Development of organic scintillators

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.

https://wwwusers.ts.infn.it/~milazzo/pubblicazioni/P42.pdf

Pulse shape analysis of liquid scintillators for neutron studies

Liquid scintillation detectors of type NE213 or BC501A are well suited and routinely used for spectrometry in mixed n-γ-fields. Neutron- and photon-induced pulse height spectra may be simultaneously recorded making use of the n/γ-discrimination capability based on pulse shape analysis. The light output functions for the detected secondary charged particles, i.e. electrons, positrons, protons and other charged reaction products, and the pulse height resolution function must carefully be determined. This can be done experimentally, in part via an iterative procedure by comparison with calculations. The response functions can then be reliably calculated by Monte Carlo simulations. Photon response functions calculated with the PHRESP code, which was developed on the basis of the EGS4+PRESTA program package, are in very good agreement with calibrations up to 17 MeV, both in shape and absolute scale. Similarly, neutron response functions calculated with the NRESP7 code well describe the pulse height spectra for monoenergetic neutrons up to 20 MeV, although with some limitations for neutron energies beyond 10 MeV. In the case of measurements in mixed fields, the photon pulse height spectrum has to be corrected for neutron-induced photons generated in the detector assembly. The corresponding response functions may be determined numerically (with the MCNP and the PHRESP codes) or experimentally. Hence, the response matrices necessary for the deconvolution of measured pulse height spectra by the various unfolding procedures included in the HEPRO package are finally calculated with at least six functions per FWHM of the corresponding pulse height resolution. With appropriate counting statistics of the measured pulse height spectra, excellent energy resolution is achieved, e.g. about 20% of the pulse height resolution at the corresponding Compton or recoil proton edge. However, these specifications require that the entire detector system, i.e. scintillator, photomultiplier and associated electronics, exhibits a gain stable over time and independent of count rate and temperature.

Neutron and photon spectrometry with liquid scintillation detectors in mixed fields

Fast and unambiguous identification of shielded neutron sources is of paramount importance in nuclear nonproliferation and international safeguards applications. Usually, the identification of neutron spectra relies on unfolding procedures that are unstable and time consuming. In this paper, we present a new application of an existing technique, which can be used for neutron source identification by direct analysis of the pulse height spectrum, i.e., without resorting to energy spectrum unfolding. This technique is based on the acquisition of neutron and γ-ray pulses by using a liquid scintillation detector and a fast waveform digitizer. An optimized digital pulse shape discrimination method based on standard charge integration is used to discriminate neutrons from γ-rays. The neutron pulses are then analyzed to generate a pulse height distribution. The accuracy of the neutron source identification technique was tested on three neutron sources: Cf-252, Am–Be, and Am–Li. Several source–shielding configurations were tested to assess the influence of potential shielding of the source and the sensitivity of the technique. For this investigation, lead and polyethylene shielding blocks were used. The measured pulse height distributions were compared with the distributions simulated with the MCNP-PoliMi code, and very good agreement was obtained. The results show that for both shielded and unshielded configurations the identification of Cf-252, Am–Be, and Am–Li shielded sources is easily achievable by direct analysis of the measured pulse height distributions, without the need of subsequent neutron energy spectrum unfolding.

Identification of shielded neutron sources with the liquid scintillator BC-501A using a digital pulse shape discrimination method

Thick target yields of the neutron producing reactions d + Be and p + Be were investigated for deuteron energies between 9.4 and 13.6 MeV and proton energies between 17.2 and 22.0 MeV. The spectral neutron yield per unit beam charge on the Be target was determined by using time-of-flight techniques and the (n, γ) discrimination properties of an NE213 scintillation detector. In addition, the angular dependence of the spectral neutron yield was measured between 0° and 150° for a deuteron energy of 13.54 MeV and a proton energy of 19.08 MeV. The spectral neutron yield at 0° as a function of the incident particle energy, which is approximated by a simple function obtained from a least-squares fit, is compared with previous data. Fluence-averaged mean neutron energies of the neutron spectra are given as a function of the charged particle energy and neutron emission angle for various neutron energy thresholds. The present neutron spectra differ in the low-energy portion from those published by other authors. Deviations of up to 20% are found in the total neutron yield per unit beam charge at 0°.

M.R. Maier

Papers

A simple pulse-shape discrimination circuit