Page 69 - ILL Annual Report 2019

SCIENTIFIC HIGHLIGHTS
66-67
Figure 2
FIPPS set-up. Left) FIPPS+LaBr3 set-up. Right) 16 LaBr3 detectors (IKP Cologne and IFIN HH Bucharest) installed at an angle of
45 o relative to the beam axis were coupled to the 8 FIPPS HPGe clovers placed in a central ring perpendicular to the beam axis for one reactor cycle in 2018.
We used the 115Sn(nth,g)116Sn reaction at the collimated neutron guide of the FIPPS set-up, with a neutron flux of 108 n/(cm2s) and a 1 cm diameter beam spot at the target position. The g-ray detection set-up around the target consisted of eight HPGe clovers, each made of 4 Ge crystals, and 16 LaBr3 scintillator detectors. The excellent energy resolution of the HPGe detectors permits precise selection of a g-ray from a specific triple-ggg cascade and produces clean LaBr3 coincidence spectra, as illustrated
in figure 3a for the 4+2 state of 116Sn using the
2 112 keV 2+2 → 0+1 transition. The lifetime of a nuclear excited state is deduced from LaBr3-LaBr3 time-difference measurements between the g-rays feeding and de-
exciting an excited state, as described in reference [1]. After corrections (shifts) for the energy-dependent mean zero-time response of the whole set-up and the time contributions of the Compton background, the centre
of gravity of the time distribution directly corresponded
to the lifetime of the 4+2 state of 116Sn of 42(14) ps, corresponding to a half-life T1/2 = 29(10) ps [2, 3]. Comparison of the experimental data with the results
of the Interacting Boson Model (IBM-2) calculations with mixing shows that the intruder states in 116Sn have dominant contributions of the 2 particle–2 hole configuration but are strongly mixed with normal configurations.
a)
0
420 440 460 480 500 520 540 560 580 600 620
Eγ (keV)
70 60 50 40 30 20 10
467 +−
gates:
HPGe: 2112 keV LaBr3: 417 keV
3+−>4+ 1 2
P/B = 0.9(1)
e e
10
1
b)
−1 −0.5 0 0.5 1 dt (ns)
statistics: 360 counts C = 42(14) ps FWHM = 430(5) ps
tmin tmax
Figure 3
Doubly-gated coincidence g-ray spectra generated to investigate
the quality of the 467 keV peak and the Ge (2 112 keV)-triggered time-difference distribution between the 467keV and 417 keV transitions feeding and de-exciting the 4+2 state of 116Sn.
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counts
counts per 20 ps