Beta-gamma circular polarization correlation in the decay of Sc46

[

4.E

l

Nuclear Physics

43 (1963) 537--546; ( ~

North-Holland Publishing Co., Amsterdam

Not to be reproduced by photoprint or microfilm without written permission from the publisher

BETA-GAMMA CIRCULAR POLARIZATION CORRELATION IN THE DECAY O F Se 46 R. M. S I N G R U t and R. M. STEFFEN

Department of Physics, Purdue University, Lafayette, Indiana tt Received 5 February 1963 Abstract: The fl'7 circular polarization correlation in the decay of SO 6 was studied. The source (ScCI,) was placed in an evacuated chamber and the measurements were m a d e by employing four beta detectors at different angles. The value of the asymmetry parameter A in the fl-7 circular polarization correlation expression W(O,"r)= 1+ (v/c)'rA cos 0 was found to be A = 0.194-0.03. This confirmed the existence of a large interference of the Fermi and GamowTeller components with CvMF/CAMGT = --0.154-0.05. Analysis of VW directional correlation measurements in Ti 46 indicates that static interactions cannot be held responsible for the attenuation of the fl-F circular polarization correlation observed previously.

1. Introduction

The beta-gamma circular polarization in the decay o f SC46 has been studied by various groups 1-7). Earlier work reported before 1959 by Boehm and Wapstra 1), Lundby, Patro and Stroot 2), Jiingst and Schopper a) and Steffen 4) gave fairly consistent results indicating a fl-~ circular polarization correlation parameter A > 0.24 This result implied the existence of a large term due to the interference of the Fermi and Gamow-Teller components in the fl-decay of Sc46. Recent measurements, however, by Bloom, Mann and Miskel s) and Daniel and Kuntze 6) yielded values of A that were considerably smaller than the previously reported values. These later results excluded any sizeable Fermi contribution in the mixed fl-transition from Sc46. This was interpreted as an example of the validity of the isospin conservation law in fl-decay. Boehm and Rogers 7) re-examined the fl-~ circular polarization correlation of Sc46 and confirmed the existence of a large interference between Fermi and Gamow-Teller interactions. They also found, however, a strong attenuation of the circular polarization correlation in certain chemical compounds of scandium. The present measurement was aimed to resolve the discrepancy among various results. We have obtained a value of A = 0.19±0.03 from our measurements on the scandium-chloride sources. This result again indicates a large Fermi-GamowTeller interference in agreement with the earlier results in this laboratory. t On study leave f r o m Tata Institute of Fundamental Research, Bombay, India. tt This work was supported by the U.S. Atomic Energy Commission. 537

538

R. M. SINGRU AND R, M. STEFFEN

2. Experimental Method The sources of Sc46 were prepared from ScC13 in HC1 solution obtained from Oak Ridge National Laboratory. Evaporation techniques were used to prepare the sources. A small drop of diluted insulin was first placed on a Mylar film of ~ 1.2 mg/cm 2 and the excess insulin was removed. When a drop of active ScC13 solution was placed on the wetted area, it spread uniformly. Afterwards the source was dried gently under an infra-red lamp. Usually 2-3 drops were sufficient to make a strong source. The sources of Co 6° which were used for calibration measurements were prepared from COC12 in HC1 solution obtained from Oak Ridge National Laboratory. Be'~a Counfer Maanet Coils Beta

Source ----.~¢"

~Vo

cuum Chamber

~

Lead Mallory loooMe~al

'5 Cm

Fig. 1. Polarization analyser magnet, radiation detectors and vacuum chamber. The polarization analyser used in our experiment was the one described in refs. 8, 9) with s o m e modifications (see fig. 1). In order to avoid the absorption of the lowenergy fl-rays in the/%decay of Sc 46 and Co 6° an aluminium vacuum chamber was constructed to house the source. The vacuum chamber was provided with four windows which were covered by Mylar film (thickness 3 mg/cmZ). The source was inside the chamber, which had a plexiglass lining to reduce the backscattering from the wails. The magnetic field was reversed every 15 minutes. Another significant feature of our apparatus was that we employed four ]~-detectors, thus measuring the polarization correlation at four different angles 0#r simultaneously. The Pilot B scintillation crystals used as fl-detectors were 3 m m thick and were mounted on lucite light pipes. The length of the light pipes varied from 28 cm to 33 cm. These light pipes, along with additional magnetic shields, kept the relative changes in the singles counting rate to less than 0 . 1 % under reversal of the magnetization of the analyser. Each of the four detectors looked into the window of the chamber. These four detectors were thus set at fixed angles, with the average angle 0#r between the f - r a y and the 7-ray having the values Oar = 105 °, 127 °, 143 ° and 151 °. The 7-detector was the same as described in ref. 8). All the detectors employed R C A 6342A photomultiplier tubes.

THE DECAY

539

OF Sc 4~

The electronics of the apparatus is already described in ref. 8) (see fig: 2). The four //-single-channel analysers were adjusted to accept the pulses corresponding to energies above 150 keV for both measurements on Sc46 and on Co 6°. In these two decays (see fig. 3), two 7-rays of about the same energy are emitted in cascade.

Lso

cChannel o;.o,

..........

Linear Amplifier

I

! Single-Channel Analyser

I Li IFCI

Fost Coincidence

[-~

Discriminator

"--~

Um~ter

~ L~

Circuit

Triple Coincidence Scaler

Circuit

Fig. 2. Block diagram of electronic system for measurement of the beta-gamma circular polarization correlation. 5.2

+ 84 d 4 ~EB=0.557

'

y

= 0.313 MeV /3- 4 + ' ~ ' ~ ~ 2 . 5 0 5 1' MeV

5 + ~ E 8

MeV

~

2.006 MeV

i;&:: ~+

v

0+

T~4e

---=-i-----~1.552

0

Fig. 3. Decay schemes of Sc4~ and Co6°. The single-channel analyser in the 7 channel was set to accept the composite scattered peak of these cascade 7-rays. The resolving time at which the four fast coincidence circuits operated, was 10 ns. The data were recorded automatically on IBM cards. The automatic card punch instrument is described in ref. i o). The data on the IBM cards were processed on an IBM 7090 computer. We did not attempt to measure the absolute polarization of the Sc46 "~-rays. Instead we made a relative measurement with Co 6° as a standard. The Sc46 (or Co 6°)

540

R. M. SINGRU AND R. M. STEFFEN

data were recorded continuously for a period of about ten days using fresh sources for each period. The total period of measurement extended over a period of six months. 3. Evaluation of the Data

The experimental data were evaluated to yield a quantity 5 defined by N+ - N -

5 - - N++N - '

(1)

where N + and N - are the corrected fl-? coincidence rates observed with the magnetic field in the ( + ) and ( - ) direction, respectively. The observed coincidence counting rates were divided by the product (Sa St) of the singles counting rates for normalization. They were also corrected for the presence of true 7-? coincidences and for random coincidences including higher order coincidences 11). The degree of circular polarization Pc(Oar ) of the ?-rays following the t-decay is related to ~ by the equation

5 = Pc(Oar)E(hv),

(2)

where E(hv) is the circular polarization efficiency of the analyser 12). The angular correlation between the electron and the photon in an allowed fl-? cascade is described by W(Oar ) = 1 + ~(v/c)A cos Oar.

(3)

In eq. (3), z describes the sense of circular polarization of the detected ?-ray, v/c is the ratio of the velocity of the detected electron to that of light, A is the asymmetry parameter and Oar is the angle between the fl and ? particles. The circular polarization of the ?-ray is given by 12)

Pc (Opt) = A(v/c)cos Oar

(4)

Combining eq. (4) with eq. (2), we have

6 = E(hv)A(v/c)cos Oer.

(5)

In our experiment 5 was measured separately for Sc46 and for Co 6°, the angles 0pr remaining the same. Hence we have a(Sc 46) = E(Sc46)A(Sc46)(v/c)l cos Oer , a(Co

=

cos 0p,,

(6) (7)

where the subscripts 1 and 2 refer to Sc46 and Co 6°, respectively. Comparing eqs. (6) and (7),

A(Sc,6) = E(Co6°) (vlc)2 a(Sc") a(CorO). E(Sc"6) a(Co

(S)

541

THE DECAY OF Sc46

The efficiency E(hv) can be calculated 12) from the known geometry of the apparatus and from the Compton scattering cross-sections. For our apparatus E(C°6°) - 1.145

(9)

(sc for the average ~-ray energy E~, being E 7 = 1.25 MeV and E 7 = 1.0 MeV for Co 6° and Sc.6, respectively.

In our experiment,

(v/C)z =

(v/oh

0.98.

(10)

We further assume that for the pure Gamow-Teller

(AJ =

1) transition in Co 6°

A -- -½,

(11)

in conformity with our present knowledge 13). Combining this information with eq. (8), we obtain A(Sc 46) = - 0 . 3 7 4 ~(Sc46)

(12)

~(Co6°) "

Thus the experimental determination of ~ for Sc46 and Co 6° yields the value of A for Sc46. For a particular angle we paired the periods of measurements of Sc46 and Co 6° which immediately followed each other and for each such pair we calculated the value of A(Sc46). Finally for each angle we found the weighted average of all such pairs over the period of six months. The results are presented in table 1. The final weighted average for all the angles and all the weeks is A = 0.19+0.03. As a check of our instrument, we examined the values of fi obtained for Co 60 alone. All these values yielded the parameter A within 10 % of the expected value

A=-½. TABLE 1 T a b u l a t i o n o f o u r experimental d a t a o n Sc 4° fl-Deteetor

0#~

A

1

151°4-7 °

0.197~:0.051

2

105°-4-20 °

0.193:k0.221 a)

3

127°:k 18 °

0.156:k0.064

4

1430116 °

0.201 :k0.044

W e i g h t e d average for all angles

0.19 :k0.03

a) T h e error at this angle is larger o w i n g to a smaller c o u n t i n g rate a n d a smaller value o f ~(Co°°).

542

R. M. SINGRU AND R. M. STEFFEN

4. Discussion F o r a b e t a - g a m m a cascade 11 ~ 12 -~ I 3 involving a g a m m a transition o f p u r e m u l t i p o l a r i t y L a n d for the special case 11 = I2(AI = 0 in the fl-decay), the a s y m m e t r y p a r a m e t e r A can be expressed as 1,)

A-

L~-I

--2\-~-2

1

]

(13)

CAMcTA

\CAMGT] where we have a s s u m e d t h a t the fl-decay interaction is V - A a n d t h a t the t w o - c o m p o n e n t t h e o r y 4) o f the n e u t r i n o is valid (C~ = Cx, Cs = CT = Cp = 0). In b o t h the decays Sc 46 ~ Ti 46 a n d C o 6° ~ N i 6°, we have a triple cascade t r a n s i t i o n o f the type I1 (fl) 2L(Vl) L(?2) 0 with p u r e electric q u a d r u p o l e (L = 2) g a m m a rays. F o r such a special case, the b e t a - g a m m a circular p o l a r i z a t i o n c o r r e l a t i o n o f the fl-72 p a i r is the same as t h a t o f the fl-Vt p a i r i s ) . F o r the case o f Sc 46, i f y = CvMF/CAMGT, 1 A = ½[0.25-2.24y]--. l+y 2

(14)

Fig. 4 shows a p l o t o f A as a function o f y . O u r value o f A = 0.19+_0.03 results in a value o f

CvMF -CAMGT

y -- - -

0.15--+0.05.

I n table 2 we present the s u m m a r y o f the experimental results on Sc .6 b y other workers. TABLE 2

Comparison of various values of the asymmetry parameter A Ref.

A

Cv MF

Y

CAM~T

Boehm and Wapstra 1)

0.27 ±0.09 a)

--0.284-0.14

Lundby, Patro and Stroot 2)

0.29 4-0.11

--0.324-0.14

Jtingst and Schopper *)

0.24 ___0.04

--0.224-0.08

Steffen 4)

0.24 ±0.02

--0.224-0.04

Bloom, Mann and Miskel 5)

0.075±0.017

4-0.024-0.025

Daniel and Kuntze n)

0.10 4-0.02

--0.024-0.02

Boehm and Rogers ~)

0.2154-0.019 0.201 4-0.03 b)

--0.194-0.03 --0.174-0.04

Present Work

0.19 4-0.03

--0.15~:0.05

a) This value is obtained from ref. 1) by taking the ratio of Sc~s, Co 6° polarization reported therein b) The value quoted is for chloride sources.

THE DECAY OF Sc4s

54~

Our value of A for the chloride sources is in excellent agreement with the new results of Boehm and Rogers. Our present result, which indicates a large interference between the Fermi and Gamow-Teller components, cannot be reconciled',with those of Bloom et al. or Daniel et aL The chemical state of the scandium source used by Bloom et al. is not specified in their report s). Daniel et al. 6) have used a chloride source of scandium. The extensive work by Boehm and Rogers who investigated the effect of different chemical states of the source on the circular polarization correlation revealed that there is an attenuation in the oxide sources. They also found an attenuation in the chloride,

:"iiii -°.5

iI\ /

cM

/

_

0!5

.X ,

-°""

,.o C,MF

\

-0.$ -

YcA.-% ~

Fig. 4. Experimental result for SOa. The curve is the relation in eq. (14) between/t and y ~

CvMF/CAMGT.

sulphate and nitrate sources owing to the hydrolization of the ionic compounds. Our measurements were performed with ScC13 sources in vacuum, thus reducing the possibility of hydrolization. N o indication of an attenuation of the fi-7 circular polarization correlation was found in our measurements. In order to get some more information on the possibility of extranuclear perturbations of the fl-V circular polarization, the directional correlation of the Ti 46 ~-7 cascade following the fl-decay of Sc46 was studied with respect to possible extranuclear interactions in the first excited 2+(0.887 MeV) state of Ti 46. Although the critical intermediate state for an attenuation of the Sc 46 fi-V circular polarization correlation is the second excited 4 + (2.006 MeV) state of Ti 46, some knowledge of the extranuclear perturbation in the first excited 2 + state of Ti 46 may shed some light on the possible perturbation mechanism in the 4 + state. The advantage of studying extranuclear perturbations by means of ~-7 directional correlation measurements is the greater accuracy of such measurements. The error in directional correlation measurements is several orders of magnitude smaller as compared to the error in circular polarization correlation measurements.

544

R. M. SINGRU AND R. M. STEFFEN

Using the same type of ScC13 sources as in the/~-7 circular polarization correlation measurements, the anisotropy of the 7-7 directional correlation o f the 4+(7) 2+(?) 0 + cascade in Ti 46 was observed. The equipment used for these measurements and the methods of evaluating the data have been described before 26). The experimentally observed directional correlation is W(0)~xp = 1 +(0.101___0.002)P2(cos 0)+(0.010+0.003)P4(cos 0),

(15)

which is in excellent agreement with the theoretical unperturbed correlation for a 4(E2) 2(E2)0 ?-~ cascade

W(O)th =

1 + 0.102P2(cos 0) + 0.009P,(cos 0).

(16)

The theoretical anisotropy for the unperturbed directional correlation is Ath = Wth(180°)--Wth(90°) = 0.16667.

(17)

W,h(90°) The anisotropy of the Ti 46 7-? directional correlation can be measured with higher precision than the individual coeffticents of the correlation function. Our measurements yielded the result A(Ti*6)exp = 0.1673___0.0016. As a check of the equipment and the method of correcting for the finite solid angles o f the detectors the directional correlation and the anisotropy of the Co 6° ---}Ni 6° 7-~ cascade, which follows the same decay scheme as the Sc 46 ~ Ti 46 ~-7 cascade was also measured. The anisotropy of the Ni 6 o 7-? directional correlation was observed as A e x p ( C o 60) = 0 . 1 6 7 1 + 0 . 0 0 1 1 ,

in excellent agreement with the predicted value. F r o m the experimentally determined value of the S c * 6 ~ Ti 46 ?-7 directional correlation anisotropy, a lower limit of the attenuation factor G2(1) for the first excited Ti .6 state can be extracted G2(1) > 0.99. I f we consider the possibility of a static axially symmetric quadrupole perturbation in the I = 2 state 17, ~s ) of Ti 46, this value of the attenuation factor allows us to compute an upper limit of the interaction parameter x = wE zl, where ~0E is the fundamental quadrupole frequency, and z 1 is the lifetime of the first excited state of Ti 46 x~ = og~'q < 0.01. The lifetime of the first excited state of Sc 46 is 19) zl = 5.5 psec. The lifetime z 2 of the second excited state of Ti 46 (I = 4) is not known. However, an upper limit of 42 < 4.3 x 10 -x~ see, can be obtained from coincidence measurements 2o). Thus

THE DECAY OF S ~ °

545

the quadrupole interaction parameter x2 of this state is "/72

x2 = x l - - < 0.08. With this information we can compute the attenuation factor for the fl-~ circular polarization correlation involving the second excited state of Ti 46 with I = 4, which is given by 21) Gl(x2)=~(4

.'

4

-,

1 )2

.-.'

1

(18) 2'

where E~ are the eigenvahies of the reduced quadrupole interaction Hamiltonian The result of the computation

D = ( 1 / h c o E ) H Q.

GI(x2) > 0.90 indicates that the fi-~ circular polarization correlation cannot be significantly altered by the static quadrupole interaction in a ScC13 source. The considerations above were made under the assumption of an axially symmetric quadrupole interaction. An electric quadrupole interaction of lower symmetry has practically the same effect on the time-integrated correlation as an axially symmetric interaction 22). A similar computation assuming a static paramagnetic hyperfine interaction in the ScC13 sources leads to the same conclusion as the considerations of the electrostatic quadrupole interaction. Boehm and Rogers reported a large attenuation of the fl-? circular polarization correlation in scandium-oxide sources. The attenuation factor was found as small as G1 (2) = 0.43 + 0.15. From ~-7 directional correlation experiments, Boehm and Rogers extracted the value G2(1) >= 0.96 for the attenuation factor in oxide sources. From this value the attenuation coefficient for the fl-~ circular polarization correlation is computed as G1(2) > 0.70, if a static quadrupole interaction is assumed. Thus it seems very unlikely that the attenuation observed by Boehm and Rogers can be attributed to a static quadrupole interaction in the scandium-oxide sources. In fact, it is difficult to explain the perturbation of the Sc46 fl-~ circular polarization correlation by any interaction caused by s t a t i c crystalline or atomic fields at the nucleus. A possible explanation of the observed attenuation effects might be suggested on the basis of an after-effect of the fi-transition, which could, for a short time At << z2, cause very large fields that would be strong enough to perturb the fl-~ correlation significantly. Such an "on-off" interaction 16) would not affect the succeeding 7-~ correlation. The authors are greatly indebted to Dr. S. D. Bloom and Dr. F. Boehm for several discussions and for information about their measurements prior to publication. One of us (R. M. Singru) wishes to express his gratitude to the Purdue Research Foundation for a fellowship during the course of this work.

546

R . M. S I N G R U

AND

R. M. STEFFEN

Note added in proof'. After the completion of this paper, a report of a new series of experiments on the/~-V circular polarization correlation in Sc 46 by H. Daniel et al. (Heidelberg group) was received. These authors have studied the correlation using different chemical compounds of Sc 46. The asymmetry parameter was found to be independent of source composition or time. They report the following value of the asymmetry parameter, A = 0.113___0.008 resulting in CvMF/CAMGT = -0,040-t0.011. Their study of V-V and/~-7 angular correlation in Sc46-Ti 46 indicated no presence of any attenuation, in agreement with our results. We wish to thank the Heidelberg group for communicating their results to us. References 1) 2) 3) 4) 5) 6) 7) 8) 9) 10) 11) 12) 13) 14) 15)

F. Boehm and A. H. Wapstra, Phys. Rev. 109 (1958) 456 Lundby, Patro arid Stroot, Nuovo Cim. 7 (1958) 891 W. JiJngst and H. Schopper, Z. Naturforsch 13a (1958) 505 R. M. Steffen, Phys. Rev. 115 (1959) 980 S. D. Bloom, L. G. Mann and J. A. Miskel, Phys. Rev. 125 (1962) 2021 H. Daniel and M. Kuntze, Z. Phys. 162 (1961) 229 F. Boehm and J. Rogers, Nuclear Physics 33 (1962) 118 P. Alexander and R. M. Steffen, Phys. Rev. 124 (1961) 150 P. Alexander and R. M. Steffen, Phys. Rev. 128 (1962) 1783 P. Alexander, Nucl. Instr. 14 (1961) 288 H. Paul, Nucl. Instr. 9 (1960) 131 H. Schopper, Nucl. Instr. 3 (1958) 158 H. Schopper, Forts. Phys. 8 (1960) 327 Alder, Stech and Winther, Phys. Rev. 107 (1957) 728 M. Morita, Phys. Rev. 107 (1957) 1729; M. Morita and R. S. Morita, Phys. Rev. 109 (1958) 2048 16) R. M. Steffen, Phys. Rev. 103 (1956) 116 17) K. Alder, H. Albers-Schonberg, E. Heer and T. B. Novey, Helv. Phys. Acta 26 (1953) 761 18) A. Abragam and R. V. Pound, Plays. Rev. 92 (1953) 943 19) D. G. Alkhazov, A. P. Grinberg, K. I. Erokhina and I. K. Lemberg, Izvest. Akad. Nauk. SSSR Set. Fiz. 23 (1959) 223 20) R. E. Azuma, Phil. Mag. 46 (1955) 1031 21) K. Alder, E. Matthias, W. Schneider and R. M. Steffen, Phys. Rev. 129 (1963) 1199 22) E. Matthias, W. Schneider and R. M. Steffen, Phys. Lett. 4 (1963) 41