Recoil-distance lifetime measurements in 22Na, 22Ne, 30P and 30Si

1.E.4 :3.A

Nuclear Phy:tcs A27S (1977) 141-150; © North-Notland Pwbltahtnp Co., dnnterdo~n Not to be reproduced by photoprlnt or microfilm without written permlaion from the publirhv

RECOIIrDLSTANCE LIFETIIVIE MEASUREMENTS IN ==Na, s=Ne, 3°P AND 3aSi D. C. RADFORD and A. R. 1?OLETTI

Department of Physics, Uni°erstty of Auckland, Auckland, New Zealand

Received 30 August 1976 Abstract : Lifetimes of ten excited states in "Na,'=Ne,'°P and'°Si have been remeasured by the recoildistance method . The values of the mean lives obtained are as follows : ==Na(0 .89 MeV) : s = 15.410.6 Ps ; ~~Na(1 .53 MeV) : s = 5.3710.27 ps ; "Na(1.98 MeV) : s g 2.4610.22 pa ; ~'Na(2.21 MeV) : s = 21 .210 .8 ps ; ~~Na(2.57 MeV) : t = 8 .0910.45 ps ; =~Ne(1 .28 MeV) : s = 5.6210.20 Ps ; a°P(0.71 MeV) : s = 49.111 .2 ps ; a°P(1 .45 MeV) : s = 6.0110.23 ps ; '°P(1 .97 MeV) : s = 3.1310.35 ps ; and 3 °Sî(3.79 MeV) : s = 12 .510 .9 ps . Theseresults are discussed and compared to previous measurements. E

NUCLEAR REACTIONS "F(a, ny), ' 9 F(a, py), E = 5.65 MeV; ~'AI(a, ny), E = 6.18 MeV; ='Al(a, py), E = 3.92 MeV; measurod E~(B~ = 0), Doppler shift, recoil distance. 2:Ne, = 3 Na levels deduced T~ ~2 . 3°Sî , 30P levels deduced Tl~_, B(it). Natural targets.

1. Introdnctlon In the nuclei s°Si and s°P, the lifetime measurements that have been made of a number of levels are in strong disagreement. For the mean life of the 3.79 MeV level in 3°Sî, Beck et al. t) obtainod a value of z = 15.6f 1 .2 ps, whereas Haas et al. ~) obtained s = 24.2f 2.0 ps and Anyas-Weirs et al. 3) obtained T = 6.8 f2.0 ps. These measurements were all made using the rocoil-distance method (RDM In 3°P, for the mean life of the 1 .45 MeV level, Hensonet al. 4), ; using the RDM, obtained s = 9.2 f0.5 ps, and Pixley and Poletti s) measured T = 8 f 5 ps by the Doppler-shift attenuation method (DSAM). However, Sharpey-Schafer et al. 6~ again with the DSAM, measured s = 2.90f 1.25 ps, and Anyas-Weirs et al. (RDM) report s < 1.5 ps. For the 1 .97 MeV level, Henson et al. report T = 6.8f 0.4 ps, Sharpey-Schafer report T = 2.1 f 0.7 ps, and Pixley and Poletti report s > 8 ps. In order to resolve these discrepancies, it was decided to remeasure the mean lifetimes of these three states, and also that of the 0.71 MeV level in s°P, using the RDM. The RDM is preferable to the DSAM for lifetimes of this order, since it is independent of poorly known stopping powers, and hence is intrinsically more accurate . We also present the results of an earlier experiment measuring five lifetimes in ~~Na and one in 2~Ne . All but two of these lifetimes have been previously measured 141

142

R. D. RADFORD AND A. R. POLETTI

by the RDM a number of times, but the states at 1.98 MeV and 2.57 MeV in ZZNa have each only once been previously reported as having their mean life measured by the RDM. These measurements were made by Anyas-Wéiss et al . 3), who obtained i = 2.40f0.25 ps for the 1.98 MeV level, and Haas et al.'), with a value of ti = 8.8 ± 0.9 ps for the 2.57 MeV level. This earlier experiment was performed for two reasons: firstly, in order to try to improve on the RDM measurements of these two relatively poorly known lifetimes, and secondly, as a test of a newly constructed RDM target chamber - the same chamber as was later used for the s°Si and 3°P experiments. 2. ExPerimemal method and results 2.1 . THE ~°Si LIFETIME

The apparatus and methods of analysis that were used have been described previously 8). The 3.79 MeV level was populated via the reaction Z'Al(a, p). The târget was made by evaporating approximately 50 pg/cm2 of natural aluminium onto a nickel foil 1.02 ~m thick, and the resulting foil was then stretched flat as described in ref. s). This target was positioned so that the aluminium was on the downstream side of the beam, and then bombarded with 4.54 MeV a-particles from the folded tandem accelerator at the University of Auckland. This energy was chosen after considerable experimentation as giving the best yield of the 3.79 MeV level. Therecoiling excited nuclei were stopped by a stretched nickel foil, of3.8 pm thickness, mounted accurately parallel to the target by the use of a laser light-lever technique, and the de~xcitation y-rays were detected at 0° to the beam axis in a 10 ~ efficient Ge(Li) detector, about 10 cm from the target. Liquid nitrpgen cold-traps around the collimators and target assembly ensured reasonably good .vacuum conditions. Using the method described by Alexander and Bell ~, the capacitance, and thus the distance, between the target and stopper foils was monitored throughout the experiment. This provided an accurate and convenient measurement of the smaller target-stopper separations, where very small but not negligible deviations from the distance shown by the micrometer screw were evident. These were apparently caused by expansion effects due to ambient temperature changes and beam heating effects. Slow changes could have a significant effect because the experiment took so long in order to get sufficient data, with a beam current of 150 nA, it was necessary to run för about five hours at each distance. Three runs were made, and in each run spectra were taken first at a large distance and then at decreasing distances, the micrometer always being screwed inwards to the requited setting in order to eliminate backlash. A far-distance spectrum was also taken at the conclusion of the third run. These far-distance spectra show a slight but significant rise in the "background" ratio R~ as time progressed, probably due to slow carbon build-up on the target foil during the eight-day experiment . However, by analysingeach run separately and including in the points tobe fittod the far distance

== Na, == Ne, a° P AND a°Si

143

Fig. 1 . The experimental ratio R plotted as a function of target-stopper distance, D, for the'°Si 3 .788 MeV level. The solid line shows the least-squares fit used to determine the mean life . Also shown are y-ray spectra for the full-energy component of the s°Si 3 .788 - . 2.236 MeV transition, at the target-stopper distances indicated, and with an energy dispersion of 0 .489 keV/ch .

points both at the beginning and end of each run, the eliect of this build-up was minimised. Typical spectra at large and small target-stopper separations are shown in fig. 1 for the full-energy component of the s°Si 3 .79 -+ 2.24 MeV transition . Included in the spectra is the a°Si 3.77 ~ 2.24 MeV transition, which is always Doppler-shifted since it has a very short lifetime. For the transition of interest (3.79 -. 2.24 MeV) two peaks are seen : that of lower energy is the unshifted peak with intensity 1°, and that of higher energy is the shifted peak, with intensity I,. The difference in energy gives the mean recoil velocity, which is in this case 0.456 ~ of c. In order to obtain the intensities I° and I,, a general least-squares gaussian fitting program was used to fit thedata. For the two spectra in fig. l, the fit obtained is shown by the dots. Also shown in fig. 1 is a semilogarithmic plot of the experimental ratio R = I°/(1° +1~ against D, the target-stopper separation, obtained from the data taken in the second of the three runs that were made. The solid line shows a leastsquares fit ofthe results to a function which includes corrections for the spread in the component of the recoil velocity parallel to the beam axis, for changes in detector efficiency with energy, and for the relativistic change in detector solid angle for the

144

R. D. RADFORD AND A. R. POLETTI 1 Measured lifetimes in ~°Si and 3°P TeA~

Nucleus

t

Level (MeV)

previous measurements 15 .6 t1 .27 24 .2 t 2 .0 ~ 6.8 }2.0 ~ 55 f 7 ~ 54 t 6 ~ 53 t 10 °) 22 t 5 7 ~ 1 .5 ~ 8 t57 9.2 t0.5 ~) 2.90t 1.25 ~ > 87 6.8~f 0.4~) 2.1 t0.7 ~

3.788

12.5 f0.9

aoP

0.709

49.1 f 1 .2

aoP

~ Ref. 3).

1.454

6.01 t0.23

1 .973

3.13 t 0.35

~ Ref. s) .

(Ps)

present work

aosi

30P

') Ref. 1).

F

~ Ref. ").

') Ref. °) .

~) Ref. `) .

~ Ref. 6).

recoiling nuclei 8). The lifetime value obtained from this fit was also corrected by a factor obtained from a calibration of the change in target-stopper distance applied by a unit change in the setting of the micrometer screw. The three runs gave mean-life values of s = 11 .6 t 1 .1 ps, t = 12.5 ± 1.0 Ps, and s = 13.8 f 1.3 ps, respectively . A weighted average of these yields r = 12.5 f 0.6 ps. We increase the uncertainty to 0.9 ps in order to allow for possible systematic errors, and quote as our measurement T

= 12.5 t 0.9 ps.

This value is tabulated together with previous measurements in table 1. 2.2 . THE '°P LIFETIMES

The mean lives of the 0.71 MeV, 1.45 MeV and 1.97 MeV levels in 3°P have been measured using exactly the same apparatus and techniques described above for the '°Si experiment . The states were populated via the Z'Al(a, n) reaction, at a bombarding energy of6.2 MeV (allowing for about 300keV energy loss in thenickel target backing). This energy was chosen as giving a good yield to all of the states ofinterest, yet being below the threshold of the 2.84 MeV state, which has a lifetime of the order of 1 ps [ref. 6)] and feeds both the 1.45 and 0.71 MeV levels. Two runs were made : the first used the same target as was used for the s°Si measurement, and the second used one of about 30 hg/~ z aluminium, again on a 1 .02 ~m

aa Na, aape,'oP AND '°3i

145

thick nickel foil. This time the background ratio, Rm,did not increase with timeduring either run. The capacitance txhnique was again used to measure the shorter targetstopper distances. Typical . spectra at near and far distances are shown in fig. 2 for the 3°P 1 .45 -" 0 MeV docay, which is the transition used in the measurement of the lifetime ofthat state. For the 1.97 MeV level, the transition used was the 1.97 -" 0 MeV 49 branch, the 1.97 ~ 0.79 MeV 51 % branch's being partly obscured by the a°Si

Fig. 2 . Gamma-ray spectra for the full-energy component of the '°P 1 .454 ~ 0 MeV transition, at the target-stopper distances indicated. The energy dispersion is 0.493 keV/ch . The position of the 1 .461 MeV y-ray from `°K decay is also shown . 1A 0 .5 0.3 0.2 R

0 .1

0.05 0.03 0 .02 0.01

D tNm )

Fig. 3 . The experimental ratio R plotted es a function of target-stopper distance, D, for the'°P 1 .454 MeV and 1 .973 MeV levels. The solid lines show the least-squares fits used to determine the mean lives . The decays which were used in the measurements are indicated by the heavier arrows .

146

R. D. RADFORD AND A. R . IPOLIETI'I

3.50 ~ 224 MeV transition. The 0.71 MeV level decays wholly to the ground state. In the fitting of the spectra for the 1.45 MeV level, a peak Gom natural 4 °K radioactivity was included in the background under the shifted peak . The position and width of this peak was determined from background y-ray spectra accumulated at the conclusion of each run, and the height was calculated from the time of accumulation at each distance. The position of the `°K peak is indicated in fig. 2, and its area was generally around 10 ~ of that of the 1 .45 MeV shifted peak . For the second run, and for the 1.97 and 1.45 MeV levels, R [= I°/(I° +I~] is shown plottod against D (target-stopper separation) in fig. 3, the solid lines indicating the least-squares fits used to obtain the mean lives . The function fitted to the data again included the corrections mentioned above for the a°Si measurement, and in the case ofthe 0.71 MeV level, a correction for the feeding from the 1 .97 and 1 .45 MeV levels. The amount of feeding was calculated from the known branching ratios t °), the ratios of detector elTiciencies at 1 .97, 1.45 and 0.71 MeV, and the observed ratios of 1.97 -+ 0, 1 .45 ~0 and 0.71 -. 0 MeV decays . Also as for the s°Si measurement, weighted averages of the mean lives determined from each of the two runs had their uncertainties enlarged to allow for possible systematic errors. The results which we quote as our measurements are shown in table 1, together with previous measurements. For all levels, the lifetimes obtained from the two runs were in reasonable agreement.

Fig . 4. Gamma-ray spectra for the full-energy " component of the' 2 Na 2 .572 -. 0 MeV transition, et the target-stopper distances indicated. The energy dispersion is 0 .493 reV/ch.

=~Na, "Ne, 3 °P AND ~°Si

147

2 .3 . THE '~Na AND "Ne LIFETIMES

The mean lives offive excited states in 2 ~Na, at 0.89,1.53,1.98, 2.21 and 2.57 MeV, and of the first excited state in 22Ne, at 1.28 MeV, have been measured using the same techniques and apparatus as described above. The reactions i9F(a, n) and ' 9F(a, p) were used to populate these states, using 6 MeV a-particles to bombard a target of approximately 100 pg/cm 2 of CaFZ on a nickel backing of 1 .02 pm thickness. The stopper was a stretched gold foil, about 3.5 mg/cm 2 thick. Fig. 4 shows spectra at close, medium and far target-stopper distances for the ~ZNa 2.57 -" 0 MeV transition, and figs . 5 and 6 show graphs of R against D for four of the lifetimes measured . The transitions used for the measurements are also

Fig . 5 . The experimental ratio R plotted aa~ a function of target-stopper distance, D, for the ='Na 1 .528 MeV aad 2 .211 MeV levels . The solid lines show the least-squares fits used to determine the mean lives . The decays which were used in the measurements are also indicated .

n ~ ~n ~ r

75 100 D(yrm)

Fig. 6. Aa for fig. 5, for the 1 .983 MeV and 2.572 MeV levels in "Na.

148

R. D. RADFORD AND A. R. POLETTI T~i.e 2

Measured lifetimes in' 2Na and ~'Ne Nucleus

Level (MeV)

~~Na '~Na "Na =sNa ~=Na "Ne ') Ref.').

0.891 1 .528 1 .983 2.211 2.572 1 .275 b)

previously adopted

present work

14.7 10 .7') 4.8 10.3') 2.40 t0.25 ') 21 .2 11 .0') 8.8 t0.9') 4.9 10.3 ")

15 .4 10 .6 5.3710 .27 2.4610 .22 21 .2 10 .8 8.0910.45 5.6210.20

Ref.'s.

indicated. Small corrections taken into account include all of those mentioned in subsect. 2.1 above, and in the case of the 0.89 MeV level, a correction for feeding from the 1 .53 MeV level, Care was also taken to add the two-escape shifted and unshifted peaks from the ZZNa 2.57 ~ 0 MeV transition to the background Under the fullenergy peak of the 22Na 2.21 -" 0.66 MeV transition during the fitting for I° and 1,. The final mean-life values obtained are shown in table 2, together with previously adopted values. The errors quoted by us include allowance for both statistical and possible systematic errors. 3. D~on For the levels in 22Na and 2ZNe it will be seen that our results agree very well with the adopted averages of previously measured values' and are in all cases more accurate . Haas et al.') have recently made a detailed comparison of all previous mean-life measurements for the 2~Na levels, and both they and Spear et al. tz) have discussed the theoretical description of these levels. The 22Ne lifetime has been measured many times, by diBerent . methods, and reasonable agreement obtained for most results. For the s°Si lifetime, we are in clear disagreement with both Haas et al. z) and Anyas-Weirs et al. 3). With the exception ofthe measurement by Pixley and Poletti'), all of the mean-life results for the 0.71 MeV level in a°P are in good agreement, but agreement for the 1 .45 MeV level is only obtained with Pixley and Poletti') and for the 1.97 MeV level with Sharpey-Schafer et al. 6). The. disagreement with DSAM results can possibly be explained in terms of insdfficient knowledge of the stopping processes of ions, but the RDM measurements are harder to reconcile. Some possible reasons could be : The measurements by Anyas-Weirs et al. 3) for the 3 °Si 3.79 MeV and 3°P 1 .45 MeV levels are very much lacking in statistics and in sufficient points for the 3 °Si R(D) curve. Henson et al. `) have much better statistics, but their energy resolution is rather poorer, aad again they are slightly lacking in

=~Na, ~=Ne,'°P AND 3 °Si

149

points for their R(D) curves . It would also seem that they failed to take account adequately of excited nuclei which are stopped within the target, that is, that have a recoil velocity of zero . For the'°P 1.45 MeV level, another possible source of error, both for Anyas-Weirs et al. 3) and Henson et al. `) is the 1.46 MeV y-ray from `°K electron capture that underlies the sh:.ïed peak, and of which we found it necessary to take account (see subsect. 2.2). Indoed, it seems entirely possible that Anyas-Weirs et al . mistook this peak for the shifted 1.45 -" 0 MeV peak, and were not exciting the 1 .45 MeV level at all until they turned their target around in order to attempt a DSAM measurement. The disagreement with Haas et al. z) over the a ° Si lifetime is somewhat more difficult to explain, but the lack of a capacitance monitor for their target-stopper distance may have had some effect, especially for their closer distances. Also, the resolution of their detector was again rather poor. The agreement between different runs for each lifetime lends us extra confidence in our results, as does the good agreement with previously established lifetime values in the 22Na and ZxNe nuclei . Althôugh the uncertainties quoted by us seem rather small, they include in each case quite a large contribution for possible small unknown systematic errors. Two groups t3 . ta) have recently measured the EO branching ratio for the '°Si 3.79 ~-decay to the 0* ground state. A weighted average of their results (r,~r = 2.66t 0.33) combined with our measurement of the lifetime of this level leads to a partial EO lifetime of 4.7±0.7 ns which corresponds to 0.068±0.010 single-particle emits t s . t s). TABLE

3

Transition strengths (in Weiaaropf units) for the levels in ' °Si and '°P 100 x B(M1) (W .u.)

B(E2) (W.u .) Nucleus

E' ~ E~

(MeV)

J~ -. J~

theory

theory

experiment shellmodel unified model ~) b) ~

'°P

0.71 ~ 0

1* ~ 1*

a°P '°P a°P '°P '°P '°Si

1 .45 y 0 1 .43 y 0.68 1 .45 y 0.71 1 .97 y 0 °) 1 .97 ~ 0.71 ~ 3.79 -. 2.24

2' 2* 2* 3* 3* 0*

~ 1* ~ 0* y 1* y I* ~ I* ~ 2*

0.7610.12 (or 15.610.5) 0.1610.3 < 1 .7 ~ 6.3 0.6310.10 8.411 .2 1.2810.09

0.42 0.7 5.0 0.02 0.05 3.4 0.39 0.90

1 .1 0.0 0.3 0.3 0.0 1 .0

experiment

1 .17110 .004 (or 0.08410 .004) 0.13410 .006 2.7 0 0 0 ~ 0.07 0 .05 0 6.7 0 0 0.5

shellmodel unified model ~) b) `) 0.55 1.0

0.03

0.01 0

0.16 0 0.22 0 0

0 0 0

0.1 0 1.5 0 0 0

') wldenthal et al. ") using FPSDI calculation. ~ Glaudemans et al.' 6) using case 4 (E2) and case 2 (M1) calculations . ~ Singh et al. 's). °) From analysis of our combined :pears, we were able to place an upper limit of 2 ~ on the branching ratio for the 1.97 ~ 1 .45 MeV transitions, in agreement with ref.'s.

15 0

R D. RADFORD AND A. R POLETTI

The E2 and M1 strengths for transitions from the s°Si and s°P levels studied,in the present work are compared to theoretical predictions in table 3. Glaudemans, Endt and Dieperink t6) and Wildenthal et al. t') have each made shell-model calculations using a number of different models : the figures quoted in the table are from the calculations which seem to give the best overall agreement to the transition strengths The calculations made by Singh et al. t e) are made on the basis of aunified model, and the experimental values are calculated using our lifetime values, the branching ratios compiled in ref. 1°), and Mixing ratios from Harris, Hyder and Walinga t~. It will be noticed that none of the calculations does very well in reproducing the general trend of the experimental strengths The unifed model is the only one to prodict the experimentally large value of the E2 strength for the 3 °P 1.97 -. 0.71 MeV transition ; yet it predicts too large an F.2 strength for the 3°P 1.45 -" 0 MeV transition. Neither the shell model nor the unified model reproduces the very low B(M1) observed for the 3 °P 1 .45 -~ 0.71 MeV transition. We should like to thank M. J. Keeling, W. B. Wood and C. R. Young for their respective parts in the commissioning of the helium ion source at Auckland. Our thanks also go to J. R. Southon who assisted with data collection. Refereaees 1) F. A. Heck, T. Byraki, P. Engelsfein and J. P. Vivien, Nucl . Phys. A228 (1974) 393 2) F. Haas, B. Heuach, G. Frick, A. Gallmann and D. E. Albarger, Nucl. Phys . Als6 (1970) 385 3) N. Anyaa-Weiss, R. Griftitha, N. A. Jelley, W. Randolph, J. Szücs and T. Ii;. Alexander, Nucl . Phya. A201(1973) 513 4) S. H. Henson, S. Cochavi, M. Marmor aad D. B. Foasan, Phys. Rev. C3 (1971) 191 5) R. E. Pixley and A. R Polani, Bull. Am . Phya. Soc. 14 (1969) 125 6) J. F. Sharpey-Schafar, P. R. Alderaon, D. C. Hailey, J. L. Durell, M. W. Greene and A. N. James, Nucl . Phya. A167 (1971) 602 7) F. Haas, R. M. Freeman, J. FernandezCaatillo and A. Galhnann, Phya . Rev. G (1973) 2169 8) D. C. Radford and A. R Polani, NucLPhys. A2S4 (1975) 183 9) T. K. Alexander and A. Bell, Nucl . Inatr. 81 (1970) 22 10) P. M. Endt and C. van der Leun, Nucl . Phya . A214 (1973) 1 11) F. D. Snyder, Phys . Rev. C6 (1972) 204 12) R. H. Spear, R. A. I. Hell, M. T. Eaat, P. R Gardner, D. C. Kean and A. M. Baxter, Phys. Rev. Cll (1975) 742 13) E. K. Warburton and D. E. Albarger, Phya. Rev. C10 (1974) 1570 14) J. C. AdlotT, K. H. Souw, D. Diether, F. Scheibling, P. Chevallier and Y. Wolfaon, Phys. Rev. C10 (1974)1819 15) D. H. Wilkinson, Nucl. Phys . A133 (1969) 1 16) P. W. M. Glaudemans, P. M. Endt and A. E. L. Dieperink, Ann. of Phys. 63 (1971) 134 17) B. H. Wildenthal, J. B. McGrory, E. C. Halbart and H. D. Grabar, Phys . Rev. G(1971)1708 18) B. P. Singh, B. Cartel, K . P. Johnstoneand K. W. C. Stewarf, Phys. Rev. CS (1972) ti613 19) G. I. Harris, A. K. Hyder, Jr. and J. Walinga. Phya. Rev. 187 (1969) 1413