Precise excitation energy and γ-ray decay of the lowest T = 2 state in 40K

Nrrclear PhysW A290 (1977) 200204 ; © Nortb-Hofland PublWilno Co., Amur" Not to be reproduced by photoprint of microfilm wlthoot wrMten permialou from the yubliehw

PRECISE EXCTTATfON ENERGY AND 7-RAY DECAY OF THE LOWEST T = 2 STATE IN "K W. A. STERRENBURG, Ci . VAN MIDDELKOOP and J. A. ß. DE RAEDT Fyadwh Labomtorium, Rijluuntoaraiteit, UtreMt, The Netherlmtda

Received 9 May 1977 (Revised 29 June 1977) Abstract : Theexcitation seamy of the lowest T = 2state in 40K has been determined as Fj - 4384,0± 0.3 loeV from n-y and yry coincidence experiments. The state was populated with the 40Ar(p, n)40K reaction at F.r = 8.30 MeV. Gamma-gamma angular correlation measurements yield unambiguous spin assignments J = 0 and 1 for the 1 .64 and2.290 MeV states, respectively. The excitation energy of the T- 2, J' - 0+ state leads to a calculated mass excess of -9120± 150 keV for 4011. B

NUCLEAR REACIYONS 40K deduced levels J, a 40Ar(p, ny), E - 8.30 MeV; measured ny-, yy-coin, yy(0). 40 Ti cakulated mass excess.

Atomic mass excesses of proton-rich nuclei can be calculated with the (quadratic) isobaric mass equation ifthe energies of at least three members of isobaric multiplets are known 1). The validity of the mass equation for sd shell nuclei can be checked in nine independent cases (eight quadruplets and one quintuplet). In these cases the energies offour members are known. In fig. l the difference AE between the calculated and the measured mass excess of the most proton-rich member is plotted versus mass number. The mean value of AE, 7 f 12 keV, indicates that the calculation is reliable to within about 20 keV. In the A = 40, T = 2 quintuplet the lowest T = 2, J'° = 0+ states are known for 4oAr(g.s.), 40K(4379f 16keV) and 40Ca(111978±25 keV) [ref. 2)]. From a simple calculation one finds the following expression for the 40Tí mass excess M(40n) = 6{m(4oCaY-i"E=( 4oCa)) -8{m(40K)+EX(40 K))+3m( 4OAr), where m denotes the mass excess and F . the excitation energy of the lowest 7' = 2 state. Since the 4°K excitation energy appears with a weight factor of eight, it seemed worthwhile measuring the 4oK excitation energy precisely via its y-ray decay. 2. Experinumt For this experiment we chose the 40Ar(p, n)40K reaction because the wave func tions ofthe 40Àr and 4 oK T - 2 states are expected to have alarge overlap. Neutron200

25 30 -ATOMIC MASS NUMBER

Fig. 1. The energy difference between calculated and nwasured mass excesses of proton-rich nuclei in the ad shell.

y and 7-7 coincidence experiments were performed. In the latter also 7-7 angular correlations were measured . Protons were accelerated to an energy of Ep = 8:30 MeV with the Utrecht 7 MV tandem Van de Graaff generator. The beam current was maintained at 20 nA. In the n-y experiment the target consisted of a 10 cmlong by 7mmdiameter stainless steel cylinder closed off at both ends by a 2 jrm thick Ni foil and filled with I°Ar gas at apressure of 100 Ton. A gold sleeve inside the cylinder prevented scattered protons from hitting the steel tube. Gamma rays were detected in a NaI(TI) detector and a large volume Ge(Li) detector, both at 90° to the beam direction. Neutrons coincident with 7-rays were detected in a liquid seintillator at 0° and at 150 cm from the target. The 7-ray detectors were shielded by lead collimators to prevent detection of direct radiation from the foils and the beam stop . ,. . In the yy7 angular correlation experiment a vertical cylindrical gas target of 6 mm diameter was used. In this case the beam passed through two 1 .5 pm Tafoils and the gas pressure was 200 Torr. Gamma rays were detected in three large-volume Ge(Li) detectors at 8.3 cm from the target. Two detectors were fixed at 45° and 135° to the beam direction. The third detector was movable and was set alternately at -45°, -90° and -135°. Gamma-gamma coincidences between each detector pair were stored on magnetic tape for subsequent off-line analysis. The coincidences recorded between the two fixed detectors served for normalization . The I-ray absorption was measured with "Co and 19'Ir radioactive sources placed at the target position. 3. Reselts and dlscumlon The n-7 experiment revealed that the main y-ray decay. of the T = 2 state is to the 2 .290 MeV level. The insert in fig. 2 shows a time-of-flight neutron spectrum at Ep = 8.30 MeV. At this energy the-T = 2 state was found to be strongly populated.

w. w. STERRBNBURG er al.

202

TAM 1

Fo-4

Gamma-ray branching ratios in 40K

(MeV)

Jl' -,. J,*

Branching ratio M.)

4.38 --> 2.290 -> 2.73

0+

1+ 1

7613 2413

2.73 -+ 0.80 -+1.64 -> 1.96

1

20+ 2+

413 73f9 2318

ó00

800

1000

1200

-~ EyCkOV)

1400

1600

Fig. 2. Gamma-ray spectra coincident with primaryy-mys from the T - 2 stateat 4.38 MeV. The up. per spectrum is coincident with the 4L38 -2.290 MeVtransition, the lower with the4.38 --)-.2.73 MeV and 2.34 -r 0.89 MeV transitions (see teat). The insert shows a time-of-flight neutron spectrum coincident with ally-radisxion. The arrow indicates the position of the neutron group corresponding to the 4.38 MeV state.

The arrow indicates the position of the neutron group corresponding to this state. The other bump corresponds to many lower-lying levels . From the y-y experiment two ?-ray branches in the decay of the T = 2 state were found. The major branchto the 2.290 MeVlevel has (76±3) % and the second branch to the 2.73 MeV level (24t3) % of the total intensity (see table 1). The 7-ray spectra coincident with these primaries are shown in fig. 2. The spectra are the Doppler-shift corrected sums of coincident -t-ray spectra from all detector combinations, except from the two fixed detectors, corrected for randoms and background. In the decay of the 2.73 MeV level a hitherto unknown branch to the 1.96 MeV level was found. The energy of the 2.73 -. 1.96 MeV transition (0.77 MeV) practically coincides with the energy of the 0.80 -" 0.03 MeV transition (ace lower part of fig. 2) and was therefore probably not observed in earlier singles work 3). The branching ratios for the 7-ray decay of the 2.73 MeV level are given in table 1 .

Fig.

3 . The " angular correlation for the 4.38 -b- 2.290 --> 1.64 MeV cascade. The M line corresponds to the calculated correlation fora 0 -" 1 --" 0 spin sequence. The daded anddotted curves are the best lite for the spin sequences 0 - " 1 --1-1 and 0 -> 2 -r 1, respectively.

The 892 keV y-ray seen in the lower part of fig. 2 corresponds to the 0.89 --+ 0 MeV transition in 4*K . This transition is the secondary in the decay of the 2.54 MeV state with JI = 7+ [ref. 4)]. The primary, which has F,,, = 1 .65 MoV, coincides in energy with the 4.38 -. 2.73 MeV transition . It seems strange that such a high spin state would be populated in the (p, n) reaction at Ev = 8.30 MeV. From the grazing collision model s) one finds that only levels with J :!g 4 can be populated directly . Therefore a -f-ray spectrum coincident with the peak at 892 keV was generated. In this spectrum y-rays corresponding to the 2.88 -o- 0.89 MeV transition were observed. The 2.88 MeV level, which has J'° = 6+, is known to decay with 64 % and 36 % to the 2.54 and 0.89 MeV levels, respectively 6). The intensity of the 2 .88 -" 0.89 MeV transition implies that the 2.54 MeV state is populated about 50 % via the 6+ state.

204

W. A. STERRENBURG et al.

It seems likely that the 6* and 7+ states are fed from other states with lower spin. Since the lower level of the .y-ray spectra was set at 550 keV these presumably lowenergy transitions were not detected. The excitation energy of the T = 2 state was found from y-ray singles spectra at 90° in both the experimental set-ups described in sect. 2. These spectra were calibrated with a radioactive "Co source. The calibration was checked with some well-known 2) y-ray energies in 4°K. The excitation energy then is obtained by adding the primary and secondary q-ray energies after correction for recoil . The final result (F., = 4384.0 t0.3 bDV) is.the mean value of the results of the two experiments. Gummargamma angular correlation coefficients for the 4 .38 -. 2.290 -" 1.64 and 4.38 -. 2.73 -+ 1.64 MeV cascades were found as As = O.-42±0.10 and As = 0.42t0.19, -respectively. The correlation of the former is given in fig. 3. The measured -/-ray intensities are consistent with a spin sequence 0 -+ 1 -. 0. Other possible spin sequences 0 -+ 1 -+ 1 and 0 -+ 2 -+ 1 are ruled out at the 0.1 % probability limit. In the analysis the mixing ratio of the secondary transition was varied with the restriction that the transition strength of the quadrupole component did not exceed. the recommended upper limit 7). The 0 -+ 2 -+ 0 spin sequence is ruled out because this would lead to an E2 strength of the secondary of at least 6000 W.u. The J = 0 and 1 assignments to the 1 .64 and 2.290 MeV levels are in agreement with previous model-dependent assignments') . The parities of these states are known to be even z). The results for the cascade through the 2.73 MeV level are consistent with the known value J(2.73) = 1 . From the measured excitation energy of the T = 2, Jx = 0+ state the mass excess of the 'I 'M ground state can be estimated as -9120±150 keV. The error is entirely due to the uncertainty in the energy of the "Ca multiplet member. The authors appreciated the assistance of A. Holthuizen, A. J. Rutten and P. C. Zalm during the experiments. This work was performed as part of the research program of the "Stichting voor Fundamenteel Onderzoek der Materie" (FOM) with financial support from the "Nederlandse Organisatie voor Zuiver-Wetenschappelijk Onderzoek" (ZWO). Refanuoes

1) 2) 3) 4) 5) 6) 7)

s. Weinberg and S . B. Treiman, Phys . Rev. 166 (1959) 465

P. M. Enctt and C. van der Leun, Nucl . Phys. A214 (1973) 1 A. M. F. Op don Kamp and A. J. Spits, Nucl. Phys . A180 (1972) 569 M. F. Thomas et al.. J. of Phys. A7 (1974) 1985 H. V. Klapdor et ai., Pbys. Lett . 49B (1974) 431 H. H. Eggenhnisen et ab, Nucl. Phys . A2W (1977) 167 P. M. Enctt and C. van der Leun, Nucl . Phys . A235 (1974) 27