Lifetimes of low-lying levels in 18F

Nuclear Physics A386 (1982) 333-345 © North-Holland Publishing Company

LIFETIMES OF LOW-LYING LEVELS IN' 8F G. C. BALL, T. K. ALEXANDER, W. G. DAVIES, J. S. FÖRSTER and I. V. MITCHELL Atomic Energy of Canada Limited, Chalk Rü~er Nuclear Laboratories, Chalk River, Ontario, Canada KOJ IJO

and J. KEINONEN and H . B. MAK Department of Physics, Queen's University, Kingston, Ontario, Canada K7L 3N6

Received 16 March 1982 Abstract : Mean lifetimes of levels in `sF have been measured using the Doppler-shift attenuation method and the inverse reaction 3 He(' 60, p)' s F. Targets of 3 He implanted into AI, Nb, and Au foils were employed in the measurements. The Doppler-broadened lineshapes observed at 0° to the beam were analyzed to obtain the following lifetime values :0.971 f 0.030, 0 .605 f0.029 and 0.435 t 0 .041 ps for the 1 .70(1 *), 2.52(2* ) and 3 .36(3 * ) MèV members of the K` = 1 * rotational band, 5.12 f 0.56, 0 .403 f 0.018 and 1.91 ±0 .17 ps for the 2.10(2 - ), 3 .13(1 - ) and 3 .79(3 - ) MeV members of the K* = 0 - bands, and (1 .2, 2 .7±i :; and 20±2 fs for the 3.06(2*, T = 1), 3.72(1*) and 3.84{2*) MeV states, respectively. E

NUCLEAR REACTIONS'He(' 60, p), E = 19.4 MeV ; measured DSA, py~oin. 'sF levels deduced T}.

1. Introduction Recent interest in the possibility of investigating the weak hadronic interaction by measuring parity mixing between the 1 .08 (Jx = 0- ; T= 0) and the 1 .04 (0 +, 1) MeV levels') in'8F has motivated a series ofexperimental z - ') and theoretical e-9) studies of the properties of the low-lying levels in mass-18 nuclei . Earlier calculations' °-tz) with truncated (2s, ld) shell-model wave functions predicted only the general features of these levels and quantitative comparisons with experimental results were far from satisfactory. While such wave functions are useful in estimating parity-mixing effects' 3- 's), better shell-model wave functions are needed in order to deduce information on the weak nucleon-nucleon interaction from the experimentally measured parity-non-conserving matrix element. Experiments within the last two years have concentrated on the 1.08 and 1.04 MeV states as well as the analogues of the 1.04 MeV level. There are striking discrepancies between calculated and experimental results. The ß+ decays,' 8Ne(0+,1) -. ' BF(0+, 0) 333

334

C. C. Ball et al. / ' BF

and 18 F(0 +, 0) -.' 80(0 +, 1), are a factor of two slower than the decay rates deduced from the analogue M1 transition z.3 .16), 1 .04(0+ , 1) MeV -. 0.0(0 +, 0) MeV, in 18F. Moreover, the 1aNe(0 + , 1) =.' a F(0- , 0) ß+ decay rate 4 5) is much slower than the value calculated from shell-model wave functions employed in a recent analysis of the parity-non-conserving matrix element' -e ). Haxton has shown that it is possible to quench the ß+ decay rate ofthe 18 Ne(0+ , 1) -~ 18 F(0 - , 0) transition by enlarging the basis of the shell-model wave functions, allowing more admixture into the levels in question . It is of interest to test these wave functions for other low-lying levels in ' aF. Accurate measurements ofthe lifetimes ofthese levels provide useful information on the mixing of2p-Oh and 4p-2h configurations in the positive-parity states, and of 3p-lh and Sp-3h configurations in the negative-parity states . Previously 3 ) we reported the result of a measurement ofthe lifetime ofthe 1 .04 MeV level in 18F determined by the Doppler-shift attenuation method using the inverse reaction 3He( 160, p)1aF . In the present work, accurate lifetime values have been obtained for nine other low-lying levels in 18F . The statistically well determined lineshapes for the transitions from the 1.70 MeV level have allowed us to verify that the electronic stopping powers used in our previous centroid shift analysis ofthe 1.04 MeV level (T = 2.7 f 0.7 fs) are consistent within ± 5 ~ for F in Al, Nb and Au. Ofparticular interest in the present experiment is the lifetime limit ofi < 1.2 fs for the 3.06 MeV (2 + , 1) level in 1aF . When this is combined with the branching ratio value 16) for the 3.06 -. 1.04 MeV transition, a limit of > 5.8 W.u. is obtained for the strength of the isoscalar component in the decay of the first excited 2+ , T= 1 level in mass 18. It will be shown that this is consistent with the strength deduced from the analogue transitions in 18Ne and 180. ~

6)

2. Experiment and results The apparatus and general experimental procedure were the same as those used in our previous lifetime measurements 3) in 18F and are described in more detail in refs. "~ 18) . A beam of 19.4 MeV 16 0 3 + ions from the CRNL MP tandem accelerator was used to bombard targets of3 He implanted at 35 keV into Au, Nb, and A1 foils by the CRNL 70 kV isotope separator . Characteristics ofthe targets are summarized in table 1 and discussed in detail in ref. 3). The 160 beam energy was chosen to maximize the yield to the 3.06 MeV level . The target foils were thick enough to stop the oxygen beam and 1aF ions recoiling from the 3He(160, p)18F reaction. Protons were identified in a dE-E telescope consisting of 50 pm and 1 mm thick surface barrier counters positioned at 0° to the beam direction. The angle subtended by the proton counter was ± 17° ; the correlated 18 F ions recoiled with an average initial velocity ofabout 3.9 ~ the velocity of light into a cone with half-angle ~ 2.7° . The Doppler shifted y-rays were detected in a 100 cm3 Ge(Li) detector positioned at 0°. The detector was located 5.0 cm from the target and shielded from low-energy y-rays

G. C. Ball et al. /'aF

33 5

TABLE 1

Characteristics of the implanted 3 He targets ; the implantation energy was 35 keV Backing material') Au(4) Nb(7) Al(4)

Dose b) {10" atoms/mz)

Q `)

{pm)

co a) (atomic ~)

aN ~ `) (ps- ')

4.0 7.0 4.1

0.05 0.06 0.10

54 80 27

2.80 2.10 1 .08

') The numbers in parentheses refer to the 3He dose and are used to label the targets. b) The implanted area was 1 cmz. Nuclear micro-analysis using the 3He(d, p)`He reaction showed that the retained 3He and the 3He fluentes were linearly related for all targets and that the implant dose was uniform across each target . `) The adopted standard deviations from refs . i 9-2 '). The values correspond to normal density. a) co is the peak atomic concentration of 3He in the material assuming a gaussian range distribution and 100 ~ retention. `) The slowing down parameter for' aF at c = 0.039c" in the unimplanted material is calculated from the stopping powers of ref. z2) and of ref. 23) with the modification outlined in ref. ~s).

and X-rays by 1 mm of lead. The gain ofthis detector was stabilized during playback by analyzing yy coincidences from BBY and 6 °Co sources with a second Ge(Li) detector which was shielded from the target by 10 cm of lead. The yy coincidences from the BB Y and 6 °Co sources were also used to generate intrinsic y-ray lineshapes or detector response functions for the DSA analysis. Lineshape data from each target were accumulated over 24 hr bombardment periods at an average beam current of 150 nA of 16 03 + . Fig. 1 shows a typical coincidence proton spectrum . The proton energy resolution was approximately 160 keV and was adequate to ensure direct feeding of a particular level and permit proper identification of primary and secondary y-transitions.

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Fig. 1. Coincidence proton particle spectrum from '60 bombardment of 3He implanted in Al foil .

33 6

G. C. Ball et al. / 'sF

Figs. 2 to 5 show the background-corrected experimental lineshape data for some of the y-transitions from low-lying levels in t eF. The data have been stripped by using the experimentally determined intrinsic lineshapes and were analyzed as described in refs. t ' ~ t s ) to determine lifetimes by comparison with calculated lineshapes. Experimental iz) electronic stopping powers were used in the present analysis for F slowing down in Au. The values for F in Nb and A1 were obtained by scaling the tabulated values of Northcliffe and Schilling Zs) (NS) according to the discrepancy between NS and measured values sa) for 4He ions. Specilïcally ZS), (dE~dPx)F = (dE/dPx)FS(dE/dPx)âH~/(dE/dPX)âHa at each energy per nucleon considered. The results of this procedure are illustrated in fig. 6. Fig . 2 shows nine experimental lineshapes and the best fits for transitions from the 1 .70 MeV level for stopping in Al (first row), Nb (second row) and Au (third row). Since the lifetime of the 1.04 MeV level is very short (T = 2.7±0.4 fs), the lineshape of the

W Z Z

a x U W

a z 0 U

Fig. 2. Experimental background~orrected Doppler-broadened lineshapes for y-transitions from the 1 .70 MeV level . The dispersion is 0.625 keV/ch . Solid lines are best fits to the data.

33 7

G. C. Ball et al. / ' BF

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Fig. 3. Best fits to the lineshape data for (left to right) the 2.52 -. 0.0 MeV transition (dispersion = 1.25 keV/ch), the 1 .04 -. 0.0 MeV, secondary from the 3.13 decay (dispersion = 0.625 keV/ch) and the 3.36 ~ 1 .70 MeV transition (dispersion = 1 .25 keV/ch) .

1.04 -. 0.0 MeV transition (second columns fed by the 1.70 MeV level, has also been analyzed and the values deduced are in good agreement with those obtained from the 1.70 -. 1 .04 MeV (first column) and 1.70 ~ 0.0 MeV (third column) primary transitions. In addition, there are only small systematic differences in the lifetime values deduced from dif%rent stopping materials. These data show that the errors in the scaled stopping powers used for F in A1 and Nb are small. As a result, uncertainty in the lifetime measurements resulting from such errors was assumed to be ~ ± 5 ~. The excess ofcounts near thefull Doppler-shift ofthe 1.04 -. 0.0 MeV lineshape igcaused by the 1 .08 -i 0.0 MeV, s = 28 ps, transition and has been taken into account in the analysis. It is more pronounced for the Au target since Au is the fastest stopper of the three. The nine measurements of the lifetime of the 1.70 MeV level are summarized in table 2 and give an average value of 0.971 f0 .030 ps . The lineshape data and best fits to the 2.52 --> 0.0 MeV, (3.13 -. ) 1 .04 ~ 0.0 MeV and 3.36 -i 1.70 MeV transitions are shown in fig. 3. Again, each row has data from the

338

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20

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Fiß . 5 . Representative lineshapes observed for the three fast transitions 3.06 ~ 0.94 MeV, 3.72 ~ 1 .04 MeV and 3 .84 ~ 3.06 MeV. The dispersion is 0 .625 keV/ch . The solid lines are fits to the data for the lifetime values indicated and include the eflbcts of swelling of We implanted layer ").

Al, Nb or Au targets, respectively . The 1 .08 -. 0.0 MeV line is again present near the full Doppler-shift of the 1 .04 -. 0.0 MeV transition and has been excluded from the fit. The results are summarized in table 2. Representative best fits to the 2.10 -~ 0.0 MeV and 3.79 -~ 2.10 MeV, slower transitions are shown for the A1 target in fig. 4. These results, as well as the results from the Nb and Au targets, are summarized in table 2. The experimental lineshapes observed for the 3.06 ~ 0.94 MeV, 3.72 -. 1.04 MeV and 3.84 -~ 3.06 MeV short-lived transitions are shown in fig. 5. In these cases the calculated lineshapes must also take into account the swelling of the target layer t7 ). For the levels at 3.06 and 3.72 MeV, the broadening results mainly from the spread in

33 9

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angle subtended by the y-ray detector and the lineshapes are rather insensitive to the nuclear lifetimes. As a result, only limits of i < 5 fs and < 8 fs, respectively, were obtained. A value of T = 21 ±2 fs was determined from least square fits to the experimental lineshapes of the 3 .84 -. 3.06 MeV transition . For the very short-lived levels at 3 .06, 3.72 and 3.84 MeV, the centroid shifts were also used to determine their lifetimes. For short lifetimes measured in a homogeneous medium, the average y-ray energy ~Ey~ observed at 0° is related to the mean lifetime r by the expression 26)

TABLE

2

Summary of lifetimes from lineshape analysis E~

E~

1 .70

2 .10 2 .52 3 .13 3 .36 3 .79 3.84

r (ps) ") AI

Nb

Au

average

Published values 'e) s (ps)

1 .035±(0.020) 1 .030 ± (0.020) 0 .980 ± (0.040) 1 .022± 0 .053 5 .60 t 0.75 0.650+_ 0 .048 0.390 +_ 0 .028 0.450± 0.055 1 .92 f 0 .20 0.022 t 0 .004

0 .935±(0 .035) 0 .925 f (0 .025) 0 .910 ± (0 .065) 0 .926± 0 .050 4 .20 t 0 .93 0 .600+_ 0 .054 0 .405 +_ 0 .032 0 .415± 0 .073 1 .88 ± 0 .37 0 .021 f 0 .002

0 .970±(0.050) 0 .975 f (0.030) 0 .945 ±(0.070) 0.967± 0 .054 6 .2 ± 2 .2 0.565+_ 0 .049 0.425 +_ 0 .037 0.420f 0.120 1 .86 ± 0 .79 0.022 ± 0 .003

0 .971±0 .030 5 .12 ±0 .56 0 .605+0 .029 0 .403 + 0 .018 0 .435±0 .041 1 .91 ±0 .17 0 .021 t 0 .002

1 .09 f0 .1 4 .3 f 1 .4 0 .68 _+0 .11 0 .32 +_ 0 .10 0 .49 ±0 .07 0 .224±0 .035 0 .029 t 0 .009

E~

(MeV) (MeV) (MeV) 1 .70 1 .04 1 .04 0 1 .70 0 average 2 .10 0 2 .52 0 1 .04 0 3 .36 1 .70 3 .79 2.10 3 .84 3 .06

') Values in brackets are statistical errors only while the total error includes a ±5 °ô uncertainty in electronic stopping .

340

G. C. Ball et a1. / ' 8F

where EY (t = 0) is the fully shifted y-ray energy, dEy is the full Doppler-shift and aN r = (1/v)(dv/dt) is the characteristic slowing down time of the material . For an implanted target the characteristic slowing down time in the region of the swollen material is given by l ') a- t = aN' (1 + S)/(1 + Ac), where c is the local atomic concentration of 3 He, A = 0.75±0.25 is an experimentally determined proportionality constant and 1 + S is the correction factor to the stopping power for the added 3 He. In the present analysis, the implanted targets were approximated by a two-layer medium in which the gaussian implant profiles with peak concentration co and standard deviations tr were taken to be rectangular profiles with a constant concentration c o and a width of 2ntr . The effective slowing down parameter a~rf for such a medium is given by r') Aco

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G . C. Ball et al. / ' eF

34 1

a~ff = 0 are the full relativistic Doppler-shifts calculated from the known energies of the transitions and recoil velocities determined by the kinematics of the reaction. The attenuation due to the solid angles ofthe detectors was applied to the calculated value. The solid lines in fig. 7 are least squares fits to the four data points for each case. The fit for the 3.06 -+ 0.94 MeV transition gives a positive slope, 0.12 ±0.11 keV ~ ps consistent with zero slope (only zero or negative slopes are meaningful) yielding a 2a limit of r < 1 .2 fs. The lifetime ofthe 3.84 MeV level, deduced from the centroid shift, 17.4 ± 3.6 fs, is in good agreement with the value deduced from the lineshape analysis, 21 ± 2 fs. The weighted average is 20±2 fs. The centroid shift results are summarized in table 3. T~HI,s 3 Summary of lifetimes from centroid shift analysis F, F (MeV)

E; (MeV)

E~ (MeV)

3 .06 3 .72 3 .84

3 .06 3 .72 3 .84

0 .94 I .04 3 .06

Present work') Xz

z (fs)

Previous value n) z (fs)

1 .2 1 .8 0 .5

< 1 .2 `) 2 .7±4.1 17 .4 ± 3 .6

< 1 .5 4f2 29 f 9

') Total errors include statistical errors as well as uncertainties in the average recoil velocity, the stopping power and the target density . n) Ref . ' °) . `) The 2a limit based on r = -1 .2t 1 .2 fs .

3. Discussion The lifetime values obtained in the present experiment for nine low-lying levels in t BF are compared with previous data'6) in tables 2, 3. With the exception of the 3.79 MeV level, the results are in agreement with previous values ; however, the uncertainties in most cases have been substantially reduced . Our result for the lifetime of the 3.79 MeV level is a factor of 10longer than the value reported by Rolfs et al. s') who studied this level via t4N(a, y)te F and t 'O(p, y)teF resonances . At Queen's University, the t4N(a, y)t gF resonance at E = 2.35 MeV was studied with a 120 ug/cmZ thick TiN target . The F (i) value obtained from measurements at 0° and 90° was less than 3 ~, confirming the lifetime value deduced from the high recoil velocity measurement but at variance with the value F (i) _ (27±6) ~ published by Rolfs et al. z'). In the latter experiment, consistent results -were obtained in six separate measurements . The reasons for this discrepancy are not clear. Part of the argument Z') for assigning a J~ of3- to this level was based on the y-strength of this level to the 3.06 MeV level, but reduction by a factor of 10 in strength would still allow a 3 - assignment, the spin and parity favoured independently by the y-decay to this level from levels at higher excitation energies . The y-transition rates deduced from the present work and published branching

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ratios 16) are listed in table 4. For comparison, the transition rates from earlier theoretical calculations 1°-iz) are also listed . There are large discrepancies between the experimental results and the predictions of these different models. The structure ofthe lowlying levels has been discussed in detail by Rolfs et a1 .2''Z9'3°). The complicated ydecay schemes for these levels, especially the large cross-band E2 transitions, indicate substantial configuration mixing in the shell-model wave functions. The limit of r < 1.2 fs for the lifetime ofthe 3.06 MeV 2+, T= l level in t8 F from the present experiment and the measured branching ratio 16 ), (0.1110.03) ~, yields ~M (E2)~Z > 5.8 W.u. for the dT = 0, E2 transition to the 1.04 MeV 0+, T= l level. The analogue transitions in ' 80 and 1BNe have ~M (E2)~Z = 3.4 f 0.08 W.u. [ref. 3 t )] and 18.2± 1 .6 W.u. [ref. 32 )], respectively, implying 33) that for teF, ~M (E2)~2 = 9 .3±0.9 W.u., the isoscalar part for mass 18 in the absence of isospin mixing. Recent 3 ") shellmodel calculations predict a value of 5.9 W.u. These dT = 0, E2 analogue transitions are of interest to the interpretation of recent n+ inelastic scattering experiments 35 .36) . With the exception ofsome very weak branches, the electromagnetic transition rates for most of the low-lying levels in 18F have been measured accurately. The results should provide a rather severe testing ground for any new shell-model wave functions . It is essential that any set of wave functions used to calculate the parity-mixing effects between the 1 .04 and 1.08 MeV levels also predict the electromagnetic transition rates of most, if not all, the low-lying levels in t rF. References 1) C. A. Barnes, M. M. Lowry, J. M. Davidson, R. E. Marrs, F. B. Morinigo, B. Chang, E. G. Adelberger rind H. E. Swanson, Phys . Rev. Lett. 40 (1978) 840 2) J. Kainonen, H. B. Mak, P. Skensved, J. R. Leslie and W. McIatchie, Phys . Rev. 22C (1980) 351 3) J. Kainonen, H. B. Mak, T. K. Alexander, G. C. Ball, W, G. Davies, J. S. Forstar and I. V. Mitchell, Phys . Rev. 23C (1981) 2073 4) E. C. Adelberger, C. D. Hoyle, H. E. Swanson and R. D, Von Lintig, Phys . Rev. Lett. 46 (1981) 695 5) T . J . Bowler, J . Browne, P. Lisowski, M. Steps and R. G. H. Robertson, Bull . Am . Phys . Soc. 26 (1981) 6) 7) 8) 9)

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