Compton Edge Research Articles

Energy calibration of plastic scintillators for low energy electrons by using Compton scatterings of γ rays

An energy calibration of plastic scintillators for electrons in the energy region 0.5–3 MeV is shown to be made by means of Compton scatterings of γ-rays. The shape of the Compton spectrum was analytically derived and the availability of the shape function was tested by fitting the simulated spectrum. The method to derive the energy and energy resolution at the Compton edge of the γ-ray is described.

Experimental measurements with a LiF(W) scintillator

We have constructed a scintillation detector using LiF(W) and studied its response to neutron and gamma sources. We could not detect any signals from low energy photons because its scintillation efficiency is only 6–7% of the efficiency of NaI(Tl). We did observe the Compton edges due to 137Cs and 40K sources. We also observed thermal neutrons by the reaction 6Li(n, α) 3H. The thermal neutron peak had a resolution (FWHM) of ≈17%. Finally, we observed events due to the cosmic ray muon flux in our laboratory.

Gamma-ray pulse height spectrum of241Am-Be source by ‘Li-BC501 (n-γ) spectrometer system

Neutrons from a source are moderated by means of hydrogenous materials such as polyethylene (PE) or water to reduce the energy of fast neutrons and to increase the fluence rate of moderated neutrons. The rise-time and γ-ray pulse height spectrum from a PE moderated241Am-Be neutron-gamma (n-γ) mixed source were measured by using6Li-BC501 scintillation detector and pulse shape discriminator (PSD) system. The difference in rise-time between γ and neutron signals tumed out to be 18.5 ns for the6Li-BC501 (n-γ) spectrometer system. The figure of merit (FOM) for this separation was estimated to be 1.52, and this was compared with the published results. From this comparison, the6Li-BC501 system has much superior characteristics in (n-γ) separation to other detector systems. Two Compton edges at around 1.87 and 3.99 MeV which are produced by H(n,γ)D reaction and by the first excited state of12C* from Be(α, n)12C* reaction were also investigated.

Energy resolution measurements of benzene, toluene and cyclohexane scintillators for monoenergetic γ-rays

Energy resolution and difference of the maximum energy of Compton electrons from the half-height of the Compton edge was measured experimentally for benzene (C 6H 6), cyclohexane (C 6H 12) and toluene scintillators for monoenergetic γ-rays using a γ-γ coincidence technique. For 0.5–1.2 MeV γ-rays, the energy resolution for C 6H 6, C 6H 12 and toluene scintillators varies from 23.8−12.6%, 28.6−2.5% and 23.5−4.6%, respectively. The corresponding energy difference between the maximum Compton electron energy and the half height of the Compton edge for C 6H 6, C 6H 12 and toluene scintillators varies from 13.2−7.6%, 17.7−10.2% and 10.2−6.8%, respectively. The energy resolution of the C 6H 6 detector agrees within 5–10% of that obtained with an identical C 6D 6 scintillator.

A modular 4π counter for β-delayed neutrons

We report on the construction of a detector array for low-energy β-delayed neutrons consisting of sixteen NE 213 scintillation counters. Three pulse-shape discrimination techniques were compared, with special emphasis on the discrimination of neutrons from γ-rays in the energy region down to 50 keVee (keV electron equivalent). The efficiency as a function of neutron energy was obtained by comparing the precisely known neutron spectrum from the spontaneous fission of 252Cf and the neutron distribution measured in coincidence with the fission events. The neutron energy was determined with the time-of-flight (TOF) technique. In order to examine the light output function L( E p) for recoil protons, a careful energy calibration was done by fitting the Compton edges from standard γ-ray sources. Neutron efficiencies were also calculated with a Monte Carlo code. Agreement between measured and calculated efficiency curves was observed for two differently shaped detectors and neutron flight paths of 1 and 3 m. This correspondence justifies the efficiency calculation for the 4π array with the source in the center. For neutron energies above 800 keV an error of less than 5% in detection efficiency can be reached.

Gamma-ray detectors from thermally annealed Bridgman-grown CdTe

The characteristics of small γ-ray probe detectors (hemispherical contact geometry) made from Cl- and In-doped Bridgman-grown crystals of CdTe are described. A slow, low-temperature anneal of as-grown CdTe ranging from 485 to 85°C, and cooled at a rate of < 20°C/h, was found to greatly improve detector performance. Best photopeak to Compton edge ratios were 2:1 for 662 keV γ-rays. Typical values were 1:1 with energy resolutions of 5% full width at half maximum. Energy resolution was ∼ 13.5% for 185 keV γ-rays from a 235U (93%) source.

Compton scattering, the electron mass, and relativity: A laboratory experiment

Compton scattering in a semiconductor detector is used to ‘‘discover’’ the relativistic relation between energy and momentum and to demonstrate the dependence of p, E and γ on β. The motivation is to measure the (rest) mass of the electron, and this can be done to within 1 keV with a commonly available set of gamma ray sources. To determine precisely where the Compton edge occurs in a spectrum, a Monte Carlo calculation of detector response is described which also helps the student to understand the physics of the detection process.

Compton continuum suppression by analog pulse shape analysis

We present experimental results on Compton continuum suppression and Compton edge flattening by analog pulse shape analysis. The local peak-to-Compton ratio was increased by a factor of 1.7.

Compton continuum suppression by digital pulse shape analysis

We present experimental results on a technique for flattening the Compton edges and suppressing the Compton continuum in gamma ray spectra with digital pulse shape analysis. The peak-to-Compton ratio can be improved by more than a factor 2.

Simulation of gamma-ray current pulse shapes produced in a high purity germanium detector

The authors present results of a simulation of gamma-ray interactions in a high-purity germanium detector (HPGe). Gamma-ray interactions have been divided into Compton events which leave only part of their energy in the detector were examined on a pulse-by-pulse basis. The simulation showed that rejecting all waveforms with trailing edges of less than 50 ns improved the peak-to-Compton ratio by a factor of two and in addition, flattened the remaining Compton edges.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Compton edge energy calibration of organic detectors

A method for energy calibration of liquid scintillators, plastic neutron detectors and other organic detectors utilizing the Compton effect is described. The proposed procedure accounts for the finite energy resolution of the detector and also for multiple scattering. A convenient graph is proposed which connects the Compton edge position relative to the observed Compton peak and the energy resolution of the detector. The method can be applied to detectors for which there is no total absorption peak, e.g. organic scintillators.

Gamma calibration of organic scintillators

Abstract The electron light responses of three organic scintillator detectors – a stilbene (3.81 × 1.27 cm2) and two NE102A (5.0 × 2.5 cm2 and 5.0 × 7.6 cm2) — were systematically studied by means of a set of gamma sources. The edge maximum and half-height point energies of the Compton distributions have been determined by the use of a fast/slow coincidence arrangement, which permits taking adequate spectra of the maximum energy Compton electrons. According to these measurements, the maximum energy Compton electrons are located 21.1 ± 1.4% below the half-height Compton edge point for stilbene, and 4.8 ± 1.4% and 6.6 ± 1.5% above the Compton edge maxima for the NE102A scintillators, respectively.

Limitation of the Compton suppression in Ge-BGO Compton suppression spectrometers

A detailed study of the NORDBALL Ge-BGO Compton suppression spectrometers has been performed. Special emphasis has been given to photoelectron yields of the scintillators, optimisation of charge integration in the electronics and the low energy thresholds of the BGO detectors. The time relation between the Ge detector and the BGO scintillator logic pulses has been studied in detail. Maximum Compton suppression factors of 12.0 and 12.7 have been measured for 60Co and 137Cs γ-rays respectively. Photon events observed between the full energy peak and the Compton edge suggest that other effects than multiple Compton scattering are involved. Such effects are usually not included in Monte Carlo simulations and could be the explanation of the factor of 2 deficiency between calculated and measured suppression ratios.

Extension of the semiempirical germanium detector response function to low energy gamma rays

A model for the spectral response function of large volume germanium gamma-ray detectors in the range of incident gamma-ray energies from 60 keV to 6.2 MeV has been developed and applied to a 39% high purity germanium detector. Functional forms for describing the various features of the response function are based either on empirical functions or on analytical expressions of the interaction mechanisms. Improved functions are formulated to give good agreement to the experimental spectra near the Compton edge in the Compton continuum of the primary and annihilation photons. To form the complete detector response function only nine linear and six nonlinear parameters are needed. Validity of the model is demonstrated by comparing the regenerated spectra of 241Am, 51Cr, 137Cs, 54Mn, 28Al, 37S, and 16N and the synthesized spectra of 24Na and 133Ba using the response function to the equivalent experimental spectra.

Nondestructive evaluation of jades by means of 14 MeV neutron activation

Fourteen MeV neutron activation of jades was used to test the authenticity of jades: to ascertain whether they are genuine jadeite and whether their greenish colour is genuine, both of which are important criteria for the high market value of jadeite. For given activation and measurement conditions of the jades, the gamma-ray spectrum was observed to change as a function of the type of the jades; the counting ratio of the photopeaks from the two most prominent constituent elements varies greatly as a function of the type of the jades. The ratio of counts for the Compton edges of the 1.779 MeV gamma-ray of28Al, from28Si(n,p)28Al reaction, and the 1.434 MeV gamma-ray of52V, from52Cr(n,p)52V reaction, varied as a function of the type of the jades and also as a function of the color. These results can be applied for a fast and nondestructive evaluation of jades.

Gamma-calibration of NE 213 scintillation counters

A method is described of calibrating the light output of NE 213 scintillation detectors by means of “monoenergetic” photon sources. The position of the Compton edge can be precisely determined by comparing the measured pulse-height spectrum with a Monte Carlo simulated distribution properly folded with the pulse-height dependent resolution. For the various sizes of scintallator systems the position of the Compton edge relative to the position of the maximum and the half maximum of the distribution is tabulated in dependence on relevant detector resolutions. The results may be used to determine the resolution function and to check the linearity of the light output.

Energy calibration of NE-213 scintillation counters by δ-rays

An accurate energy calibration of organic scintillation counters by means of γ-rays is described. The position of the Compton edge has been precisely determined comparing measured with calculated spectra. The calculation includes wall and light pipe effects and the influence of channel dependent pulse height resolution. Relations are given for the position of the maximum and the half maximum of a Compton spectrum as a function of the incident photon energy and the resolution of the detector. Results are given with uncertainties smaller than 1 %.

Serious Degradation Characters of CdTe Nuclear Detector by Electron or Gamma Irradiations

We present the degradation characteristics of M-S-M (metal-semiconductor-metal) type CdTe nuclear detectors fabricated from undoped p-type THM crystals under successive irradiations of 60Co gamma rays and 2 MeV electrons at room temperature. Radiation-induced degradations on detector performances are investigated by the evaluations both of the photopeak and Compton edge of 241Am and 137Cs gamma rays. The FWHM energy resolution and counting rate of the photopeak become worse with increasing gamma ana electron fluences. It is thus concluded that the ?? product in the electrons and holes are decreased in comparison with that before irradiation, probably due to the carrier trapping effect. Comparing with the radiation effects of a Si surface-barrier detector, we discuss the annealing behavior of the irradiated CdTe detector.

Comparative study of different experimental methods for measuring the scintillation intensity of a liquid scintillator

The scintillation intensity of an organic liquid scintillator has been measured as a function of solute concentration in terms of four different quantities, d.c. current, photopeak pulse height, Compton edge and total count rate. The first three methods are found to be equivalent and the method of total counts is shown to be erroneous. The limitations of the photopeak method and the Compton edge technique are discussed.

An analysis method of gamma-ray pulse-height distributions obtained with a Ge(Li) detector

A method is proposed for analysis of the gamma-ray pulse-height distribution (PHD) obtained with a Ge(Li) detector, which makes use of a fitting technique based on a nonlinear least-squares method. The successive steps of analysis are thus made from peak search to estimations of gamma-ray peak areas, peak locations, and their errors. Gamma-ray peaks and background continuum are expressed in a Gaussian distribution function; an exponential function is connected smoothly with the Gaussian at a certain point on the low-energy side of a gamma-ray peak, a newly defined Valley Count Function (VCF) expressing the flat-background continuum located between Compton edge and centroid of peak, and a quadratic polynomial expressing the ordinary background continuum. From the results of the analysis obtained with a computational code, it was found that, to obtain meaningful solutions in gamma-ray PHD analysis, the full width at half maximum (fwhm) must contain three or more channels, and the separation between the peaks should be larger than half the fwhm in the analysis of a composed PHD having two peaks. The validity of the method is shown by an analysis of the gamma-ray spectrum of Ta isotopes.