Energy dependence of the beta asymmetry in the decay of 8Li

Nuclear Physics A483 (1988) 1-8 Nosh-Holland, Amsterdam

ENERGY DEPENDENCE

OF THE BETA ASYMMETRY IN THE DECAY OF *Li

J.R. HALL’, D.L. CLARK’, S.J. FREEDMAN3

and S.S. HANNA

De~ar~~enc of Physics, Stanfard Univ~rsiiy, Stanford, CA 94305, USA

Received 16 September 1986 (Revised 4 January 1988) Abstract: The energy dependence of the asymmetry in the beta decay of ‘Li to the first excited state of *Be has been measured. Expectations based on the conserved-vector-current hypothesis and the spherical she11model are in good agreement with the measured asymmetry for beta energies above =5 MeV. No second class currents are needed to explain the data but the present experiment is not a very sensitive test.

E

There are now several measurements of the energy dependence of the angular d~st~bution of beta particles from vector polarized and tensor-aligned nuclei sensitive enough to detect recoil order terms in beta decay ‘). We have used a different method “) for producing polarized beta emitters, applied here to study recoil order terms in the beta decay asymmetry of ‘Li. Polarized *Li is produced in a reaction involving a polarized deuteron projectile. This method produces a copious supply of ==12% polarized *Li appropriate for the present experiment which requires good statistical accuracy. The angular distribution of the emitted betas relative to the axis of vector polarization has the form W~e)~l+(~/~)~A~{l+S(~)]cose,

(1)

where P is the polarization of the parent nuclei, A0 is the usual beta decay asymmetry factor in the nuclear allowed approximation (A0 = -f for *Li decay), z, is the velocity of the beta particle, and S(E) is the energy-dependent quantity of interest here containing terms reflecting properties of the weak interaction and the internal structures of the parent and daughter nuclei 3). We do not directly measure the ’ Present address: Physics Department, Brookhaven National Laboratory, Upton, NY 11973. ’ Present address: Research and Engineering, Eastman Kodak Co., Rochester, NY 14650. 3 Present address: Physics Division, Argonne National Laboratory, Argonne, IL60439-4843. 0375-9474/88/%03.50 @ Elsevier Science Publishers B.V. (Noah-Holland Physics ~blishi~g Division) June 1988

2

J.R. Heir et al. / Energy ~e~e~de~ce

parent nucleus polarization. Since the major part of the asymmetry comes from A0 the polarization can be estimated by neglecting S(E). Our final analysis of the dependence of the asymmetry with energy, to study S(E), is independent of the actual value of the polarization. The smallness of S(E) implies that very high precision is needed in the measurement. The theoretical form of S(E) is given in the work of Holstein “). In the mass-8 system, there are mirror beta decays of 8Li and ‘B to a broad (2+, 2.9 MeV) state of *Be which then decays into two alpha particles. The half-life of *Li is 844 ms and the nominal end-point kinetic energy of the decay is about 13 MeV. Since the final state is broad the spectral shape of the beta particles deviates significantly from the allowed form. The beta-alpha correlation in ‘Li was originahy measured “) to verify the first-order isotropy expected for allowed decay. Later the correlations for both ‘Li and *B were measured “) more precisely to be sensitive to recoil as a test of the conserved-vector-current hypothesis (CVC). There have since been very precise 6*7)measurements of these two angular correlations along with measurements of the analog gamma decay “) as tests of CVC and for the possibility of second-class-currents (SCC’s) in the weak interaction. The accumulated evidence is consistent with CVC and no SCC’s. The first observation of the beta asymmetry from 8Li decay employed polarized neutron capture on ‘Li [ref. “)I. The early experiments using this technique were only sensitive enough to detect the gross asymmetry. The present investigation was undertaken to detect the recoil order terms in the beta asymmetry. The experimental procedure consisted in measuring the beta spectrum in two detectors positioned at 0 and 180” with respect to the polarization direction of the 8Li nuclei. Spectra were obtained for two opposite vector polarizations. Computing the asymmet~ from the results for these two measurements for each detector separately eliminates the problem of gain matching the two detectors precisely. The final result combines the asymmetries from the two detectors. The vector polarized ‘Li source was produced by accelerating a 6-MeV beam of polarized (0.45 < Pd < 0.50) deuterons from the Stanford Tandem onto a target of enriched ‘Li metal. Polarization was transferred to *Li nuclei via the ‘Li(~,~)*Li reaction. The target was thick enough (35 mg/cm’) to stop the recoil *Li nuclei. Selection of the host medium, isotopically pure ‘Li metal in this case, is important in preserving nuclear polarization. The deuteron beam was interrupted periodically by a mechanical chopper. While the beam was off the data collection system was active as the nuclei in the target decayed. During the beam-on portion of the cycle the data collection system and the total energy detectors were made inactive. The polarization of the beam was reversed after a preset number of beam cycles. The two identical beta detector systems consisted of a 15.2 cm diam x 11.4 cm thick total energy E detector, a 0.95 cm diam x 0.100 cm thick AE detector, and an active collimator-antibackscattering detector, all made of NE102 plastic. One of the detec-

J. R. Hall et al. / Energy dependence

tor assemblies

is shown

in fig. 1. A stainless

3

steel target

chamber

provided

two

0.0025 cm Al beta windows (top and bottom). The target was cooled with liquid nitrogen to help preserve the polarization. A magnetic field was maintained at the source perature

(target)

by coils placed

a field

above

and below.

of 300 G was sufficient

It was found

to preserve

that at room tem-

the polarization

for times

comparable to the nuclear half-life, but at liquid nitrogen temperatures a much lower field of only 10 G was adequate. A valid beta event consisted of an E signal, not vetoed by pile-up, in coincidence with a AE signal not vetoed by the active collimator. The (AE, E) coincidence

I

48

cm

LIGHT

PIPE

E DETECTOR

BACKSCATTER ANTICOINC DETECTOR

1

I

~ACRYUC

LIGHT

LEAD SHIEL

BEAM WINDOWJ

Fig. 1. Target

chamber

and one of the detector asymmetry

hAGNETYOKE TARGET

assemblies in nuclear

used to detect the energy P-decay.

dependence

of the

4

.l. R. Hall et al. f Energy dependence

requirement disc~minated against gammas and helped to define the beta’s direction. The active collimator veto localized events to the center of the E-detector and discriminated against betas which back-scattered out of the E-detector. The alignment of the above elements ensured, to the extent that multiple scattering can be neglected, that events could not originate from the walls of the target chamber. The veto pile-up signal was generated whenever any two E-detector pulses above a minimum size occurred within 1.1 b.s of each other. The gains of the E-detectors were stabilized with light emitting diodes (LED). The online computer system identified the incoming signals (E-detector or LED, top or bottom detector, polarization state, period in beam cycle, etc.), performed gain stabilization (based on the LED pulses), and stored the processed data. Both gain corrected spectra and gain-uncorrected spectra were collected. It should be noted that gain shifts correlated with count rate are potentially serious systematic effects, but it turned out that the gain-stabilized and gain-unstabilized data gave consistent results. The data acquisition was divided into four successive time periods after irradiation to study possible time-dependent systematic effects arising from the changing count rate or from the beta decay of impurities. The principal effect of computer dead times resulting from the systematic count rate differences for the two different polarizations is an energy-independent shift in the observed asymmetry. The effect is visible in our data because the ADC for the top detector is serviced with higher priority and thus there is a difference in the gross asymmetry determined by the two detectors. However, to the precision of the present experiment, dead time effects have a negligible effect on the energy dependence of the asymmetry. The detector systems were energy calibrated by fitting to an effectively unpolarized *Li beta decay spectrum (obtained by adding the spectra for the two opposite polarizations). To ensure the reliability of the resuhs, severai tests were carried out: (i) All beta spectra were satisfactorily fitted by a theoretical beta spectrum. The simpler allowed shape from 12B decay was also observed with the same apparatus. (ii) As can be seen in fig. 2, the raw asymmetry curves from the two independent detectors were visually the same, except for a small energy-independent displacement between the curves that is due to the dead-time effect described above. (iii) Similarly, the asymmetry curves were independent of the time after irradiation as was evident in the data from the four successive time intervals. (iv) As a second check on systematic gain shifts, the data from the two polarization states (-I- and -) in each time interval were summed and these “unpolarized” spectra for different time intervals were compared; no energy dependence was observed. (v) As a third check, pseudo asymmetry curves were generated by producing unpolarized ‘Li nuclei but alternatety varying the length of the beam-on periods in a long-shop-long-shod . . . sequence. Asymmetries of about 5% were generated that were independent of energy. (vi) A contaminant beta decay was looked for by appropriately normalizing unpolarized data from the fourth time interval and subtracting it from unpoiarized data from

J.R. Hall et al. / Energy dependence

5

2.5

l Top o Bottom X Top-Bottom

0

t

b

-I 0.4

1

It _____2__~_________i__ $,t, **r,‘tpit

OS2

0

I

'-0.2 5

IO

15

20

25

E/me Fig. 2. The asymmetry measured in the P-decay from sLi as a function of total energy. The data are normalized to the energy averaged asymmetry. The small shift in the data between the top and bottom detectors is due to a small systematic dead time difference arising from differences in the interrupt priorities of the two ADC’s. These differences should be energy independent as demonstrated by the difference data.

the first interval. was observed. There remains

Only a small beta-decay the question

contaminant

below 2 MeV, probably

of the fall-off of the asymmetry

17F,

below the theoretical

expectation of a U/C factor at energies below about 4 MeV (see fig. 2). We attribute this to systematic effects in the experiment. A small fall-off is expected from contamination beta decays with end point energies, but as noted above, no serious contamination was found, Instead, the bulk of the effect is due to multiple scattering of low energy betas in the target, the detectors, and surrounding material (primarily the steel target chamber). Since the detector solid angles are small (~1% of 4n), the effects of multiple scattering can be substantial, particularly at low energies. While we can qualitatively explain the consequence of multiple scattering a detailed calculation is very sensitive to the assumed geometry. However, in essentially all models of low energy electron plural scattering one expects distortions which

6

J. R. Halt et al. ,I Energy ~e~ende~ee

go like l/p2 where p is the electron momentum. Thus, to account for multiple scattering in the asymmetry we introduce a correction factor with the simple form (1 - C/$). Here C is a constant chosen to fit the low-energy asymmet~ data. The effect of applying this empirical correction factor can be seen in fig. 3b, where the final data have been fitted with various theoretical curves based on eq. (1) (discussed below) and the multiple scattering correction factor. It is emphasized that this factor has a negligible effect for E / m, B 10 and our conclusions are unchanged if we concentrate only on these data, and use no scattering correction as in fig. 3. The final data presented in fig. 3 are the sum of six separate runs and contain 2.9 x lo9 beta counts. The errors are statistical; systematic errors and a *5% uncertainty in the energy calibration are not included.

K”RATH ---BAKER -BOYARKINO

4

3c 1

OW E/m,

Fig. 3. The asymmetry in the P-decay of *Li compared to calculations made with the different nuclear wave functions indicated. In (a) the data are fit above E/M,= 13 with the normalization as a free parameter. In (b) a multiple scattering correction is included (see text). The error bars are statistical; systematic errors and a *5% unce~ainty in the energy calibration are not included.

7

J. R. Hall et al. / Energy dependence

Using the formalism present

measurement,

of ref. 3, and including

only terms that are significant

in the

we have

A(E)=A,(l+S(E))=-f $/g j, +- Azcmz (E*-iEEo) ”

1

(2)

,

where E(E,) is the total beta (end-point) energy, m, is the nucleon mass, A = 8, and c is the Gamow-Teller matrix element which is given by the measured ft,,, value lo). The induced form factors, at zero momentum transfer, are represented by the constants b, 6,) d, and j2. At the level of the present comparison with theory we can neglect the complications from the width of the final state in *Be. We assume CVC and the values of b and 6r are obtained by using the measured values of r,, and T,,/T,, for the analog electromagnetic transitions in ‘Be [ref. “)I. The induced coupling d is generated by both first-class and second-class currents. We assume no second-class currents (d,, = 0) and calculate d, and the second forbidden coupling j, from the various shell-model wave functions discussed in refs. 6,7). The results are listed in table 1. The dominant contribution to the energy dependence of the asymmetry is from the term with j, in eq. (2); the magnetic dipole and electric quadrupole terms (terms with b and 6,) from CVC largely cancel. Fig. 3 shows fits to the various shell model predictions and the data; the empirical multiple scattering correction discussed above is included. Aside from the multiple scattering parameter C, the only free parameter is the polarization. TABLE

Shell model

Barker “) Boyarkina b, Kurath ‘)

predictions

1

for j,/A*c

and d,/Ac

jJA2c

d,lAc

-351.59 -452.918 -310.578

5.10 4.89 4.41

“) Ref. I’). b, Ref. “). ‘) Ref. 13).

All the models give reasonable fits to the measured beta asymmetry. The prediction of the Boyarkina model seems favored but the statistical and systematic errors are too large to draw a clear conclusion. The empirical multiple scattering correction seems to describe the deviation of the data at low energy. More precise data for the energy dependence of the beta-decay asymmetry in both *Li and 8B decay can be used in combination with other measurements to understand more completely the magnitudes of all the induced couplings in the

8

J.R. Hall et al. / Energy dependence

mass-8 system. For example, the beta-alpha correlation is sensitive to other combinations of the same induced terms discussed here. We believe that the systematics associated with multiple scattering in the present experiment can be significantly reduced by careful attention to geometry and construction materials. We acknowledge the support of the US National Science Foundation. References 1) K. Sugimoto, I. Tanihata and J. Goring, Phys. Rev. Lett. 34 (1975) 1533; F.P. Calaprice, S.J. Freedman, W.C. Mead and H.C. Vantine, Phys. Rev. Lett. 35 (1975) 1566; P. Lebrun, Ph. Deschepper, L. Grenacs, J. Lehmann, C. Leroy, L. Palffy, A. Possoz and A. Maio, Phys. Rev. Lett. 40 (1978) 302; H. Brindle, L. Grenacs, J. Lang, L.Ph. Roesch, V.L. Telegdi, P. Truttmann, A. Weiss and A. Zehnder, Phys. Rev. Lett. 40 (1978) 306; Y. Masuda, T. Minamisono, Y. Nojiri and K. Sugimoto, Phys. Rev. Lett. 43 (1979) 1083; T. Minamisono, K. Matsuta, Y. Nojiri and K. Takegama, J. Phys. Sot. Jpn. 55 (1986) Suppl. p. 382 2) T. Minamisono, J.W. Hugg, D.G. Mavis, T.K. Saylor, S.M. Lazarus, H.F. Glavish and S.S. Hanna, Phys. Rev. Lett. 34 (1975) 1465 3) B.R. Holstein, Rev. Mod. Phys. 46 (1974) 789 4) S.S. Hanna, E.C. LaVier and C.M. Class, Phys. Rev. 95 (1954) 110 5) M.E. Norberg, F.B. Morinigo and C.A. Barnes, Phys. Rev. 125 (1962) 321 6) R.E. Tribble and G.T. Garvey, Phys. Rev. Cl2 (1975) 967 7) R.D. McKeown, G.T. Garvey and C.A. Gagliardi, Phys. Rev. C22 (1980) 738; and Phys. Rev. C26 (1982) 2336 8) T.J. Bowles and G.T. Garvey, Phys. Rev. Cl8 (1978) 1447; C26 (1982) 2336 9) M.T. Burgy, W.C. Davidon, T.B. Novey, G.J. Perlow and R. Ringo, Bull. Am. Phys. Sot. 2 (1957) 206; A.H. Wapstra and D.W. Connor, Nucl. Phys. 22 (1961) 336 10) F.Ajzenberg-Selove, Nucl. Phys. A413 (1984) 1 11) F.C. Barker, Nucl. Phys. 83 (1966) 418 12) A.N. Boyarkina, Izv. Akad. Nauk USSR (ser. fiz.) 28 (1964) 327 13) S. Cohen and D. Kurath, Nucl. Phys. 73 (1965) 1