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Lifetime of the second excited state of 43K
Nuclear Physics A250 (1975) 257--259; ( ~ North-Holland PubllsMng Co., Amsterdam Not to be reprod,__t,99d__by photoprint or microfilm without writtlm pefmiuion from tha publisher
L I F E T I M E O F T H E S E C O N D EXCITED STATE O F ° K E. BOZEK *, C. G E H R I N G E R , C. J A E G E R and J. C. M E R D I N G E R
Centre de Recherches Nucktalres et Univer$1tdLouis Pasteur, Strasbourg, France Received 19 June 1975 Abs~act: The lifetime of the 738 keV level of 't3K, excited by means of the 4eAr(or, p)43K reaction, has been deduced from the distribution of time delays between protons and decay y-rays. The half-life, 205-t-10 ns, favors a j,r = t - assiloament. E [
I
N U C L E A R REACTION SOAr(,,, Pg'), E = 11.7 MeV; measured E~,(O), P'9' delay. 4SK level deduced T | , J, ~.
I. Introdnctlon Among the low-lying states of odd-A nuclei in the upper half of the s-d shell, states with J~ = ] - have been identified and generally interpreted within the framework of the shell model as ft single-particle states. The F-decay of these states to the J~ = 3 ÷ ground states proceeds via M2/E3 transitions whose partial M2 lifetimes are retarded by about an order of magnitude as compared to single-particle estimates. So the lifetimes of these states are long and typically lie in the nanosecond time range when the energy of the transition is of the order of I MeV, as it is the case in 4tK for example t). The 43K second excited state, at 0.74 MeV, might be a J " = 5 - state. Santo et al. 2) have shown that the 44Ca(t, ce)'t3K reaction populates this state via lp = 3 pick-up, implying J " = 3- or 5-- In the 44Ca(p, d)4aCa reaction, Martin et ai. 3) identified the analogue states corresponding to 43K states and their angular distributions lead to an 1o = 3 assignment for the analogue state of the 0.74 MeV state in 43K. Since this level decays to the J~ = 3 + ground state 4) the half-lives are expected to be vastly different for the two cases: ~--,3}+ by E l / M 2 and ] - - - . ~ * by M2/E3. We report here the results of a half-life measurement of this state.
2. Experlmeatal method A Doppler-shift attenuation measurement was first performed in order to identify the F-transitions in 43K which correspond to states with long lifetimes. Protongamma coincidence spectra from the reaction 4°Ar(~, pF)4~K were measured at E, = 11.7 MeV using a gas target chamber. Pure (99.99 %) natural argon gas at a pressure o f 2 arm retained by a 4 mg/cm 2 gold foil was used. Protons were detected t Permanent address: Institute of Nuclear Physics, Cracow, Poland. 257
258
E. BOZEK et
aL
in two identical S00 #m thick surface barrier detectors each placed at an angle o f 90 ° with respect to the incident beam direction. These detectors were covered with 53 mg/cm 2 thick tantalum foils to stop elastically scattered ~-particles. Two particle counters, whose output signals were summed for both energy and timing channels, were used because of the low coincidence yield. The 84 cm ~ Ge(Li) ?-ray detector could be rotated between 0~ = 0 ° and 90 ° at 7 cm from the entrance window of the target chamber. The lifetime of the 738 keV level was determined by measuring the distribution of time delays between protons feeding this state and the 738 keV },-rays in a time-topulse-height conversion system. The same detectors as for the DSA measurement were used both for the protons and the 7-ray detection, Time spectra gated by slowcoincidence energy conditions imposed on both particle detectors and 7-rays were accumulated in 2048 channels on the time axis. Kinematic energy spreads did not allow separation of the proton groups and the slow-coincidence conditions of the particle detectors were adjusted to accept all the reaction protons while eliminating
2'
Y
2¸ 10, 5,
24
level
ttt ,l
loo, 5¸
a
Z'3K:738keV
tt' itItI t i ' !!j!!Ili T l l 2 = 2 0 5 "L'-10n$
l
P
5O
100 150 CHANNEL NUHBER
200
I!
250
Fig. 1. The experimental decay curve for the 738 keV level in 4SK obtained in run 2. A uniform background has been subtracted from these data. Since the proton groups from the '*°Ar(~, p)ZSK reaction were not resolved, the time spectrum contains a prompt component. The line drawn through the portion of the data between the arrows, illustrates a calculated fit to these data. The time calibration for this distribution is 9.26 × 10-9 s/channel.
aSK LIFETIME
259
most of the//-rays originating from the "3K ground-state decay. On the 7-ray side a narrow window was set on the photopeak of the 738 keV y-ray. In order to measure the system response to a prompt event, another time spectrum was recorded simultaneously for which the y-ray window was set just on the high-energy side of the 738 keV photopeak. This window corresponded to Compton scatterings from y-transitions arising from upper states, with lifetimes short compared with the electronic time resolution. The data for the lifetime measurement were taken over about 35 h for each of two independent runs. 3. Results and discussion
From the DSA measurement, the excitation energy of the second excited state was determined to be 738.4-t-0.5 keV in good agreement with the value found from the radioactive decay of *3Ar [ref. *)]. Due to the long slowing-down time of the recoiling nuclei at the gas pressure used, all y-lines were fully shifted in the 0 ° spectrum, except for the line at 738 keV which showed the same energy and shape as in the 90 ° spectrum. This leads to a lower limit ~ > 6 ns for the lifetime of this state, while all other observed 43K levels should have lifetimes shorter than 150 ps. The decay curve for the 738 keV level, observed in one of the electronic-timing measurements, is shown in fig. 1. From a least-squares fit of a straight line to the logarithmic slope superimposed on a constant background, half-life values of 199+ 14 and 211 4- 15 ns were obtained for the two runs. This results in a half-life value of 205+ 10 ns for the 738 keV level. Statistical and time calibration uncertainties are included in the error limits. The lifetime gives strong evidence for a d" = ~- assignment for the 738 keV level. in fact the M2 strength of this g.s. transition would be 0.06 W.u., quite close to the corresponding strength for well identified ~- ~,} + transitions, whereas the E 1 strength of the same transition would be 6 x 10 - 9 W. u. which is four orders of magnitude smaller than the weakest El transition strength known s) in this mass region. References I) P. M. Endt and C. van der Leun, Nacl. Phys. A214 0 9 7 3 ) 1 2) R. Santo, R. Stock, J. H. Bjerregaard, O. Hansen, O. Nathan, R. Chapman and S. Hinds, Nucl. Phys. A I I 8 (1968) 409; A139 (1969) 711 (erratum) 3) P. Martin, M. Buenerd, Y. Dupont nnd M. Chabr¢, Nuci. Phys. Alg5 (1972) 465 4) J. Hudis, E. Hageb6 and P. Patzelt, Nucl. Phys. A l S l (1970) 634 5) P. M. Endt and C. van der Leun, Nucl. Phys. A235 0974) 27
L I F E T I M E O F T H E S E C O N D EXCITED STATE O F ° K E. BOZEK *, C. G E H R I N G E R , C. J A E G E R and J. C. M E R D I N G E R
Centre de Recherches Nucktalres et Univer$1tdLouis Pasteur, Strasbourg, France Received 19 June 1975 Abs~act: The lifetime of the 738 keV level of 't3K, excited by means of the 4eAr(or, p)43K reaction, has been deduced from the distribution of time delays between protons and decay y-rays. The half-life, 205-t-10 ns, favors a j,r = t - assiloament. E [
I
N U C L E A R REACTION SOAr(,,, Pg'), E = 11.7 MeV; measured E~,(O), P'9' delay. 4SK level deduced T | , J, ~.
I. Introdnctlon Among the low-lying states of odd-A nuclei in the upper half of the s-d shell, states with J~ = ] - have been identified and generally interpreted within the framework of the shell model as ft single-particle states. The F-decay of these states to the J~ = 3 ÷ ground states proceeds via M2/E3 transitions whose partial M2 lifetimes are retarded by about an order of magnitude as compared to single-particle estimates. So the lifetimes of these states are long and typically lie in the nanosecond time range when the energy of the transition is of the order of I MeV, as it is the case in 4tK for example t). The 43K second excited state, at 0.74 MeV, might be a J " = 5 - state. Santo et al. 2) have shown that the 44Ca(t, ce)'t3K reaction populates this state via lp = 3 pick-up, implying J " = 3- or 5-- In the 44Ca(p, d)4aCa reaction, Martin et ai. 3) identified the analogue states corresponding to 43K states and their angular distributions lead to an 1o = 3 assignment for the analogue state of the 0.74 MeV state in 43K. Since this level decays to the J~ = 3 + ground state 4) the half-lives are expected to be vastly different for the two cases: ~--,3}+ by E l / M 2 and ] - - - . ~ * by M2/E3. We report here the results of a half-life measurement of this state.
2. Experlmeatal method A Doppler-shift attenuation measurement was first performed in order to identify the F-transitions in 43K which correspond to states with long lifetimes. Protongamma coincidence spectra from the reaction 4°Ar(~, pF)4~K were measured at E, = 11.7 MeV using a gas target chamber. Pure (99.99 %) natural argon gas at a pressure o f 2 arm retained by a 4 mg/cm 2 gold foil was used. Protons were detected t Permanent address: Institute of Nuclear Physics, Cracow, Poland. 257
258
E. BOZEK et
aL
in two identical S00 #m thick surface barrier detectors each placed at an angle o f 90 ° with respect to the incident beam direction. These detectors were covered with 53 mg/cm 2 thick tantalum foils to stop elastically scattered ~-particles. Two particle counters, whose output signals were summed for both energy and timing channels, were used because of the low coincidence yield. The 84 cm ~ Ge(Li) ?-ray detector could be rotated between 0~ = 0 ° and 90 ° at 7 cm from the entrance window of the target chamber. The lifetime of the 738 keV level was determined by measuring the distribution of time delays between protons feeding this state and the 738 keV },-rays in a time-topulse-height conversion system. The same detectors as for the DSA measurement were used both for the protons and the 7-ray detection, Time spectra gated by slowcoincidence energy conditions imposed on both particle detectors and 7-rays were accumulated in 2048 channels on the time axis. Kinematic energy spreads did not allow separation of the proton groups and the slow-coincidence conditions of the particle detectors were adjusted to accept all the reaction protons while eliminating
2'
Y
2¸ 10, 5,
24
level
ttt ,l
loo, 5¸
a
Z'3K:738keV
tt' itItI t i ' !!j!!Ili T l l 2 = 2 0 5 "L'-10n$
l
P
5O
100 150 CHANNEL NUHBER
200
I!
250
Fig. 1. The experimental decay curve for the 738 keV level in 4SK obtained in run 2. A uniform background has been subtracted from these data. Since the proton groups from the '*°Ar(~, p)ZSK reaction were not resolved, the time spectrum contains a prompt component. The line drawn through the portion of the data between the arrows, illustrates a calculated fit to these data. The time calibration for this distribution is 9.26 × 10-9 s/channel.
aSK LIFETIME
259
most of the//-rays originating from the "3K ground-state decay. On the 7-ray side a narrow window was set on the photopeak of the 738 keV y-ray. In order to measure the system response to a prompt event, another time spectrum was recorded simultaneously for which the y-ray window was set just on the high-energy side of the 738 keV photopeak. This window corresponded to Compton scatterings from y-transitions arising from upper states, with lifetimes short compared with the electronic time resolution. The data for the lifetime measurement were taken over about 35 h for each of two independent runs. 3. Results and discussion
From the DSA measurement, the excitation energy of the second excited state was determined to be 738.4-t-0.5 keV in good agreement with the value found from the radioactive decay of *3Ar [ref. *)]. Due to the long slowing-down time of the recoiling nuclei at the gas pressure used, all y-lines were fully shifted in the 0 ° spectrum, except for the line at 738 keV which showed the same energy and shape as in the 90 ° spectrum. This leads to a lower limit ~ > 6 ns for the lifetime of this state, while all other observed 43K levels should have lifetimes shorter than 150 ps. The decay curve for the 738 keV level, observed in one of the electronic-timing measurements, is shown in fig. 1. From a least-squares fit of a straight line to the logarithmic slope superimposed on a constant background, half-life values of 199+ 14 and 211 4- 15 ns were obtained for the two runs. This results in a half-life value of 205+ 10 ns for the 738 keV level. Statistical and time calibration uncertainties are included in the error limits. The lifetime gives strong evidence for a d" = ~- assignment for the 738 keV level. in fact the M2 strength of this g.s. transition would be 0.06 W.u., quite close to the corresponding strength for well identified ~- ~,} + transitions, whereas the E 1 strength of the same transition would be 6 x 10 - 9 W. u. which is four orders of magnitude smaller than the weakest El transition strength known s) in this mass region. References I) P. M. Endt and C. van der Leun, Nacl. Phys. A214 0 9 7 3 ) 1 2) R. Santo, R. Stock, J. H. Bjerregaard, O. Hansen, O. Nathan, R. Chapman and S. Hinds, Nucl. Phys. A I I 8 (1968) 409; A139 (1969) 711 (erratum) 3) P. Martin, M. Buenerd, Y. Dupont nnd M. Chabr¢, Nuci. Phys. Alg5 (1972) 465 4) J. Hudis, E. Hageb6 and P. Patzelt, Nucl. Phys. A l S l (1970) 634 5) P. M. Endt and C. van der Leun, Nucl. Phys. A235 0974) 27