J. L. Tain

Pulse shape analysis of liquid scintillators for neutron studies

The acquisition of signals from liquid scintillators with Flash ADC of high sampling rate ð 1G S=sÞ has been investigated. The possibility to record the signal waveform is of great advantage in studies with g’s and neutrons in a high count-rate environment, as it allows to easily identify and separate pile-up events. The shapes of pulses produced by g-rays and neutrons have been studied for two different liquid scintillators, NE213 and C6D6: A 1-parameter fitting procedure is proposed, which allows to extract information on the particle type and energy. The performance of this method in terms of energy resolution and n=g discrimination is analyzed, together with the capability to identify and resolve pile-up events. r 2002 Elsevier Science B.V. All rights reserved.

New antineutrino energy spectra predictions from the summation of beta decay branches of the fission products

In this Letter, we study the impact of the inclusion of the recently measured beta decay properties of the (102;104;105;106;107)Tc, (105)Mo, and (101)Nb nuclei in an updated calculation of the antineutrino energy spectra of the four fissible isotopes (235,238)U and (239,241)Pu. These actinides are the main contributors to the fission processes in pressurized water reactors. The beta feeding probabilities of the above-mentioned Tc, Mo, and Nb isotopes have been found to play a major role in the γ component of the decay heat of (239)Pu, solving a large part of the γ discrepancy in the 4-3000 s range. They have been measured by using the total absorption technique, insensitive to the pandemonium effect. The calculations are performed by using the information available nowadays in the nuclear databases, summing all the contributions of the beta decay branches of the fission products. Our results provide a new prediction of the antineutrino energy spectra of (235)U, (239,241)Pu, and, in particular, (238)U for which no measurement has been published yet. We conclude that new total absorption technique measurements are mandatory to improve the reliability of the predicted spectra.

Simulation of light collection in scintillators with rough surfaces

Abstract A simple model to evaluate the light collection in scintillators with rough surfaces by Monte Carlo simulation is proposed. The model has a single parameter related to the degree of roughness. The results are compared with measurements for a long tapered BGO crystal.

Beta decay of 103In: Evidence for the Gamow-Teller resonance near 100Sn

Abstract The β decay of the neutron-deficient isotope 103 In was investigated by using total absorption γ-ray spectrometry on mass-separated sources. The measurement reveals a high-lying resonance of the β-decay strength in striking disagreement with high-resolution γ-ray data. The result is discussed in comparison with shell-model predictions.

Enhanced γ-Ray Emission from Neutron Unbound States Populated in β Decay.

Total absorption spectroscopy is used to investigate the β-decay intensity to states above the neutron separation energy followed by γ-ray emission in (87,88)Br and (94)Rb. Accurate results are obtained thanks to a careful control of systematic errors. An unexpectedly large γ intensity is observed in all three cases extending well beyond the excitation energy region where neutron penetration is hindered by low neutron energy. The γ branching as a function of excitation energy is compared to Hauser-Feshbach model calculations. For (87)Br and (88)Br the γ branching reaches 57% and 20%, respectively, and could be explained as a nuclear structure effect. Some of the states populated in the daughter can only decay through the emission of a large orbital angular momentum neutron with a strongly reduced barrier penetrability. In the case of neutron-rich (94)Rb the observed 4.5% branching is much larger than the calculations performed with standard nuclear statistical model parameters, even after proper correction for fluctuation effects on individual transition widths. The difference can be reconciled by introducing an enhancement of 1 order of magnitude in the photon strength to neutron strength ratio. An increase in the photon strength function of such magnitude for very neutron-rich nuclei, if it proves to be correct, leads to a similar increase in the (n,γ) cross section that would have an impact on r process abundance calculations.

Enhanced and quenched B(E1) transition rates between parity doublet bands in 227Ra

Abstract The fast timing βγγ ( t ) method has been used to measure level lifetimes in the parity doublet bands in 227 Ra populated in the β − decay of 227 Fr. In particular T 1 2 = 254(9) ps , 236(30) ps , ⩽41 ps and 16(13) ps have been measured for the 3 2 − and 5 2 − members of the K π = 3 2 − band and for the 1 2 − and 3 2 − members of the K π = 1 2 − band, respectively. These results indicate surprisingly fast B (E1) rates in this octupole transitional nucleus with the average | D 0 | value of ⩾ 0.11 e ·fm for the K π = 1 2 − band and | D 0 |= 0.10 e ·fm for K π = 3 2 − band. The general enhancement (and occasional quenching) of E1 intra-doublet and some enhanced extra-doublet transitions in 227 Ra are interpreted within the quasiparticle-plus-phonon model with the inclusion of Coriolis coupling. Similar calculations, performed for 223 Ra, are found to be in good agreement with the experimental data.

Accuracy of the Pulse Height Weighting Technique for Capture Cross Section Measurements

The accuracy of the pulse height weighting technique for the measurement of neutron capture (n,γ) cross sections is investigated. Detailed Monte Carlo simulations of capture measurements are compared to experimental data. Several causes of systematic deviation are investigated and their effect is quantified.

Octupole correlations in 229Ra

Abstract The structure of 229 Ra has been studied in the β − decay of 229 Fr. Spins and parities have been determined from the conversion electron measurements, while half-lives for the 137.5, 142.7, 168.8, 213.0 and 479.0 keV levels have been measured in the ps and ns ranges via the fast timing βγγ ( t ) method. Our data confirm the previous results and enrich the knowledge of the structure of this nucleus by 25 new states and several strong transitions. Structural similarities are established between 229 Ra and its N = 141 isotone 231 Th. The experimental results are compared with the theoretical predictions of the quasiparticle-phonon model with inclusion of Coriolis coupling. The reduced transition probabilities, and especially the moderately fast B (E1) rate for the 137.5 keV transition connecting the band-heads of the K π = 5 2 ± parity doublet bands, reveal the presence of significant octupole correlations in 229 Ra.

The nuclear structure of 227Fr

The γ-rays following the β− decay of 227Rn have been investigated by means of γ-ray singles and γγ-coincidence measurements using an array of 12 Compton-suppressed Ge detectors. The fast-timing βγγ(t) method has been used to measure six level lifetimes. Multipolarities of 32 transitions in 227Fr have been established by measuring conversion electrons with a mini-orange electron spectrometer. Most of the observed transitions have been placed in a level scheme comprising 38 excited states of 227Fr. The low-lying levels are interpreted in terms of seven rotational bands. The observed E1 strengths for two transitions connecting the Kπ = 32± bands are consistent with the transitional character of 227Fr, and confirm the presence of octupole correlations in this nucleus.

Quenched E1 transition rates in 231Th

Abstract The fast timing βγγ(t) method has been used to measure lifetimes of the low-lying levels in 231Th populated in the β− decay of 231Ac. The half-life of the K π = 5 2 − band-head at 185.7-keV was measured as T 1 2 = 1073(79) ps , yielding D0 = 1,76(8) × 10−2 e·fm and 1.65(6) × 10−2 e·fm for the E1 transitions to the 7 2 + and 5 2 + members respectively of the K π = 5 2 + ground state band. Furthermore, T 1 2 ⩽ 74 ps and ⩽ 74 ps were measured for the K π = 3 2 + and 1 2 + band-heads at 221.4- and 247.6-keV, and T 1 2 = 503(12) ps and 98(36) ps for the 1 2 − and 3 2 − states at 554.7- and 593.6-keV, members of the K π = 1 2 − band. The latter results consistently yield D0 values of the order of 1.1 × 10−2 e·fm for three transitions connecting the K π = 1 2 ± bands. The observed transition rates have been interpreted within the quasiparticle-phonon model with the inclusion of Coriolis coupling.