 1.0 MeV. For the gamma rays, a 3.7 cm x 2.5 cm NaI(T1) crystal was used. The experimental chamber, containing the solid state detector fixed in position and the source, was constructed with cylindrical symmetry about the axis of rotation of the moveable scintillation counter in order to minimize the possible anisotropies introduced due to scattering. The source was held vertically in the geometrical centre of the vacuum chamber 1.7 cm from the solid-state electron detector. The source used was 131Ba(ll.5 d) obtained from O R N L in the form of BaC12 and was sublimed on to a 180 #g/cm 2 aluminium foil. The source had a radiochemical purity of 98 + Yo and a specific activity of approximately 20 Ci/g at the start of the run. 3. Experiments The experiments consisted of a pulse-height analysis of the electrons of correct energy, which were in coincidence with the gamma rays detected also of correct ener131 Bo (11.5d)
Spin
I~ .-~--~
1500-
I _1
ZP,
2L" I 1o
200.
496
(.3 IZ
(a)
0 LO
l Cs131
24 K
~ I000-
620
(b)
500-
I00-
Z
124 L,M
[ ~
(.3 Z
496 L,M 486 KI
0 40
50
60
70
80
90
250
~ 270
, 290
CHANNEL NUMBER Fig. 1. The 13~Cs conversion electron spectrum (a) in coincidence with the gamma spectrum in the 450-550 keV energy range and (b) in coincidence with the gamma spectrum in the 110-140 keV energy range for the angle of 135 ° between the detectors. The numbers denote the transition energies in keV.
44
s . K . SEN AND D. A. DOHAN
gy. The coincidence gated conversion electron spectra were taken at 15° intervals from 90 ° to 180° for 2 h, 40 min at each position. The complete angular range (in all seven angles) was scanned three times alternately in opposite directions. A singles gammaray spectrum taken at each angle was used to normalize the coincidence gated spectrum at that angle. After each experiment the discriminator of the time-to-amplitude converter (TAC) used for the coincidence were set for a region away from the true coincidence region. In this way the random coincidences were determined but were found to be negligibly small. Corrections were made for the decay of the source over the time of the angular correlation experiments. The total number of coincidences collected at each position was approximately 13 000 in the y-e experiment and approximately 4000 in the e-y experiment. Fig. 1 shows the 124 keV K conversion electrons in coincidence with the 496 keV gamma ray and the 496 keV K conversion electrons coincident with the 124 keV gamma ray. The 133 keV K electron--486 keV y-ray and the 133 keV y-ray--486 keV K electron coincidences can also be seen in the figure although the 133 keV and 486 keV gamma rays were not resolved from the 124 keV and 496 keV y-rays, respectively, by the scintillation counter.
4. Analysis of the data
In order to maximize the efficiency of the statistics, the data were analysed by a method due to White s). The experimentally obtained quantities W(O~) taken at angles 90 °, 135 ° and 180° were used to solve the Legendre polynomial series W(O~) = C o + C2P2(cos 0~) + C4P4(cos 0i),
i = 1, 2, 3.
The coefficients C2 and C4 for both e-y and y-e angular correlation functions and the errors in these coefficients were determined by a computer. These were then corrected for finite solid angle using the tables of Yates 6) and are given in table 1. The TABLE l
Experimental values of An and A** coefficients
A.i Aa
Electron-gamma
Gamma-electron
--0.1134-0.031 q-0.140q-0.036
--0.0124-0.012 +0.0144-0.014
coefficients A22 and A44 thus obtained were used t o plot the curves shown in fig. 2 at 5 ° interval. An independent check on each of these curves was made by finding the least-squares fit of the seven experimental points. The two curves were found to be similar.
e-7 AND y-e CORRELATIONS
45
5. Interpretation of the results The main features of our results (table 1) are that the coefficients A~4 are greater than A22 and although the ?-e correlation is negligibly small, we can, generally, say (this is evident from fig. 2) that A22 values are negative while A44 values are positive and have non-zero values, which excludes pure dipole radiations for the cascade. This also means that the intermediate level has spin greater than 3. 1.10496
K -
1247"
496
7" "
124 K
1.00.
0')
v
0.90-
1.10-
1.00
0.90' 90
120
150
180
0 (degrees) Fig. 2.
W(O)/W(90)versus 0. The
dotted lines represent the best fit curves to the experimental points
and their errors. From shell-model considerations, and from the log f i values determined by Kelly and Horen x) the spins and parities of x31Cs are limited to the values <~-+ for the 620 keV level and to < ~+ for the 124 keV level. The ground state of x3~Cs is known 7) to be definitely ~z+. The K conversion coefficient determined by Kelly and Horen 1) suggests the 496 keV transition to be predominantly M 1 with a possible small mixture of E2 while the measurement 1) of L-subshell ratios (L~/L./LI.) for the 124 keV transition supports an E2 transition with a possible 8 ~o admixture of M1 transition. The theoretical values of A22 coefficients were calculated for all possible spin assignments, within the limits prescribed by the shell model and for all possible values of mixing of the 496 keV transition and using 8 ~o admixture 1) (62 = +0.28) of M1 radiation in the predominantly E2 radiation of the 124 keV transition. The spin sequence 3 + ~ ~+ ~ ~+ is the only one which gave the coefficients /144 positive for both e-? and ?-e angular correlations. This spin sequence is in agreement with the ?-?
46
S. K. SEN AND D. A. DOHAN
angular correlation measurements of Haskins 3). The sign and the magnitude of the A22 coefficient for the e-~ experiment also agree with this spin sequence if the mixing ratio 62 for the 124 keV radiation is taken as -0.28 and the mixing ratio 6~ for the 496 keV transition is -~v.z,~_oilo. . . . . +o ~2 Such value of 61 corresponds to a 23_+7~o of E2 admixture to the predominantly M1 transition of 496 keV and gives the value of ~tK still in agreement with the measurement of Kelly and I-Ioren ~). For the A22 coefficient of the e-~ experiment, we used the relation a22(el~2)
=
h2(el)A2(~2),
where according to Church et al. s) m
A2(el)
=
m
b2 F2
2 •
•
(Ya)+ 2pI b2F2(~I) +Pl bz F~(~I) 1 +p21
e
a2(Y2) = F~ (Y2)+ 262 F2(Y2) +
2 m
6 2 F 2 (]~2)
1+6 Here the b2 are particle parameters, and Pl is given by
The K conversion coefficients ctK(M1) and ~K(E2) for M1 and E2 transitions, respectively, are determined by graphical interpolation of the values given by Sliv and Band 9). The theoretical value of the A22 coefficient for the y-e correlation was found to be correctly negative for the spin sequence ½ + ~ { + ~ ½+ with 61-- -0.28 and 62 = +0.54. But its magnitude is larger than that of A22(yte2) contrary to the experimental finding. This discrepancy can, however, be removed only if an admixture of electric monopole transitions E0 in competition with M 1 and E2 electric transitions for the 124 keV radiation is introduced s). We can then write, A22(yIe2)
=
A2(~l)A2(e2)
,
where s) e
A2(e2)
=
e
2 m
m
b2F~(~2)+2p2b2F2(~2)+p2b2 F~ i +
()~2)+q2b
0
Here p~ = 6~(~K(MI)/~K(E2)), q~ is the ratio of the E0 conversion electron to the E2 conversion electron intensities and q2bo the EO-E2 interference term which, in this case, is positive so that A2(e2) has a small negative value. The values of the particle parameters b 2 were obtained from the graphical extrapolations of the values given by Biedenharn and Rose lo) and are given in table 2. The F-coefficients which are functions of the level spins/1, I and 12 and of the multipolarities L1, L~, L 2, L~ (inset fig. 1) were obtained from the tables of Ferentz and Rosenzweig 1x).
e-y AND ~,-e CORRELATIONS
47
An estimate of q 22was made by using the relation, A#4(yle2)
--
62 b,].L~.(?2) . 1+62 F~(?I) 1 .q P 22.[ q22
The particle parameter b~ was calculated from the recurrence formula 1o) b,~ = (1.4-b~)2.5. The value of q22 thus obtained is -,.,,-4.s.a ~+~.s Our experiment, because of small ?-e angular correlation, does not give the value of q~ with a better degree of accuracy. TABLE 2 Particle p a r a m e t e r values
bam b~ b2e
124 keV transition
496 keV transition
0.17 0.06 1.91
0.49 0.51 1.51
However, we can say that it indicates strongly the presence of monopole electric transition. Support for our observation of the existence of a non-vanishing q 22can be obtained independently by using the values of ~xp and of 622for the 124 keV transition by Kelly and Horen 1) in the expression ~¢xp = ~K(E2)(1 +q~)+622~K(M1)
i+6 and finding that the experimental values ~exP and 6~ of Kelly and Horen, are compatible only if a mean value of q~ = 0.13 is introduced. The comparison of the experimental value of A44(el 72) with the relation, Aa~(el?2)_
p2 b~F~(?l)F~(?2) 1 +P2
1 + 62
yielded the correct sign of b~ for the 496 keV transition, but gave too large a value. This is not well understood. The e-? measurements were repeated after the ?-e measurements and the same result was obtained. Moreover the fractional error in the e-y measurements was better than ?-e measurements and the results are considered to be genuine. 6. External perturbations The source was sublimed on to an aluminium foil and hence the attenuation of angular correlation due to hyperfine structure interaction was considered to be absent.
48
s.K. SEN AND D. A. DOHAN
The 124 keV level of 1alCs has a half-life ~ 4 nsec and hence an attenuation of the angular correlation might be expected due to quadrupole interaction. The experiments were performed with sources in two different physical forms, namely (i) evaporated drop source and (ii) sublimed source, as so to introduce quaddrupole interaction, if present, in different strengths. However, we found that the values of A22 and A44 in both cases agreed within the experimental errors, and therefore we assumed that the attenuation effect, if present, is very small. Furthermore, any correction for the attenuation of the directional correlations would tend to enlarge the discrepancy in the results of the A44 coefficient in the e-v experiment. 7. Summary and discussion In these measurements, a high-resolution electron detector system was used so that K electrons were completely separated from the L electrons. This also enabled us to eliminate the effect from the interfering cascade 486-133 keV which was present in the ?-? angular correlation measurements of the previous workers 2, 3). The desirable feature of the measurement of the directional correlation involving conversion electrons is that it is more sensitive to the minute admixture of multipole radiations than are the ?-? correlations. The very fact that the detectors acted as particle identifiers resulted in the interference between the counters, such as backscattering and Compton events which may distort the angular correlation, being much reduced. The present work clearly establishes that the 496 keV transition is not a pure M1 transition but has an admixture of E2 transition, and also suggests the existence of electric monopole transitions in the internal conversion channel between the 124 keV level and the ground level. These new observations are consistent with those of Kelly and Horen 1) made from the measurements of the conversion coetticients. The measurements of the 7-e and e-v angular correlation functions helped to limit the choice of spin sequences to a small number. The authors acknowledge gratefully the support from the National Research Council of Canada and wish to thank Mr. S. I. H. Rizvi for checking the numerical calculations.
References 1) 2) 3) 4) 5) 6) 7) 8) 9) 10) 11)
W. H. Kelly and D. J. Horen, Nuclear Physics 47 (1963) 454 T. Lindqvist and E. Karlsson, Ark. Fys. 12 (1957) 519 J. R. Haskins, ARGMA-Tr-IC32, No. R (1959) E. Bodenstedt et al., Nuclear Physics 20 (1960) 557 D. H. White, Nucl. Instr. 21 (1963) 209 M. J. L. Yates, in Alpha-, beta- and gamma-ray spectroscopy, Vol. 2, ed. by K. Siegbahn (North-Holland Publ. Co., Amsterdam, 1965) E. H. Bellamy and K. F. Smith, Phil. Mag. 44 (1953) 33; W. A. Nierenberg et aL, Bull. Am. Phys. Soc. 1 (1956) 343 E. L. Church, M. E. Rose and J. Weneser, Phys. Rev. 109 (1958) 1299 L. A. Sliv and I. M. Band, in Alpha-, beta- and gamma-ray spectroscopy, Vol. 2, op. cit. L. C. Biedenharn and M. E. Rose, Revs. Mod. Phys. 25 (1953) 729 H. Ferentz and N. Rosenzweig, in Alpha-, beta- and gamma-ray spectroscopy, Vol. 2, op. cit.
2.B
1
Nuclear Not
Physics A96
(1967) 42-48;
@ North-Holland
Publishing
Co., Amsrerdam
to be reproduced by photoprint or microfilm without written permission from the publisher
ELECTRON-GAMMA AND GAMMA-ELECTRON DIRECTIONAL CORRELATION EXPERIMENTS ON THE 496 keV-124 keV CASCADE IN 131Cs S. K. SEN and D. A. DOHAN r Department
of Physics,
University
of Manitoba,
Received 2 January
Winnipeg,
Canada
1967
Abstract: Careful studies were made of the directional correlations between the 496 keV K-conversion electron and the 124 keV gamma ray and between the 496 keV gamma ray and the 124 keV Kconversion electron in the 620-124-o keV cascade of iJICs formed by the electron capture decay of islBa. The coefficients ASS = -0.113~0.031 and A,, = +0.140&0.036 obtained from the K electron-gamma angular correlation measurements and the coefficients & = -0.012~0.012 and A,( = +0.014&0.014 obtained from the gamma-K electron angular correlation measurements together with the previously measured’) mixing ratio for the 124 keV transition give the spin sequence ++(Ml +E2)Q+(E2 + Ml)*+ for the cascade. The experiments were performed with a system, capable of electron resolution of 5 keV FWHM at 1.O MeV, allowing the K conversion lines to be unambiguously separated from the L lines. Evidence is presented for the presence of electric monopole EO electric transitions in direct competition with Ml and E2 internal conversion transitions from the 124 keV level. The experiments also confirm that the 496 keV transition is predominantly Ml but establishes that it has a 23&7 % E2 admixture. The new measurements together with previous measurements of the gamma-gamma angular correlation and the internal conversion coefficients give a more complete picture of the cascade. E
RADIOACTIVITY
islBa [from lsOBa(n, r)]; measured EO/E2. Enriched target.
yce(0). i*lCs deduced J, n,
1. Introduction Several authors 2-4) measured the y-y angular correlations of the 496 keV-124 keV cascade but their spin assignments for the cascade are not in agreement. Lindqvist (97 ‘4 Ml + 3 ‘AE2)3+. The measureand Karlsson 2, gave a spin sequence J+(E2)$+ 2 ments of Haskins 3, are consistent with the assignment ++(Ml)$+(E2+Ml)t+ or 3+(Ml)-t+(E2)3+, while Bodenstedt et al. 4, indicated the 496 keV-124 keV cascade to be isotropic indicating the spin of 124 keV level to be 4’. We have performed the angular correlation measurements involving conversion electrons using a high-resolution, solid-state electron detector. 2. Apparatus and the source preparation The electron detector was a gold-silicon, surface-barrier diode obtained from ORTEC. It has a circular area of 50 mm2 and the maximum depth obtained at a t Present address: Department of Physics, McMaster University, Hamilton, Ontario, Canada. 42
43
e-Y AND y-e CORRELATIONS
bias of 475 V was 1500 #m which is the range of 980 keV electrons in silicon. The pulses from the solid state detector were amplified by the Nuclear Enterprises 52315230 low-noise, amplifying system. The silicon detector was cooled to liquid nitrogen temperature to give a full width at half maximum ( F W H M ) of about 5 keV at 1.0 MeV. At room temperature (25°C), the same detector was capable of an energy resolution of 10 keV FWI-IM at 1.0 MeV. For the gamma rays, a 3.7 cm x 2.5 cm NaI(T1) crystal was used. The experimental chamber, containing the solid state detector fixed in position and the source, was constructed with cylindrical symmetry about the axis of rotation of the moveable scintillation counter in order to minimize the possible anisotropies introduced due to scattering. The source was held vertically in the geometrical centre of the vacuum chamber 1.7 cm from the solid-state electron detector. The source used was 131Ba(ll.5 d) obtained from O R N L in the form of BaC12 and was sublimed on to a 180 #g/cm 2 aluminium foil. The source had a radiochemical purity of 98 + Yo and a specific activity of approximately 20 Ci/g at the start of the run. 3. Experiments The experiments consisted of a pulse-height analysis of the electrons of correct energy, which were in coincidence with the gamma rays detected also of correct ener131 Bo (11.5d)
Spin
I~ .-~--~
1500-
I _1
ZP,
2L" I 1o
200.
496
(.3 IZ
(a)
0 LO
l Cs131
24 K
~ I000-
620
(b)
500-
I00-
Z
124 L,M
[ ~
(.3 Z
496 L,M 486 KI
0 40
50
60
70
80
90
250
~ 270
, 290
CHANNEL NUMBER Fig. 1. The 13~Cs conversion electron spectrum (a) in coincidence with the gamma spectrum in the 450-550 keV energy range and (b) in coincidence with the gamma spectrum in the 110-140 keV energy range for the angle of 135 ° between the detectors. The numbers denote the transition energies in keV.
44
s . K . SEN AND D. A. DOHAN
gy. The coincidence gated conversion electron spectra were taken at 15° intervals from 90 ° to 180° for 2 h, 40 min at each position. The complete angular range (in all seven angles) was scanned three times alternately in opposite directions. A singles gammaray spectrum taken at each angle was used to normalize the coincidence gated spectrum at that angle. After each experiment the discriminator of the time-to-amplitude converter (TAC) used for the coincidence were set for a region away from the true coincidence region. In this way the random coincidences were determined but were found to be negligibly small. Corrections were made for the decay of the source over the time of the angular correlation experiments. The total number of coincidences collected at each position was approximately 13 000 in the y-e experiment and approximately 4000 in the e-y experiment. Fig. 1 shows the 124 keV K conversion electrons in coincidence with the 496 keV gamma ray and the 496 keV K conversion electrons coincident with the 124 keV gamma ray. The 133 keV K electron--486 keV y-ray and the 133 keV y-ray--486 keV K electron coincidences can also be seen in the figure although the 133 keV and 486 keV gamma rays were not resolved from the 124 keV and 496 keV y-rays, respectively, by the scintillation counter.
4. Analysis of the data
In order to maximize the efficiency of the statistics, the data were analysed by a method due to White s). The experimentally obtained quantities W(O~) taken at angles 90 °, 135 ° and 180° were used to solve the Legendre polynomial series W(O~) = C o + C2P2(cos 0~) + C4P4(cos 0i),
i = 1, 2, 3.
The coefficients C2 and C4 for both e-y and y-e angular correlation functions and the errors in these coefficients were determined by a computer. These were then corrected for finite solid angle using the tables of Yates 6) and are given in table 1. The TABLE l
Experimental values of An and A** coefficients
A.i Aa
Electron-gamma
Gamma-electron
--0.1134-0.031 q-0.140q-0.036
--0.0124-0.012 +0.0144-0.014
coefficients A22 and A44 thus obtained were used t o plot the curves shown in fig. 2 at 5 ° interval. An independent check on each of these curves was made by finding the least-squares fit of the seven experimental points. The two curves were found to be similar.
e-7 AND y-e CORRELATIONS
45
5. Interpretation of the results The main features of our results (table 1) are that the coefficients A~4 are greater than A22 and although the ?-e correlation is negligibly small, we can, generally, say (this is evident from fig. 2) that A22 values are negative while A44 values are positive and have non-zero values, which excludes pure dipole radiations for the cascade. This also means that the intermediate level has spin greater than 3. 1.10496
K -
1247"
496
7" "
124 K
1.00.
0')
v
0.90-
1.10-
1.00
0.90' 90
120
150
180
0 (degrees) Fig. 2.
W(O)/W(90)versus 0. The
dotted lines represent the best fit curves to the experimental points
and their errors. From shell-model considerations, and from the log f i values determined by Kelly and Horen x) the spins and parities of x31Cs are limited to the values <~-+ for the 620 keV level and to < ~+ for the 124 keV level. The ground state of x3~Cs is known 7) to be definitely ~z+. The K conversion coefficient determined by Kelly and Horen 1) suggests the 496 keV transition to be predominantly M 1 with a possible small mixture of E2 while the measurement 1) of L-subshell ratios (L~/L./LI.) for the 124 keV transition supports an E2 transition with a possible 8 ~o admixture of M1 transition. The theoretical values of A22 coefficients were calculated for all possible spin assignments, within the limits prescribed by the shell model and for all possible values of mixing of the 496 keV transition and using 8 ~o admixture 1) (62 = +0.28) of M1 radiation in the predominantly E2 radiation of the 124 keV transition. The spin sequence 3 + ~ ~+ ~ ~+ is the only one which gave the coefficients /144 positive for both e-? and ?-e angular correlations. This spin sequence is in agreement with the ?-?
46
S. K. SEN AND D. A. DOHAN
angular correlation measurements of Haskins 3). The sign and the magnitude of the A22 coefficient for the e-~ experiment also agree with this spin sequence if the mixing ratio 62 for the 124 keV radiation is taken as -0.28 and the mixing ratio 6~ for the 496 keV transition is -~v.z,~_oilo. . . . . +o ~2 Such value of 61 corresponds to a 23_+7~o of E2 admixture to the predominantly M1 transition of 496 keV and gives the value of ~tK still in agreement with the measurement of Kelly and I-Ioren ~). For the A22 coefficient of the e-~ experiment, we used the relation a22(el~2)
=
h2(el)A2(~2),
where according to Church et al. s) m
A2(el)
=
m
b2 F2
2 •
•
(Ya)+ 2pI b2F2(~I) +Pl bz F~(~I) 1 +p21
e
a2(Y2) = F~ (Y2)+ 262 F2(Y2) +
2 m
6 2 F 2 (]~2)
1+6 Here the b2 are particle parameters, and Pl is given by
The K conversion coefficients ctK(M1) and ~K(E2) for M1 and E2 transitions, respectively, are determined by graphical interpolation of the values given by Sliv and Band 9). The theoretical value of the A22 coefficient for the y-e correlation was found to be correctly negative for the spin sequence ½ + ~ { + ~ ½+ with 61-- -0.28 and 62 = +0.54. But its magnitude is larger than that of A22(yte2) contrary to the experimental finding. This discrepancy can, however, be removed only if an admixture of electric monopole transitions E0 in competition with M 1 and E2 electric transitions for the 124 keV radiation is introduced s). We can then write, A22(yIe2)
=
A2(~l)A2(e2)
,
where s) e
A2(e2)
=
e
2 m
m
b2F~(~2)+2p2b2F2(~2)+p2b2 F~ i +
()~2)+q2b
0
Here p~ = 6~(~K(MI)/~K(E2)), q~ is the ratio of the E0 conversion electron to the E2 conversion electron intensities and q2bo the EO-E2 interference term which, in this case, is positive so that A2(e2) has a small negative value. The values of the particle parameters b 2 were obtained from the graphical extrapolations of the values given by Biedenharn and Rose lo) and are given in table 2. The F-coefficients which are functions of the level spins/1, I and 12 and of the multipolarities L1, L~, L 2, L~ (inset fig. 1) were obtained from the tables of Ferentz and Rosenzweig 1x).
e-y AND ~,-e CORRELATIONS
47
An estimate of q 22was made by using the relation, A#4(yle2)
--
62 b,].L~.(?2) . 1+62 F~(?I) 1 .q P 22.[ q22
The particle parameter b~ was calculated from the recurrence formula 1o) b,~ = (1.4-b~)2.5. The value of q22 thus obtained is -,.,,-4.s.a ~+~.s Our experiment, because of small ?-e angular correlation, does not give the value of q~ with a better degree of accuracy. TABLE 2 Particle p a r a m e t e r values
bam b~ b2e
124 keV transition
496 keV transition
0.17 0.06 1.91
0.49 0.51 1.51
However, we can say that it indicates strongly the presence of monopole electric transition. Support for our observation of the existence of a non-vanishing q 22can be obtained independently by using the values of ~xp and of 622for the 124 keV transition by Kelly and Horen 1) in the expression ~¢xp = ~K(E2)(1 +q~)+622~K(M1)
i+6 and finding that the experimental values ~exP and 6~ of Kelly and Horen, are compatible only if a mean value of q~ = 0.13 is introduced. The comparison of the experimental value of A44(el 72) with the relation, Aa~(el?2)_
p2 b~F~(?l)F~(?2) 1 +P2
1 + 62
yielded the correct sign of b~ for the 496 keV transition, but gave too large a value. This is not well understood. The e-? measurements were repeated after the ?-e measurements and the same result was obtained. Moreover the fractional error in the e-y measurements was better than ?-e measurements and the results are considered to be genuine. 6. External perturbations The source was sublimed on to an aluminium foil and hence the attenuation of angular correlation due to hyperfine structure interaction was considered to be absent.
48
s.K. SEN AND D. A. DOHAN
The 124 keV level of 1alCs has a half-life ~ 4 nsec and hence an attenuation of the angular correlation might be expected due to quadrupole interaction. The experiments were performed with sources in two different physical forms, namely (i) evaporated drop source and (ii) sublimed source, as so to introduce quaddrupole interaction, if present, in different strengths. However, we found that the values of A22 and A44 in both cases agreed within the experimental errors, and therefore we assumed that the attenuation effect, if present, is very small. Furthermore, any correction for the attenuation of the directional correlations would tend to enlarge the discrepancy in the results of the A44 coefficient in the e-v experiment. 7. Summary and discussion In these measurements, a high-resolution electron detector system was used so that K electrons were completely separated from the L electrons. This also enabled us to eliminate the effect from the interfering cascade 486-133 keV which was present in the ?-? angular correlation measurements of the previous workers 2, 3). The desirable feature of the measurement of the directional correlation involving conversion electrons is that it is more sensitive to the minute admixture of multipole radiations than are the ?-? correlations. The very fact that the detectors acted as particle identifiers resulted in the interference between the counters, such as backscattering and Compton events which may distort the angular correlation, being much reduced. The present work clearly establishes that the 496 keV transition is not a pure M1 transition but has an admixture of E2 transition, and also suggests the existence of electric monopole transitions in the internal conversion channel between the 124 keV level and the ground level. These new observations are consistent with those of Kelly and Horen 1) made from the measurements of the conversion coetticients. The measurements of the 7-e and e-v angular correlation functions helped to limit the choice of spin sequences to a small number. The authors acknowledge gratefully the support from the National Research Council of Canada and wish to thank Mr. S. I. H. Rizvi for checking the numerical calculations.
References 1) 2) 3) 4) 5) 6) 7) 8) 9) 10) 11)
W. H. Kelly and D. J. Horen, Nuclear Physics 47 (1963) 454 T. Lindqvist and E. Karlsson, Ark. Fys. 12 (1957) 519 J. R. Haskins, ARGMA-Tr-IC32, No. R (1959) E. Bodenstedt et al., Nuclear Physics 20 (1960) 557 D. H. White, Nucl. Instr. 21 (1963) 209 M. J. L. Yates, in Alpha-, beta- and gamma-ray spectroscopy, Vol. 2, ed. by K. Siegbahn (North-Holland Publ. Co., Amsterdam, 1965) E. H. Bellamy and K. F. Smith, Phil. Mag. 44 (1953) 33; W. A. Nierenberg et aL, Bull. Am. Phys. Soc. 1 (1956) 343 E. L. Church, M. E. Rose and J. Weneser, Phys. Rev. 109 (1958) 1299 L. A. Sliv and I. M. Band, in Alpha-, beta- and gamma-ray spectroscopy, Vol. 2, op. cit. L. C. Biedenharn and M. E. Rose, Revs. Mod. Phys. 25 (1953) 729 H. Ferentz and N. Rosenzweig, in Alpha-, beta- and gamma-ray spectroscopy, Vol. 2, op. cit.