Lifetime of the 22Na 3708 keV level

Nuclear Physics A136 (1969) 160----164; (~) North-Holland Publishino Co., Amsterdam Not to be reproduced by photoprint or microfilm without written permission from the publisher

LIFETIME OF THE 22Na 3708 keV LEVEL E. K. WARBURTON t, L. E. CARLSON, G. T. GARVEY, D. A. HUTCHEON and K. P. JACKSON Nuclear Physics Laboratory, Oxford, England Received 23 July 1969

Abstract: The mean lifetime of the 22Na 3708 keV level was measured by the Doppler-shift attenuation method. The 19F(ct, n)22Na reaction at alpha energies between 9 and 11 MeV was used to populate the state. Observations of gamma rays de-exciting the state were made using a Ge(Li) detector in singles and also as the center detector of a combination anti-Compton and pair spectrometer. A mean lifetime of 524-17 fs was obtained.

E

NUCLEAR REACTION: 19F(~t,n); E = 9, 11 MeV; measured ET, Doppler shift attenuation. 22Na level deduced T~. Natural target, Ge(Li) detector.

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1. Introduction The nuclei between masses 19 and 25 have rather highly developed rotational structure and m u c h effort has gone into study o f this phenomenon. In recent years a large fraction o f the experimental effort has been to locate the high spin states of rotational bands. Since these states usually lie in a region o f rather high level density it is important to find a reaction that excites them selectively. The present w o r k was undertaken to explore the possibility of forming and identifying high spin states o f 22Na by means: o f the lgF(~, n)22Na reaction. It has been f o u n d 1,2) that the 19F (ct, p)22Ne reaction ( a = 1698 k e V ) f o r m s the 6.35 MeV J~ = 6 + state of the g r o u n d state (J~ = 0 ÷) band of 22Ne quite strongly at E~ - 12 MeV. It was hoped that at comparable alpha energies high spin states of the three lowest rotational bands 3) of 22Na would be formed by the similar 19F(~, n)22Na reaction (Q = - 1 9 5 1 keV).

2. Experimental procedure and results The first part of the present study o f g a m m a - r a y emission following ~-bombardment of 19 F was undertaken using a 30 cm 3 Ge(Li) detector inside a split NaI(T1) annulus operated simultaneously as a three crystal pair spectrometer and as an antiC o m p t o n spectrometer 4). The target consisted o f a slurry of SrF2, ~ 5 m g / c m z thick, deposited on a thick gold backing. Spectra were recorded at 0 ° and 90 ° to an 11.0 MeV, 50 n A beam o f 4He+ + f r o m the Oxford Van de G r a a f f accelerator. t u.s. National Science Foundation Postdoctoral Fellow 1968-9. 160

161

LIFETIME OF 22Na

The total running time was ,,~ 12 hours. Over 70 resolved gamma-ray transitions with energies less than 5.8 MeV were observed in the pair spectra and even more were seen in the anti-Compton spectra. We shall concentrate on those lines pertinent to the discussion of sect. 1. The levels in 2ZNa at 891 keV and 1528 keV have been identified as the J= = 4 + and 5 + members of a rotational band based on the 3 + ground state. It is suspected 3) but not established, that the state at 3712+6 keV [ref. 5)] is the 6 + member of this band. The gamma-ray decay of this level to those at 891 keV (65 + 10 ~o) and 1528 keV

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Fig. ]. Portions of the 0 ° and 90 ° pair spectra showing the gamma-ray energy region from 2.8 to 3.2 MeV. The five peaks are identified in the text and in table 1. Peak E, from the decay of the 2~Ne 4.46 MeV level to the 1.27 MeV level, has a stopped component (apparent in the 0 ° spectrum) due to feeding by the long-lived 22Ne 5.14 MeV level 7).

(35+ 10 7oo) had previously been observed in the 2°Ne(z, p)22Na reaction using a gas target 6). The energy regions of the pair spectra in which the transition to the 891 keV level is anticipated are shown in fig. 1. The corresponding portions of the anti-Compton spectra exhibited the same 5 peaks evident in fig. 1. In table 1 are listed the measured energies of these five transitions, the observed Doppler shifts of the centroids of the peaks and the values of F(z), the ratios of these shifts to the full shifts calculated assuming (cos 0 .... ) = 0.0±0.33. The assignments of the gamma-ray transitions are

162

E.K. WARBURTONet al.

based on the energy measurements, on Ge(Li) singles spectra taken at 1 MeV intervals in alpha bombarding energy between 7 and 11 MeV, and on Ge(Li)-NaI(T1) gamma-gamma coincidence measurements taken at E~ = 9, 11 and 11.5 MeV. The coincidence measurements confirmed the assignments made to peaks A and C. The evidence from this source for the assignment made to peak B was only suggestive. The assignment to peak B is thus made from energy measurements alone and is only tentative. The 3109 keV gamma ray (peak D) was too weak to be observed in the singles or coincidence experiments. Its origin is not known but considering the Doppler shift and the line shape it is probably a transition in 22Ne or 22Na. The unshifted component of peak E results from a well-known 1,2, 7) cascade in 22Ne. This component was not included when calculating the centroid of the shifted peak. The gamma ray corresponding to the 22Na 3.71 --* 1.53 MeV transition was observed in the coincidence spectra but was obscured in all other spectra by a 2170+2 keV gamma ray which probably arises from the decay of the 22Ne 5.52 MeV state to the 3.36 MeV state 2). TABLE 1 Results from 19F+~ at E= = 11 MeV Peak A

E~ (keY)

AE r (keV)

2816.7±0.7

34.5

F(T)

Assignment

0.92i0.09

22Na 3.71 --~ 0.89

B

2951.6+1.0

38.9

0.99:~0.08

(22Na 4.47 --~ 1.53)

C

2991.9+0.7

37.4

0.94d:0.11

22Ne 6.35 --~ 3.36

D

3108.9±0.7

42.5

1.03 a)

E

3179.4-4-0.9

41.8

0.99d:0.13

9. 22Ne 4.46 ~ 1.27

a) Assuming the peak arises from X9Fq-~t with
Previous measurements 7) have established the excitation energies of the lowlying 4+(890.9+__0.2 keV) and 5+(1528.1 +_0.3 keV) levels in 22Na. These values and the measured energies of the two transitions in 22Na listed in table 1 yield excitation energies for the initial levels of 3707.6 +__0.8 and 4479.7 +__1.1 keV. The best previous estimates of these excitation energies were 3712+-6 and 4 4 7 3 _ 6 k e V [ref. 5)]. The present gamma-ray energy measurements averaged with the previous results of Warburton et al. 7) and Kutschera et al. 2) yield excitation energies for the second and third excited states of 22Ne of 3356.2+-l.4keV and 4454.3+_0.9 keV, based on an energy of 1274.52+_0.07 keV [ref. s)] for the gamma-ray decay of the first excited state of 22Ne. Combining the above excitation energy of the 4 + second excited state with the 6 + to 4 + transition energy (table 1) gives 6348.2+_ 1.6 keV for the excitation energy of the J~ = 6 + state. The F(z) values for lines A and C of table 1 suggest that both originate from levels whose lifetimes are measurable by the Doppler-shift method but, if so, the kinematical full shift must be better determined. The method used was to observe the Doppler

LIFETIME

163

OF 22Na

shift in gas (CC12F2) and solid (CaF2) targets. The former was at a low enough pressure to give the full shift and thus, F(,) = l-a, where

A = [Er(gas)oo-E~(solid)oo]/[Ev(gas)oo-E?(gas)9oo]. The energies are obtained from centroids and F(z) pertains to the solid. In principle, this method is the same as that of observing the Doppler shift for a thin target with emission of g a m m a rays into a solid backing and into vacuum. In practice, it is quite difficult to get the target thin enough so that this latter method is accurate for fast lifetimes. i

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Fig. 2. The full energy peak of the 22Na 2817 keV gamma ray from lOF(~, nT)2aNa observed at 0 ° to the beam in gas and solid targets. The centroids of the two peaks are indicated by solid lines. Note the suppressed zeros.

The measurements were made at E, = 9 MeV after a search for the optimum peak to background conditions for the 2817 keV peak from 22Na 3708 ~ 891. A 50 cm 3 Ge(Li) detector with a resolution of about 6 keV F W H M for the 6°Co lines was used. The CaF2 target was 600 #g/cm 2 thick and on a Pb backing. The CC12F2 gas was confined at a pressure of 0.I atm ( ~ 600 #g/cm 2) by a Ni foil, 1.27 p m thick. The beam energy was adjusted so that the mean alpha energy was the same in the gas and solid target, the loss in the Ni foil being ~ 330 keV. At E~ = 9 MeV the full energy peaks corresponding to peaks A, B and C of fig. 1 were evident; the 3109 keV line (peak D) was not observed. However in the gas target a line of unknown origin obscured the 2992 keV g a m m a - r a y peak C so that only peaks A and B were studied. In fig. 2 is shown full energy peaks of the 2817 keV line observed at 0 ° from the CaF2 and gas

164

E. K. WARBURTONet a[.

targets. The positions of the centroids o f the peaks are indicated. It is evident that there is a difference between the peak energies. Three spectra from each target were recorded at 0 ° in addition to a 90 ° spectrum recorded f r o m the CaF2 target. F o r these spectra internal calibration was provided by a T h C source (2614.47_+0.10 keV) [ref. 9)]. The results of these measurements were F(z) values for stopping in C a F 2 of 0.913_+0.024 and > 0.965 for the 2817 and 2952 keV lines, respectively, based on [E~(gas)oo- E~(solid)oo ] measurements of 2.9_+ 0.7 keV and < 1.25 keV, respectively. In both cases the 0°-90 ° shift in the gas target was within 2 % of that calculated for (cos 0c.m.) = 0 and these values of F ( z ) are in g o o d agreement with those extracted at E~ = 11 MeV (table 1) with this assumption. The mean lifetimes corresponding to these values o f F(z) were extracted using standard procedures 1o). In particular, the electronic stopping power was taken as 1.05 times the Lindhard-Scharff value 11), with an assigned error of 15 %. The Lindhard-Scharff-Schiatt 11) representation of the nuclear stopping cross section was used and the effects o f nuclear scattering were taken explicitly into account. The results are 5 2 + 17 fs and < 24 fs, respectively.

3. Discussion A definite new result obtained f r o m the present efforts is a lifetime estimate for the 22Na 3708 keV level, "c = 5 2 + 17 Is. This result and the k n o w n 6) branching ratios lead to partial radiative widths for decay to the 4+, 891 keV level and the 5 +, 1528 keV level o f 8.2___3.0 meV and 4 . 4 + 1.8 meV, respectively. These are both too large to be anything but predominantly dipole or E2. Thus the 3708 keV level has J = 3, 4, 5, or 6 and if J = 3 or 6 the parity is even. As noted previously 3), if the 3708 keV level were the 6 + member of the 22Na K = 3 rotational b a n d it should decay to the 4 + and 5 + states with branching ratios of ~ 66 % and ~ 34 %, respectively, and have a mean life o f ~ 70 fs. The latter prediction is in agreement with the present measurement so that the 3708 keV level is still an excellent candidate for the 6 + m e m b e r o f the 22Na g r o u n d state rotational band.

References 1) 2) 3) 4) 5) 6) 7) 8) 9) 10) 11)

S. Buhl, D. Pelte and B. Povh, l'qucl. Phys. A91 (1967) 319 W. Kutschera, D. Pelte and G. Schrieder, Nucl. Phys. A l l l (1968) 529 E. K. Warburton, A. R. Poletti and J. W. Olness, Phys. Rev. 168 (1968) 1232 R. L. Auble et al., Nucl. Instr. 51 (1967) 61 P. M. Endt and C. van der Leun, Nucl. Phys. A105 (1967) 1 A. R. Poletti, E. K. Warburton, J. W. Olness and S. Hechtl, Phys. Rev. 162 (1967) 1040 E. K. Warburton, J. W. Olness and A. R. Poletti, Phys. Rev. 160 (1967) 938 W. W. Black and R. L. Heath, Nucl. Phys. A90 (1967) 650 G. Murray, R. L. Graham and J. S. Geiger, Nucl. Phys. 63 (1965) 353 A. Z. Schwarzschild and E. K. Warburton, Ann. Rev. Nucl. Sci. 18 (1968) 265 J. Lindhard, M. Scharff and H. E. Schi~tt, Mat. Fys. Medd. Dan. Vid. Selsk. 33 (1963) No. 14