Walter R. Burrus

Calibration of an organic scintillator for neutron spectrometry

Abstract The absolute differential efficiency of a 4.60- × 4.65-cm-dia. liquid organic scintillator, NE-213, was determined for nearly monoenergetic neutrons at 20 energies between 0.2 and 22 MeV incident on the curved side of the detector. These calibrations are shown to apply to an NE-211 scintillator as well. A 5-MeV Van de Graaff generator provided 2-nsec pulses of neutrons by means of T(p,n) 3 He, D(d,n) 3 He (gas target), and T(d,n) 4 He reactions. With the aid of time-of-flight techniques and pulse-shape discrimination to eliminate spurious neutron and gamma-ray events, reliable pulse-height spectra were obtained for monoenergetic neutrons. The spectra were normalized to Monte Carlo calculations of absolute differential efficiency by utilizing the proton-recoil plateau. The accuracy of the Monte Carlo calculation was verified in the region of the proton-recoil plateau by absolute experimental calibrations carried out at neutron energies of 2.66 and 14.43 MeV and by using a scintillator geometry more suitable for such tests.

Fast-neutron spectroscopy with thick organic scintillators

Abstract A neutron spectrometer which utilizes a 4.60- by 4.65-cm-dia. NE-213 liquid organic scintillator is described. Pulses due to gamma radiation are eliminated by the use of a modified Forte-type pulse-shape discriminator circuit. The FERDoR unfolding method and code are used to transform the measured pulse-height distribution into a neutron spectrum. This code produces an estimate of the neutron spectrum with a rigorous confidence interval. The effects of nonlinear scintillation characteristics, multiple scattering, and charged-particle reactions are all taken into account. The code is based on constrained minimization and utilizes the known nonnegativity of the neutron spectrum in obtaining the confidence intervals. The scintillator arrangement, the electronic circuit, and the FERDoR code are described. Typical results obtained by unfolding continuous and discrete line spectra are given. Satisfactory overall results were obtained with neutrons from 0.5 to over 16 MeV in a gamma-ray background of ≈ 5 mR/h.

A high resolution spectrometer system with particle identification for 1 through 60 MeV hydrogen and helium particles

Abstract A coincidence semiconductor spectrometer system based on a Ge(Li) total absorption detector has been applied to the simultaneous spectroscopy of all charge 1 and 2 particles from targets bombarded with protons with energy up to 62 MeV. Output spectra cover the range from the full energy down to a 1 to 5 MeV threshold which depends on particle type. The method for choosing the thickness for the two ΔE detectors is discussed and unusual features of the system are described. Particles too slow to penetrate the first ΔE counter were sorted according to mass using flight-time vs E discrimination while the more energetic particles were separated using two sets of ΔE × E discrimination. Germanium detectors thick enough to stop 60 MeV protons were used with particles entering perpendicular or parallel to the field lines, and in either case the only significant inactive region in the path of the detected particles was the protective foil over the Ge(Li) detector. The typical pulse-height resolution of the system was about 200 keV for 60 MeV protons, although a germanium detector used alone gave 55 keV resolution at this energy. Analysis was performed after the experiments using magnetic tapes written by an on-line computer; corrections to the pulse-height spectra for reaction and collimator tails are discussed. The electronic logic system is described, including portions for event characterization, for use of an “active” detector collimator, and for pileup pulse rejection based on timing information.

A True Gamma-Radiation Spectrometer

A true gamma-radiation spectrometer is described which utilizes a standard 3- by 3-in. diam NaI(Tl) scintillation detector and outputs the gamma spectrum (in photons MeV-1 cm-2 sec-1) with all spurious peaks and tails corrected. A small digital computer is utilized to accumulate the pulse-height distribution and simultaneously perform a digital filtering operation in real time.