- Email: [email protected]
Measurement of the 7Be branching ratio
Nuclear Physics A414 (1984) 141-150 0 North-Holland Publishing Company
MEASUREMENT
OF THE ‘Be BRANCHING
RATIO
R. T. SKELTON and R. W. KAVANAGH W. K. Kellogg Radiation Laboratory,
California Institute of Technology, Pasadena, California 9112S, USA
Received 2 August 1983 Abstract: The branching ratio in the electron-capture decay of ‘Be to the 478 keV and ground states of ‘Li has been measured employing an activation technique. Three ‘Li targets were bombarded with a proton beam and the neutrons resulting from the ‘Li(p,n)‘Be reaction were counted in 4x geometry. After bombardment, the 478 keV y-activity was determined. To calibrate neutrondetection efiiciency, two -s9Y targets were bombarded, and the resulting neutrons and ensuing 909 keV y-activity counted. Comparison of the neutron yields with the y-activities results in a ‘Be branch of (10.49+0.07)‘A to the 478 keV state of ‘Li.
E
RADIOACTIVITY
‘Be; measured I;.; deduced branches. *‘Zr; measured I;., T,,,. Neutrondetection system.
1. Introduction
A recent measurement by Trautvetter et al. ‘) of the ‘Be branching ratio yielding a result of (15.4 f 0.8) %, markedly higher than the previously accepted value of (10.35 + 0.07) “/, [ref. ‘)I, has prompted several new measurements, the results of which are surveyed in sect. 5. This branching ratio has several important applications, among them the determination of the 3He(cc,y)‘Be cross section by an activitation technique, as has been done recently by several research groups 3- 5). The 3He(a, y)‘Be cross section is a key parameter in the solar-neutrino problem inasmuch as nearly all the neutrinos to which the Brookhaven solar-neutrino detector is sensitive are those arising from a branch involving this reaction. The importance of the ‘Be branching ratio has prompted this attempt to resolve the discrepancy suggested by ref. ‘), and to improve appreciably on the precision with which it is known.
2. Experimental
procedure
Proton beams of 8 to 12 nA at 2.3 MeV from the ONR-Caltech tandem accelerator were used to bombard ‘Li targets; neutrons were counted during 141
142
R. T. Skelton, R. W. Kaoanagh / ‘Be branching ratio
TABLE1 Lithium target parameters LiF target ‘Be nuclei produced ( 106) bombardment energy (MeV) beam current (nA) bombardment duration (h) mean neutron energy (MeV) neutron-counting dead-time correction electronics drift correction 478 keV y statistical uncertainty (%) branching ratio B, (“/,)
1
2
3
1188 2.260 12 6.5 0.392 1.0116 1.000 0.54
1768 2.365 12 9.0 0.484 1.0118 1.008 0.41
1836 2.304 8 9.2 0.43 1 1.0131 1.004 0.45
10.48
10.55
10.43
bombardment and the 478 keV y-activity was subsequently measured. Three different Li targets were used. Each consisted of 50 to 100 pg *cm- 2 of reagentgrade LiF evaporated on 250 pm high-purity Al and then flashed with I rng. cm-’ of gold. The targets were mounted at the end of a monitor tube of 30 mm length and 1.5 mm inside diameter, which made it possible to detect any ‘Be escaping from the target and being deposited on the inside surface of the tube. Bombardment of two preliminary targets on gold backings led to a ‘Be escape of 4% without any gold flash, and 1.2% despite a gold flash of 0.5 mg. cmm2. For the final 3 targets, ‘Be loss was < 0.1 %. Further Li target parameters are listed in table 1. Two semi-thick 89Y targets were bombarded in a very similar configuration at E, = 4.24 MeV, just below the (p, n) threshold for the first excited state of 89Zr. The first target consisted of 5.5 mg . cm -’ 99.9 “/dpurity natural Y flashed with 0.5 mg.cme2 of gold on high-purity Al. The second was made as a Au-Y-Au sandwich, prepared on a glass slide and then peeled off; high-purity Al was used to stop the transmitted beam. The Y-thickness was 4.2 mg . cme2. For both targets, 89Zr loss was c 0.1%. Further Y-target parameters are given in table 2. Neutrons were detected by a system of 12 3He-tilled proportional counters embedded symmetrically in a graphite cube 1.41 m on a side. Neutron count-rates ranged from 6400 to 7200 s-l for all except the first Y-target, for which the rate was 2660 s- 1 ; these count-rates required the small dead-time corrections tabulated. These corrections were determined by bombarding at various beam currents a LiF target which had been carefully prepared for uniformity. The thickness of the Y-targets was determined as follows: after completion of bombardment and counting in the Ge(Li) system, the second (sandwich) target was mounted over a Li target, and the combination was bombarded with protons to determine total proton energy loss by comparison of the beam energy necessary to attain the ‘Li(p, n)‘Be threshold after transmission with the known threshold energy. The total sandwich thickness, correcting for the energy dependence of the stopping
R. T. Skelton, R. W. Kaoanagh TABLE
/ ‘Be branching
ratio
143
2
Yttrium target parameters Y-target
1
2
s9Zr nuclei produced (106) bombardment duration (h) beam current (nA) neutron-counting dead-time factor electronics drift’correction target thickness (keV) mean neutron energy (MeV) 909 keV y statistical uncertainty (“/,) ratio of yield 3.64 MeV to 4.24 MeV ( “/,) b’ emitter neutron background (“/,) total neutron background (“/,)
172.7 2.2 40 1.0024 1.007 275 0.454 0.28 0.43 1.14 1.71
506.4 2.7 110 1.0112 1.028 195 0.483 0.17 0.48 0.52 1.09
12.04
12.00
neutron-detection
system efficiency (“/,)
power, was 215 keV at bombardment energy; this agrees with the value of 225 keV which was determined by weighing, 30 keV of which was Au. As the a’Y(p, n)89Zr excitation function had previously been found to increase smoothly from threshold to E, = 4.25 MeV, the thickness of the first target was calculated from the observed neutron yields. This calculation resulted in a value 20% lower than weighing had indicated ; the calculated value was preferred because weighing was not considered as reliable with the preparation procedure employed for this target. The Y-target thickness is needed only for calculating the neutron-energy spectrum. The smooth increase of the excitation function was put to service in conjunction with the energy loss in the target to produce a neutron energy spectrum peaked toward the highenergy end. A thin Li target produces a similarly shaped spectrum as a result of kinetimatic effects and the forward-angle preference of the ‘Li(p, n)‘Be reaction 6). Fig. 1 shows the calculated neutron spectrum from each of the tive targets employed. Sensitivity of the neutron-detection system to neutron angular distribution was examined with the neutrons from a highly anisotropic ‘Be&, n) source consisting of a 21‘Pb c+source and a flake of Be 2 mm away. At this high neutron energy (N 10 MeV), a 20 % asymmetry between front and back counters was observed. Displacement of an isotropic point source 2.5 cm from the center of the graphite cube produced the same counter asymmetry, but reduced overall efficiency by 5 0.1%. For the lower-energy (N 0.4 MeV) neutrons from ‘Li, there was no detectable counter asymmetry. A slight drift in electronic gains affecting neutron-detection efficiency was noted from one bombardment to another. The drift was monitored by checking response to a 252Cf source prior to each bombardment and correcting the neutron count-rate accordingly. Neutron yield per unit charge was stable for the Li targets, suggesting that the drift is a longer-term effect than the bombardment
R. T. Skelton, R. W. Kavanagh / ‘Be branching ratio
144
I I
2 60
I\ I\ \I
t
$
!
$40 z
9
.r(
$20 7 p:
r
III
OlOO
\I \I \ I I
I I:
I1
200
300
Neutron
400
Energy
500
\I I I \ \
II
600
.-
70
(keV)
Fig. 1. Neutron energy spectra and mean energies for the three Li targets (a, b and c); and the two Ytargets (d and e).
intervals. For calculation purposes, a significant change in efficiency affecting the second Y-bombardment was incorporated into this factor. A failed pre-amplifier component for one of the 3He counters was replaced with one which later was found defective. As a result, a discriminator was rejecting a fraction of the neutron counts from this counter. As the reduced efficiency was not noted until after bombardment, the efficiency was subsequently re-measured several times in the same electronics configuration, and found to be reproducible within the range of drifts noted previously. Comparison of the 89Y(p, n)89Zr neutron counts to the subsequently measured 909 keV y-activity yielded an absolute neutron-detection efficiency of (12.02 f0.08) %. An independent check of the system efficiency was made with the previously mentioned 2s2Cf source which was calibrated by direct comparison to one which had been calibrated by its manufacturer against a *‘*Cf source from the National Bureau of Standards; the efficiency was (12.47+0.40)“,, for the 2s2Cf neutron spectrum, which has an average energy of 2.348 MeV; the uncertainty in the 2s2Cf half-life (2.646kO.004 y) contributes very little to the total uncertainty despite the 13 y update included. These results suggest that the neutron-detection efficiency depends only weakly on neutron energy. The neutron spectra from all 5 targets, Li and Y, are therefore considered equivalent with respect to neutrondetection efficiency. Another independent determination of neutron-detection efficiency was made using the 48Ti(p, n)48V reaction in a manner analogous to the
R. T. Skelton, R. W. Kavanagh / ‘Be branching ratio
14.5
89Y(p, n)89Zr. This yielded an efficiency of (13 f 1) % at an average neutron energy of 0.210 MeV; the lower precision in this case stems from difficulties in accurately assessing the neutron background from contaminant (p, n) reactions, which was estimated to contribute 15 % of the neutron counts. Despite its low precision, this 48Ti result helps to rule out the possibility of a major systematic error in neutrondetection efficiency. Each neutron counter was checked for y-response with a 10 PCi 6oCo source; only - 3 x lo-’ of th e y-rays entering the counter produced a response high enough to trigger the discriminator. The y-activity from all targets was measured by a 100 cm3 Ge(Li) detector having a resolution of 2.5 keV at 1.3 MeV. Targets and sources were placed 9.24 cm from the face of the crystal. An efficiency curve was determined using a National Bureau of Standards SRM-4275 reference source, which consists of a mixture of 125Sb-125mTe, “‘Eu, and 154E~ ; this standard emits a number of yrays having absolute emission rates specified to precisions ranging from 0.6% to 1.0% Eight of the lines were used. Five separate determinations were made over a period of six weeks, with a reproducibility within statistical uncertainties (0.1% to 0.2%), including a period when Ge(Li) resolution was degraded 60 % (possibly incipient failure of a pre-amp component; removing high voltage for a few hours was found to restore resolution for around 12 hours). As a precaution, the efficiency curve was checked at the ‘j°Co energies by analysis of the photopeak summing, correcting for the well-known 6oCo yy angular correlation ‘), and with commercial ’ 52E~, r3’Cs, and 6oCo sources; good agreement within the specified precision (+ 1.2 to 2.5 %) was obtained. Small corrections (S 0.5 %) were applied for slight displacements (0.25 mm) from the reference geometry for some targets. The accuracy of these corrections was verified with the 13’Cs reference source. Reference-geometry photopeak efficiencies were 0.3248 % at 478 keV and 0.1857 y0 at 909 keV. Contaminant (p, n) reactions were estimated to be negligible for the Li targets, based on several observations: the count rate below the Li threshold, about 1 s-l was equal to the known non-beam-related (e.g. cosmic-ray-induced) background; all Li bombardments were carried out below the thresholds of the common contaminants “B, 13C, and “0; and when an Al foil identical to the backings was bombarded, neutron yield per unit charge was very low (- 5 x IO-’ the yield from a Li target). The contaminant situation with the Y required a more exhaustive analysis. Each Y-target was bombarded at E, = 3.640 MeV, just below the 89Y(p, n)89Zr threshold. The neutron yield from each target was less than 0.5 % of that at the activation energy of 4.24 MeV. After bombardment the targets were counted in a Ge(Li) system permitting a search for any contaminant signatures (y or annihilation radiation). All stable isotopes with 2 5 50 having thresholds less than bombardment energy were considered, paying special attention to those which would leave no detectable signature and to those having thresholds between 3.50 and 4.24 MeV. Of those which would leave no signature, two (*H
146
R. T. Skelton, R. W. Kavanagh / ‘Be branching ratio
and 22Ne) are rare, a nd the others have fairly low thresholds, so that enormous increases in neutron yield would not be expected between 3.64 and 4.24 MeV. Thirty-seven potential signatures were sought; only three positive signatures were noted: 13C, “B, and rs0, for which the (p, n) products lead to annihilation radiation with Tt = 10, 20, and 110 min, respectively. Since b’ emitters had been anticipated, the targets were mounted (with the backings) in the Ge(Li) system within 5 minutes of the end of bombardment, and the annihilation radiation was recorded as a function of time. After these three short-lived components had decayed away, a Ge(Li) spectrum was obtained to determine the fl’ detection efficiency (via annihilation radiation) directly from the /I’ branch of 89Zr, the ratio of 5 11 keV to 909 keV y-counts, and the absolute 909 keV efficiency. An Al foil identical to the backings of all 5 targets was bombarded to obtain further information on contaminants. Measurement of neutron yield at various energies showed a general increase with energy; bombardment at 3.41 MeV, corresponding to the check of the second Y-target at 3.64 MeV when the energy loss in the target was accounted for, indicated that - 70% of the below-threshold counts from the Y-targets came from the backings; bombardment at 4.03 MeV, corresponding to the 4.24 MeV activation of the second Y-target, showed neutron yield from the backing alone to increase by a factor of 1.8 between the two energies. The Al foil was then bombarded at 4.03 MeV for 2.0 h at 280 nA and counted in the Ge(Li) system in a manner similar to the Y-targets; it was concluded that 30% of the neutrons arose from 7Li(p, n)7Be and 25 y0 from “B(p,n)“C; < 1% arose from ‘*O(p, n)18F. Possible sources for the remaining 45 “/d include (p, n) reactions on 9Be, “N, 37Cl, and 41K. The “F and 13N activities observed on the Y-targets were sufficient to account for their belowthreshold yield excess compared to the Al alone. The total neutron background from each target is thus divided into two components: that calculated from the j?’ emitters observed and that leaving no detectable signature but presumed from the Al foil bombardment, where ‘Li(p, n)7Be falls into the latter category. The absence of a significant 15N(p, n)’ 5O contribution from the Y and Au layers is suggested by two considerations: first, the data of refs. 8,9) imply a 14N(p, c()’‘C reaction rate comparable to that of “N(p, n)“O at 4.2 MeV, but the 20 min component (“C) of annihilation radiation observed was consistent with that from “B(p, n)“C in the backing as determined by the Al foil activation; second, since nitrides would be expected to be rarer than oxides or nitrates, the low level of O-contamination observed suggests that N was not present at a significant level. The data of refs. l”sll), when combined with the “O(p, n)‘*F neutron contribution inferred, imply a neutron-production rate from 170(p, n)17F of - 0.05 ‘A of the neutron yield for the first Y-target and a negligible amount for the second. Total neutron backgrounds were 1.72 % and 1.09 %. For all five targets, a time-dependent background of one neutron count per second from sources unrelated to the beam was taken into account. Each Li target was counted for three intervals and each Y
R. T. Skelton, R. W. Kavanagh / ‘Be branching ratio
147
for 4 intervals spanning several days to several weeks with statistically consistent results.
3. Experimental results Table 1 shows the results of the three individual Li bombardments. In addition to these, data from the two preliminary targets (corrected for ‘Be on the monitor tube) were analyzed and found to imply branching ratios of 10.55 % and 10.43 % ; these data were not used in determining the results of the present work, however, because of uncertainties in the effects of their higher-energy neutron spectra, the possibility of ‘Be having escaped the monitor tube, and possible contributions from the energetically accessible ‘Li(p, na)3He reaction in the first case. The ‘Be branching ratio calculated from the three activations considered reliable is (10.49f0.07)%. The *‘Zr decay scheme was re-examined. Its half-life was measured as 78.62 +O. 17 h, consistent with the result of ref. 12) (78.43 +0.08 h). Minor branches states were measured: 2.622-X).718(17) %; 2.5665-0.120(10) % to higher 2.5300-0.080(8) %; and 1.7445-0.129(10) %. The numbers in parentheses are uncertainties in the last digits; all of these branches are consistent with those given in ref. i3).
4. Analysis of uncertainties The formula for the branching ratio B, is
where the factor
lb w (W ” = [l-exp(-It,)][l-exp(-It,)] corrects for decay during the time intervals associated with bombardment, waiting, and counting. The other symbols are as follows: C, = number of photopeak counts; C, = number of neutron counts; E, = neutron-detection system efficiency; .sy= Ge(Li) system photopeak efficiency in the reference geometry; f, corrects for slight displacements from the reference Ge(Li) geometry; j& corrects the observed neutron counts for dead-time in the neutron-detection system; fe,d, corrects for the drift in the neutron-detection system signal-processing electronics; and fn,b,
148
R. T. ~kelton,
R. W. Kavanagh
! ‘Be bran~hl~g ratio
TABLE3 Contributions
to the branching ratio uncertainty Uncertainty (S;)
Factor Ge(Li) efficiency ratio of standard source 478 keV g-statistics neutron background subtraction variations in Ge(Li) counting geometry electronics drift corrections 909 keV y-statistics photopeak integration consistency half-life dependent factors neutron-counting dead-time corrections 909 keV y-branch (0.9901) [ref. ‘“)I
0.39 0.28 0.25 0.20 0.15 0.15 0.15 0.14 0.10 0.04
quadrature sum
0.66
TABLE4 Recent measurements of the ‘Be branching ratio B, Author Trautvetter et al. Norman rt al. Balamuth et al. Donoghue et al. Davids et al. Fisher and Hershberger Mathews et al. Taddeuci et al. Knapp et ul. Evans et al. present result recommended value ‘)
Method *) D B F Dl E B El G H D B
Ref.
B,(U/,P) 15.4 kO.8 9.8 +O.S 10.10+0.45 10.6 +0.5 10.61+0.23 10.61io.17 10.7 *0.2 to.3 + 1.1 10.9 * 1.1 11.4 +0.7 10.49*0.07 10.45 +0.04
“) Method code after ref. I4), which tabulates results prior to 1983. B Pneutron yield from ‘Li(p, n)‘Be and y-yield. D -‘Be yield with Si detector from **B(p, x)“Be and y-yield. Dl -method D adding a monitor Si detector. E -‘Be yield with Si detector from ‘H(‘Li, n)“Be using a spectrograph; y-yield. El -method E, substituting transport system and telescope for spectrograph. F -accelerated ‘Be beam into Si detector and y-yield. G -transition strengths of ‘Li(p, n)‘Be at O” from 60 to 300 MeV. H -inner bremsstrahlung. b, Several values are pretiminary. ‘f Weighted mean excluding the discordant result of ref. ‘1. and including six previous results: see text.
R. T. Skelton, R. W. Kavanagh / ‘Be branching ratio
149
accounts for the neutron background. The same equations can be written for the 909 keV y-branch of 89Zr , interpreting all symbols appropriately. One equation can then be divided by the other so that the ‘Be branching ratio is expressed in terms of the 909 keV y-branch of *9Zr and the ratios of corresponding quantities. Thus, although the discussion has been in terms of absolute efficiencies for the Ge(Li) and neutron-detection systems for clarity, the absolute neutron-detection efficiency cancels in the result; Ge(Li) efficiency enters only as the ratio of photopeak efficiencies at the two energies of interest Several other potential sources of systematic error are eliminated or reduced: since the geometries for all bombardments and all y-counting were nearly identical for all 5 targets, any effects on Ge(Li) system efficiency arising from spatial distribution of the activation products (i.e. size of proton beam spot) cancel; the neutron-counter dead-time corrections enter only as their ratio; and the same photopeak integration technique was used for all Ge(Li) spectra analyzed. Table 3 lists estimated uncertainties of individual factors which are combined in quadrature in the final result.
5. Summary and conclusions The results of recent measurements are presented in table 4. The present result disagrees strongly with that reported in ref. ‘); it is consistent with all other results tabulated as well as with the previous measurement of highest stated precision, (10.35 +0.08)x [ref. ““)I. The weighted mean of modern results is (10.45 +0.04)x, with a xz per degree of freedom of 0.67. This average excludes the discordant result of ref. ‘) and includes the six measurements made between 1962 and 1974 tabulated in ref. 14). The authors thank Dr. C. A. Barnes for many helpful discussions. This work was supported in part by US National Science Foundation Grant PHY82-15500.
References 1) H. P. Trautvetter. H. W. Becker, L. Buchmann. J. G&es. K. U. Kettner. C. Rolfs, P. Schmalbrock and A. E. Vlieks, Verhl. DPG VI (1983) 1141
2) F. Ajzenberg-Selove, Nucl. Phys. A320 (1979) 1 3) H. Volk, H. Krlwinkel, R. Santo and L. Wallek. Z. Phys. A310 (1983) 91 4) J. L. Osborne, C. A. Barnes, R. W. Kavanagh, R. M. Kremer. G. J. Mathews, J. L. Zyskind, P. D.
Parker and A. J. Howard, Phys. Rev. Lett. 48 (1982) 1664 R. G. H. Robertson, P. Dyer, T. J. Bowles, R. E. Brown, N. Jarmie. C. J. Maggiore and S. M. Austin, Phys. Rev. C27 (1983) 11 C. A. Burke, M. T. Lunnon and H. W. Lefevre, Phys. Rev. Cl0 (1974) 1299 H. Appel, H. Behrens, K. Biirk, H.-W. Miiller, L. Szybisz and R. Wishhusen. Z. Phys. 268 (1974) 347 P. D. Ingalls, J. S. Schweitzer, B. D. Anderson and M. Rios, Phys. Rev. Cl3 (1976) 524
R. T. Skelton, R. W. Kavanagh 1 ‘Be branching ratio
150 9) 10) ‘1) 12) 13) 14) 15) 16) 17) 18) 19) 20)
21) 22) 23)
A. R. Barnett. Nucl. Phys. Al20 (1968) 342 J. K. Bair, C. M. Jones and H. B. Willard, Nucl. Phys. 53 (1964) 209 J. K. Bair, Phys. Rev. C8 (1973) 120 D. M. van Patter and S. M. Shafroth, Nucl. Phys. 50 (1964) 113 Table of isotopes, 7th ed., ed. C. M. Lederer and V. S. Shirley (Wiley, New York, 1978) E. B. Norman, T. E. Chupp, K. T. Lesko, J. L. Osborne, P. J. Grant and G. L. Woodruff, Phys. Rev. C27 (1983) 1728; Erratum: Phys. Rev. C28 (1983) 1409 D. P. Balamuth, L. Brown, T. E. Chapuran, J. Klein, R. Middleton and R. W. Zurmiihle, Phys. Rev. C27 (1983) 1724 T. R. Donoghue, T. C. Rinckel, K. E. Sale, E. Sugarbaker, M. Wiescher, C. P. Browne, E. D. Berners, R. W. Tarara and R. E. Warner, Phys. Rev. C28 (1983) 875 C. N. Davids. A. J. Elwyn, B. W. Filippone. S. B. Kaufman, K. E. Rehm and J. P. Schiffer, Phys. Rev. C2I? (1983) 885 S. A. Fisher and R. L. Hershberger, Bull. Am. Phys. Sot. 28 (1983) 713; private communication G. J. Mathews, R. C. Haight. R. G. Lanier and R. M. White. Phys. Rev. C28 (1983) 879 T’. N. Taddeuci. J. Rapaport, C. D. Goodman, C. C. Foster, C. A. Goulding, C. Gaarde, J. Larsen, D. J. Horen, T. Masterson, E. Sugarbaker and P. Koncz, Bull. Am. Phys. Sot. 28 (1983) 714 D. A. Knapp, A. B. McDonald and C. L. Bennett, Bull. Am. Phys. Sot. Zs (1983) 713 H. C. Evans, I. P. Johnstone, J. R. Leslie, W. McLatchie, H. -B. Mak, P. Skensved and T. K. Alexander, Bull. Can. Assoc. Phys. 39 (1983) 58 1. W. Goodier, J. L. Makepeace and A. Williams, Int. J. Applied Radiation and Isotopes 25 (1974) 373
MEASUREMENT
OF THE ‘Be BRANCHING
RATIO
R. T. SKELTON and R. W. KAVANAGH W. K. Kellogg Radiation Laboratory,
California Institute of Technology, Pasadena, California 9112S, USA
Received 2 August 1983 Abstract: The branching ratio in the electron-capture decay of ‘Be to the 478 keV and ground states of ‘Li has been measured employing an activation technique. Three ‘Li targets were bombarded with a proton beam and the neutrons resulting from the ‘Li(p,n)‘Be reaction were counted in 4x geometry. After bombardment, the 478 keV y-activity was determined. To calibrate neutrondetection efiiciency, two -s9Y targets were bombarded, and the resulting neutrons and ensuing 909 keV y-activity counted. Comparison of the neutron yields with the y-activities results in a ‘Be branch of (10.49+0.07)‘A to the 478 keV state of ‘Li.
E
RADIOACTIVITY
‘Be; measured I;.; deduced branches. *‘Zr; measured I;., T,,,. Neutrondetection system.
1. Introduction
A recent measurement by Trautvetter et al. ‘) of the ‘Be branching ratio yielding a result of (15.4 f 0.8) %, markedly higher than the previously accepted value of (10.35 + 0.07) “/, [ref. ‘)I, has prompted several new measurements, the results of which are surveyed in sect. 5. This branching ratio has several important applications, among them the determination of the 3He(cc,y)‘Be cross section by an activitation technique, as has been done recently by several research groups 3- 5). The 3He(a, y)‘Be cross section is a key parameter in the solar-neutrino problem inasmuch as nearly all the neutrinos to which the Brookhaven solar-neutrino detector is sensitive are those arising from a branch involving this reaction. The importance of the ‘Be branching ratio has prompted this attempt to resolve the discrepancy suggested by ref. ‘), and to improve appreciably on the precision with which it is known.
2. Experimental
procedure
Proton beams of 8 to 12 nA at 2.3 MeV from the ONR-Caltech tandem accelerator were used to bombard ‘Li targets; neutrons were counted during 141
142
R. T. Skelton, R. W. Kaoanagh / ‘Be branching ratio
TABLE1 Lithium target parameters LiF target ‘Be nuclei produced ( 106) bombardment energy (MeV) beam current (nA) bombardment duration (h) mean neutron energy (MeV) neutron-counting dead-time correction electronics drift correction 478 keV y statistical uncertainty (%) branching ratio B, (“/,)
1
2
3
1188 2.260 12 6.5 0.392 1.0116 1.000 0.54
1768 2.365 12 9.0 0.484 1.0118 1.008 0.41
1836 2.304 8 9.2 0.43 1 1.0131 1.004 0.45
10.48
10.55
10.43
bombardment and the 478 keV y-activity was subsequently measured. Three different Li targets were used. Each consisted of 50 to 100 pg *cm- 2 of reagentgrade LiF evaporated on 250 pm high-purity Al and then flashed with I rng. cm-’ of gold. The targets were mounted at the end of a monitor tube of 30 mm length and 1.5 mm inside diameter, which made it possible to detect any ‘Be escaping from the target and being deposited on the inside surface of the tube. Bombardment of two preliminary targets on gold backings led to a ‘Be escape of 4% without any gold flash, and 1.2% despite a gold flash of 0.5 mg. cmm2. For the final 3 targets, ‘Be loss was < 0.1 %. Further Li target parameters are listed in table 1. Two semi-thick 89Y targets were bombarded in a very similar configuration at E, = 4.24 MeV, just below the (p, n) threshold for the first excited state of 89Zr. The first target consisted of 5.5 mg . cm -’ 99.9 “/dpurity natural Y flashed with 0.5 mg.cme2 of gold on high-purity Al. The second was made as a Au-Y-Au sandwich, prepared on a glass slide and then peeled off; high-purity Al was used to stop the transmitted beam. The Y-thickness was 4.2 mg . cme2. For both targets, 89Zr loss was c 0.1%. Further Y-target parameters are given in table 2. Neutrons were detected by a system of 12 3He-tilled proportional counters embedded symmetrically in a graphite cube 1.41 m on a side. Neutron count-rates ranged from 6400 to 7200 s-l for all except the first Y-target, for which the rate was 2660 s- 1 ; these count-rates required the small dead-time corrections tabulated. These corrections were determined by bombarding at various beam currents a LiF target which had been carefully prepared for uniformity. The thickness of the Y-targets was determined as follows: after completion of bombardment and counting in the Ge(Li) system, the second (sandwich) target was mounted over a Li target, and the combination was bombarded with protons to determine total proton energy loss by comparison of the beam energy necessary to attain the ‘Li(p, n)‘Be threshold after transmission with the known threshold energy. The total sandwich thickness, correcting for the energy dependence of the stopping
R. T. Skelton, R. W. Kaoanagh TABLE
/ ‘Be branching
ratio
143
2
Yttrium target parameters Y-target
1
2
s9Zr nuclei produced (106) bombardment duration (h) beam current (nA) neutron-counting dead-time factor electronics drift’correction target thickness (keV) mean neutron energy (MeV) 909 keV y statistical uncertainty (“/,) ratio of yield 3.64 MeV to 4.24 MeV ( “/,) b’ emitter neutron background (“/,) total neutron background (“/,)
172.7 2.2 40 1.0024 1.007 275 0.454 0.28 0.43 1.14 1.71
506.4 2.7 110 1.0112 1.028 195 0.483 0.17 0.48 0.52 1.09
12.04
12.00
neutron-detection
system efficiency (“/,)
power, was 215 keV at bombardment energy; this agrees with the value of 225 keV which was determined by weighing, 30 keV of which was Au. As the a’Y(p, n)89Zr excitation function had previously been found to increase smoothly from threshold to E, = 4.25 MeV, the thickness of the first target was calculated from the observed neutron yields. This calculation resulted in a value 20% lower than weighing had indicated ; the calculated value was preferred because weighing was not considered as reliable with the preparation procedure employed for this target. The Y-target thickness is needed only for calculating the neutron-energy spectrum. The smooth increase of the excitation function was put to service in conjunction with the energy loss in the target to produce a neutron energy spectrum peaked toward the highenergy end. A thin Li target produces a similarly shaped spectrum as a result of kinetimatic effects and the forward-angle preference of the ‘Li(p, n)‘Be reaction 6). Fig. 1 shows the calculated neutron spectrum from each of the tive targets employed. Sensitivity of the neutron-detection system to neutron angular distribution was examined with the neutrons from a highly anisotropic ‘Be&, n) source consisting of a 21‘Pb c+source and a flake of Be 2 mm away. At this high neutron energy (N 10 MeV), a 20 % asymmetry between front and back counters was observed. Displacement of an isotropic point source 2.5 cm from the center of the graphite cube produced the same counter asymmetry, but reduced overall efficiency by 5 0.1%. For the lower-energy (N 0.4 MeV) neutrons from ‘Li, there was no detectable counter asymmetry. A slight drift in electronic gains affecting neutron-detection efficiency was noted from one bombardment to another. The drift was monitored by checking response to a 252Cf source prior to each bombardment and correcting the neutron count-rate accordingly. Neutron yield per unit charge was stable for the Li targets, suggesting that the drift is a longer-term effect than the bombardment
R. T. Skelton, R. W. Kavanagh / ‘Be branching ratio
144
I I
2 60
I\ I\ \I
t
$
!
$40 z
9
.r(
$20 7 p:
r
III
OlOO
\I \I \ I I
I I:
I1
200
300
Neutron
400
Energy
500
\I I I \ \
II
600
.-
70
(keV)
Fig. 1. Neutron energy spectra and mean energies for the three Li targets (a, b and c); and the two Ytargets (d and e).
intervals. For calculation purposes, a significant change in efficiency affecting the second Y-bombardment was incorporated into this factor. A failed pre-amplifier component for one of the 3He counters was replaced with one which later was found defective. As a result, a discriminator was rejecting a fraction of the neutron counts from this counter. As the reduced efficiency was not noted until after bombardment, the efficiency was subsequently re-measured several times in the same electronics configuration, and found to be reproducible within the range of drifts noted previously. Comparison of the 89Y(p, n)89Zr neutron counts to the subsequently measured 909 keV y-activity yielded an absolute neutron-detection efficiency of (12.02 f0.08) %. An independent check of the system efficiency was made with the previously mentioned 2s2Cf source which was calibrated by direct comparison to one which had been calibrated by its manufacturer against a *‘*Cf source from the National Bureau of Standards; the efficiency was (12.47+0.40)“,, for the 2s2Cf neutron spectrum, which has an average energy of 2.348 MeV; the uncertainty in the 2s2Cf half-life (2.646kO.004 y) contributes very little to the total uncertainty despite the 13 y update included. These results suggest that the neutron-detection efficiency depends only weakly on neutron energy. The neutron spectra from all 5 targets, Li and Y, are therefore considered equivalent with respect to neutrondetection efficiency. Another independent determination of neutron-detection efficiency was made using the 48Ti(p, n)48V reaction in a manner analogous to the
R. T. Skelton, R. W. Kavanagh / ‘Be branching ratio
14.5
89Y(p, n)89Zr. This yielded an efficiency of (13 f 1) % at an average neutron energy of 0.210 MeV; the lower precision in this case stems from difficulties in accurately assessing the neutron background from contaminant (p, n) reactions, which was estimated to contribute 15 % of the neutron counts. Despite its low precision, this 48Ti result helps to rule out the possibility of a major systematic error in neutrondetection efficiency. Each neutron counter was checked for y-response with a 10 PCi 6oCo source; only - 3 x lo-’ of th e y-rays entering the counter produced a response high enough to trigger the discriminator. The y-activity from all targets was measured by a 100 cm3 Ge(Li) detector having a resolution of 2.5 keV at 1.3 MeV. Targets and sources were placed 9.24 cm from the face of the crystal. An efficiency curve was determined using a National Bureau of Standards SRM-4275 reference source, which consists of a mixture of 125Sb-125mTe, “‘Eu, and 154E~ ; this standard emits a number of yrays having absolute emission rates specified to precisions ranging from 0.6% to 1.0% Eight of the lines were used. Five separate determinations were made over a period of six weeks, with a reproducibility within statistical uncertainties (0.1% to 0.2%), including a period when Ge(Li) resolution was degraded 60 % (possibly incipient failure of a pre-amp component; removing high voltage for a few hours was found to restore resolution for around 12 hours). As a precaution, the efficiency curve was checked at the ‘j°Co energies by analysis of the photopeak summing, correcting for the well-known 6oCo yy angular correlation ‘), and with commercial ’ 52E~, r3’Cs, and 6oCo sources; good agreement within the specified precision (+ 1.2 to 2.5 %) was obtained. Small corrections (S 0.5 %) were applied for slight displacements (0.25 mm) from the reference geometry for some targets. The accuracy of these corrections was verified with the 13’Cs reference source. Reference-geometry photopeak efficiencies were 0.3248 % at 478 keV and 0.1857 y0 at 909 keV. Contaminant (p, n) reactions were estimated to be negligible for the Li targets, based on several observations: the count rate below the Li threshold, about 1 s-l was equal to the known non-beam-related (e.g. cosmic-ray-induced) background; all Li bombardments were carried out below the thresholds of the common contaminants “B, 13C, and “0; and when an Al foil identical to the backings was bombarded, neutron yield per unit charge was very low (- 5 x IO-’ the yield from a Li target). The contaminant situation with the Y required a more exhaustive analysis. Each Y-target was bombarded at E, = 3.640 MeV, just below the 89Y(p, n)89Zr threshold. The neutron yield from each target was less than 0.5 % of that at the activation energy of 4.24 MeV. After bombardment the targets were counted in a Ge(Li) system permitting a search for any contaminant signatures (y or annihilation radiation). All stable isotopes with 2 5 50 having thresholds less than bombardment energy were considered, paying special attention to those which would leave no detectable signature and to those having thresholds between 3.50 and 4.24 MeV. Of those which would leave no signature, two (*H
146
R. T. Skelton, R. W. Kavanagh / ‘Be branching ratio
and 22Ne) are rare, a nd the others have fairly low thresholds, so that enormous increases in neutron yield would not be expected between 3.64 and 4.24 MeV. Thirty-seven potential signatures were sought; only three positive signatures were noted: 13C, “B, and rs0, for which the (p, n) products lead to annihilation radiation with Tt = 10, 20, and 110 min, respectively. Since b’ emitters had been anticipated, the targets were mounted (with the backings) in the Ge(Li) system within 5 minutes of the end of bombardment, and the annihilation radiation was recorded as a function of time. After these three short-lived components had decayed away, a Ge(Li) spectrum was obtained to determine the fl’ detection efficiency (via annihilation radiation) directly from the /I’ branch of 89Zr, the ratio of 5 11 keV to 909 keV y-counts, and the absolute 909 keV efficiency. An Al foil identical to the backings of all 5 targets was bombarded to obtain further information on contaminants. Measurement of neutron yield at various energies showed a general increase with energy; bombardment at 3.41 MeV, corresponding to the check of the second Y-target at 3.64 MeV when the energy loss in the target was accounted for, indicated that - 70% of the below-threshold counts from the Y-targets came from the backings; bombardment at 4.03 MeV, corresponding to the 4.24 MeV activation of the second Y-target, showed neutron yield from the backing alone to increase by a factor of 1.8 between the two energies. The Al foil was then bombarded at 4.03 MeV for 2.0 h at 280 nA and counted in the Ge(Li) system in a manner similar to the Y-targets; it was concluded that 30% of the neutrons arose from 7Li(p, n)7Be and 25 y0 from “B(p,n)“C; < 1% arose from ‘*O(p, n)18F. Possible sources for the remaining 45 “/d include (p, n) reactions on 9Be, “N, 37Cl, and 41K. The “F and 13N activities observed on the Y-targets were sufficient to account for their belowthreshold yield excess compared to the Al alone. The total neutron background from each target is thus divided into two components: that calculated from the j?’ emitters observed and that leaving no detectable signature but presumed from the Al foil bombardment, where ‘Li(p, n)7Be falls into the latter category. The absence of a significant 15N(p, n)’ 5O contribution from the Y and Au layers is suggested by two considerations: first, the data of refs. 8,9) imply a 14N(p, c()’‘C reaction rate comparable to that of “N(p, n)“O at 4.2 MeV, but the 20 min component (“C) of annihilation radiation observed was consistent with that from “B(p, n)“C in the backing as determined by the Al foil activation; second, since nitrides would be expected to be rarer than oxides or nitrates, the low level of O-contamination observed suggests that N was not present at a significant level. The data of refs. l”sll), when combined with the “O(p, n)‘*F neutron contribution inferred, imply a neutron-production rate from 170(p, n)17F of - 0.05 ‘A of the neutron yield for the first Y-target and a negligible amount for the second. Total neutron backgrounds were 1.72 % and 1.09 %. For all five targets, a time-dependent background of one neutron count per second from sources unrelated to the beam was taken into account. Each Li target was counted for three intervals and each Y
R. T. Skelton, R. W. Kavanagh / ‘Be branching ratio
147
for 4 intervals spanning several days to several weeks with statistically consistent results.
3. Experimental results Table 1 shows the results of the three individual Li bombardments. In addition to these, data from the two preliminary targets (corrected for ‘Be on the monitor tube) were analyzed and found to imply branching ratios of 10.55 % and 10.43 % ; these data were not used in determining the results of the present work, however, because of uncertainties in the effects of their higher-energy neutron spectra, the possibility of ‘Be having escaped the monitor tube, and possible contributions from the energetically accessible ‘Li(p, na)3He reaction in the first case. The ‘Be branching ratio calculated from the three activations considered reliable is (10.49f0.07)%. The *‘Zr decay scheme was re-examined. Its half-life was measured as 78.62 +O. 17 h, consistent with the result of ref. 12) (78.43 +0.08 h). Minor branches states were measured: 2.622-X).718(17) %; 2.5665-0.120(10) % to higher 2.5300-0.080(8) %; and 1.7445-0.129(10) %. The numbers in parentheses are uncertainties in the last digits; all of these branches are consistent with those given in ref. i3).
4. Analysis of uncertainties The formula for the branching ratio B, is
where the factor
lb w (W ” = [l-exp(-It,)][l-exp(-It,)] corrects for decay during the time intervals associated with bombardment, waiting, and counting. The other symbols are as follows: C, = number of photopeak counts; C, = number of neutron counts; E, = neutron-detection system efficiency; .sy= Ge(Li) system photopeak efficiency in the reference geometry; f, corrects for slight displacements from the reference Ge(Li) geometry; j& corrects the observed neutron counts for dead-time in the neutron-detection system; fe,d, corrects for the drift in the neutron-detection system signal-processing electronics; and fn,b,
148
R. T. ~kelton,
R. W. Kavanagh
! ‘Be bran~hl~g ratio
TABLE3 Contributions
to the branching ratio uncertainty Uncertainty (S;)
Factor Ge(Li) efficiency ratio of standard source 478 keV g-statistics neutron background subtraction variations in Ge(Li) counting geometry electronics drift corrections 909 keV y-statistics photopeak integration consistency half-life dependent factors neutron-counting dead-time corrections 909 keV y-branch (0.9901) [ref. ‘“)I
0.39 0.28 0.25 0.20 0.15 0.15 0.15 0.14 0.10 0.04
quadrature sum
0.66
TABLE4 Recent measurements of the ‘Be branching ratio B, Author Trautvetter et al. Norman rt al. Balamuth et al. Donoghue et al. Davids et al. Fisher and Hershberger Mathews et al. Taddeuci et al. Knapp et ul. Evans et al. present result recommended value ‘)
Method *) D B F Dl E B El G H D B
Ref.
B,(U/,P) 15.4 kO.8 9.8 +O.S 10.10+0.45 10.6 +0.5 10.61+0.23 10.61io.17 10.7 *0.2 to.3 + 1.1 10.9 * 1.1 11.4 +0.7 10.49*0.07 10.45 +0.04
“) Method code after ref. I4), which tabulates results prior to 1983. B Pneutron yield from ‘Li(p, n)‘Be and y-yield. D -‘Be yield with Si detector from **B(p, x)“Be and y-yield. Dl -method D adding a monitor Si detector. E -‘Be yield with Si detector from ‘H(‘Li, n)“Be using a spectrograph; y-yield. El -method E, substituting transport system and telescope for spectrograph. F -accelerated ‘Be beam into Si detector and y-yield. G -transition strengths of ‘Li(p, n)‘Be at O” from 60 to 300 MeV. H -inner bremsstrahlung. b, Several values are pretiminary. ‘f Weighted mean excluding the discordant result of ref. ‘1. and including six previous results: see text.
R. T. Skelton, R. W. Kavanagh / ‘Be branching ratio
149
accounts for the neutron background. The same equations can be written for the 909 keV y-branch of 89Zr , interpreting all symbols appropriately. One equation can then be divided by the other so that the ‘Be branching ratio is expressed in terms of the 909 keV y-branch of *9Zr and the ratios of corresponding quantities. Thus, although the discussion has been in terms of absolute efficiencies for the Ge(Li) and neutron-detection systems for clarity, the absolute neutron-detection efficiency cancels in the result; Ge(Li) efficiency enters only as the ratio of photopeak efficiencies at the two energies of interest Several other potential sources of systematic error are eliminated or reduced: since the geometries for all bombardments and all y-counting were nearly identical for all 5 targets, any effects on Ge(Li) system efficiency arising from spatial distribution of the activation products (i.e. size of proton beam spot) cancel; the neutron-counter dead-time corrections enter only as their ratio; and the same photopeak integration technique was used for all Ge(Li) spectra analyzed. Table 3 lists estimated uncertainties of individual factors which are combined in quadrature in the final result.
5. Summary and conclusions The results of recent measurements are presented in table 4. The present result disagrees strongly with that reported in ref. ‘); it is consistent with all other results tabulated as well as with the previous measurement of highest stated precision, (10.35 +0.08)x [ref. ““)I. The weighted mean of modern results is (10.45 +0.04)x, with a xz per degree of freedom of 0.67. This average excludes the discordant result of ref. ‘) and includes the six measurements made between 1962 and 1974 tabulated in ref. 14). The authors thank Dr. C. A. Barnes for many helpful discussions. This work was supported in part by US National Science Foundation Grant PHY82-15500.
References 1) H. P. Trautvetter. H. W. Becker, L. Buchmann. J. G&es. K. U. Kettner. C. Rolfs, P. Schmalbrock and A. E. Vlieks, Verhl. DPG VI (1983) 1141
2) F. Ajzenberg-Selove, Nucl. Phys. A320 (1979) 1 3) H. Volk, H. Krlwinkel, R. Santo and L. Wallek. Z. Phys. A310 (1983) 91 4) J. L. Osborne, C. A. Barnes, R. W. Kavanagh, R. M. Kremer. G. J. Mathews, J. L. Zyskind, P. D.
Parker and A. J. Howard, Phys. Rev. Lett. 48 (1982) 1664 R. G. H. Robertson, P. Dyer, T. J. Bowles, R. E. Brown, N. Jarmie. C. J. Maggiore and S. M. Austin, Phys. Rev. C27 (1983) 11 C. A. Burke, M. T. Lunnon and H. W. Lefevre, Phys. Rev. Cl0 (1974) 1299 H. Appel, H. Behrens, K. Biirk, H.-W. Miiller, L. Szybisz and R. Wishhusen. Z. Phys. 268 (1974) 347 P. D. Ingalls, J. S. Schweitzer, B. D. Anderson and M. Rios, Phys. Rev. Cl3 (1976) 524
R. T. Skelton, R. W. Kavanagh 1 ‘Be branching ratio
150 9) 10) ‘1) 12) 13) 14) 15) 16) 17) 18) 19) 20)
21) 22) 23)
A. R. Barnett. Nucl. Phys. Al20 (1968) 342 J. K. Bair, C. M. Jones and H. B. Willard, Nucl. Phys. 53 (1964) 209 J. K. Bair, Phys. Rev. C8 (1973) 120 D. M. van Patter and S. M. Shafroth, Nucl. Phys. 50 (1964) 113 Table of isotopes, 7th ed., ed. C. M. Lederer and V. S. Shirley (Wiley, New York, 1978) E. B. Norman, T. E. Chupp, K. T. Lesko, J. L. Osborne, P. J. Grant and G. L. Woodruff, Phys. Rev. C27 (1983) 1728; Erratum: Phys. Rev. C28 (1983) 1409 D. P. Balamuth, L. Brown, T. E. Chapuran, J. Klein, R. Middleton and R. W. Zurmiihle, Phys. Rev. C27 (1983) 1724 T. R. Donoghue, T. C. Rinckel, K. E. Sale, E. Sugarbaker, M. Wiescher, C. P. Browne, E. D. Berners, R. W. Tarara and R. E. Warner, Phys. Rev. C28 (1983) 875 C. N. Davids. A. J. Elwyn, B. W. Filippone. S. B. Kaufman, K. E. Rehm and J. P. Schiffer, Phys. Rev. C2I? (1983) 885 S. A. Fisher and R. L. Hershberger, Bull. Am. Phys. Sot. 28 (1983) 713; private communication G. J. Mathews, R. C. Haight. R. G. Lanier and R. M. White. Phys. Rev. C28 (1983) 879 T’. N. Taddeuci. J. Rapaport, C. D. Goodman, C. C. Foster, C. A. Goulding, C. Gaarde, J. Larsen, D. J. Horen, T. Masterson, E. Sugarbaker and P. Koncz, Bull. Am. Phys. Sot. 28 (1983) 714 D. A. Knapp, A. B. McDonald and C. L. Bennett, Bull. Am. Phys. Sot. Zs (1983) 713 H. C. Evans, I. P. Johnstone, J. R. Leslie, W. McLatchie, H. -B. Mak, P. Skensved and T. K. Alexander, Bull. Can. Assoc. Phys. 39 (1983) 58 1. W. Goodier, J. L. Makepeace and A. Williams, Int. J. Applied Radiation and Isotopes 25 (1974) 373