Determination of neutron energy spectra inside a water phantom irradiated by 64 MeV neutrons

A NE230 deuterated liquid scintillator detector (25 mm diameter x 25 mm) has been used to investigate neutron energy spectra as a function of position in a water phantom under irradiation by a quasi-monoenergetic 64 MeV neutron beam. Neutron energy spectra are obtained from measurements of pulse height spectra by the NE230 detector using the Bayesian unfolding code MAXED. The experimentally measured energy spectra are compared with spectra calculated by Monte Carlo simulation using the code MCNPX.     

Experimental determination of absolute efficiency and energy resolution for NaI(Tl) and germanium gamma ray detectors at energies from 2.6 to 16.1 MeV

Measurements have been made to characterize the response of NaI(Tl) and Ge(Li) gamma ray detectors to gamma rays in the energy range of 2.6 to 16.1 MeV. Both absolute efficiency and energy resolution are reported. At 16.1 MeV the absolute full energy efficiency of a 10 cm × 10 cm NaI(Tl) detector is about 2% and the energy resolution is also about 2%. For a 65 cm³ Ge(Li) detector, the full energy peak absolute efficiency at 16.1 MeV is 0.2% (the escape peak efficiencies are 5 times larger), and the fwhm energy resolution is about 0.1%.     

A detection system for energetic light heavy ions

A light heavy ion detection system which consists of a gas-filled ionization chamber (IC) connected to a scattering chamber via a time-of-flight (TOF) system has been constructed. The entrance window of the IC has an area of 14 × 40 cm², the active depth is 115 cm. Filled with CF₄ at a pressure of 350 Torr, the energy range for ¹²C and ⁴⁰Ar is 5–20 MeV/A and 6–30 MeV/A, respectively. The TOF system consists of two parallel plate avalanche counters with a flightpath of 70 cm in between. The IC has been tested with ¹²C ions at an energy of 39 MeV. The energy resolution of the IC (1.1%) is mainly determined by the energy straggling in the foils of the TOF system and the ionization chamber. The energy-loss resolution is 3.5%, the horizontal position resolution varies between 6 and 20 mm and the vertical position resolution is 2 mm. The time resolution of the TOF system ranges from 800 ps for ⁴He at 5.0 MeV, to 280 ps for ²⁸Si at 55 MeV.     

Characterization and improvement of thin natural diamond detectors for spectrometry of heavy ions below 1 MeV/amu

In our previous paper [V.Kh. Liechtenstein, N.V. Eremin, R. Golser, W. Kutschera, A.A. Paskhalov, A. Priller, P. Steier, Ch. Vockenhuber, S. Winkler, Nucl. Instr. and Meth. A 521 (2004) 203], first results on the evaluation of thin natural diamond-based detectors (NDDs) as an energy spectrometer for heavy ions in the energy range below 1 MeV/amu were presented. Although results were promising, the energy resolution of the detector was limited by an unexpected high-energy loss in the “dead layer” of the entrance window. In this paper, we report a significant improvement in the spectrometric properties of two highly selected and carefully treated NDDs with electrical contacts made of carbon and gold films as thin as about 10 and 20 μg/cm², respectively, instead of much thicker aluminum contacts used before. In particular, for the NDD with thin carbon contact an energy resolution of 7.6% for ¹⁹⁷Au-ions at 20.6 MeV was obtained. The energy cut-off of the detectors was reduced to 0.9 and 1.5 MeV for carbon and gold contact, respectively. The measured data on energy cut-off for different projectiles are compared with calculations, which yields an estimate of the thickness of the dead layers. Long-term irradiation runs proved stable spectroscopic performance of the detectors, in spite of the inherent “pumping” effects and imperfections of pulse height distributions. Our data suggest that NDD-based spectrometers might outperform other detector types in applications where very fast detectors with high radiation tolerance are required.     

Neutron detection efficiency of a thick lithium glass detector

The neutron detection efficiency of a 9.55 mm thick NE-912 lithium glass scintillator has been determined for the energy range from 25 keV to 2 MeV. It was measured relative to a thin, 0.835 mm thick NE-908 glass, which was determined by Monte Carlo calculations. The measured efficiency curve shows the marked effect of the ¹⁶O resonance at 442 keV, and a strong increase with energy for E_n > 1.2 MeV, due to (n,n′γ) reactions.     

A three dimensionally position sensitive large area detector

A large area detector consisting of a parallel plate avalanche counter (PPAC) and a trapezohedral ionization chamber (TIC) is described. Its active area is 184 cm². The time resolution of the PPAC is 175 ps. The energy resolution of the TIC is 0.4%, the energy loss resolution 2.8%, the nuclear charge resolution 2.3%. The TIC is position sensitive in three dimensions. The position x is measured via a saw-tooth anode with a resolution of 0.7 mm; the drift time coordinate shows a resolution of δy ? mm. The range z is determined by a new technique, a graded density Frisch grid. It enlarges the dynamic range of the charge measurement down to the Bragg maximum at E/A ～ 1 MeV. The resolution is δZ/Z ? 3.5%     

The neutral detector at VEPP-2M

The design and characteristics of the neutral detector based on NaI(Tl) crystals with a total weight of 2.6 t for experiments at the electron-positron storage ring VEPP-2M are described. The solid angle of the detector is 65% of 4π sr. The energy resolution for photons in the energy range 50–1000 MeV is from 20% to 10% (fwhm). For events with the detection of all final particles a further improvement of the detector resolution by means of the kinematic reconstruction method is possible. For the decay ? → ηγ → 3γ an effective mass resolution of 10% is obtained for the η-meson mass. By the energy deposition in NaI(Tl) layers an electron/pion separation of 0.001 is achived at an energy of 500 MeV.     

A multiwire chamber system for measurements of neutron spectra

A new type of detector for measuring neutron flux and energy over a wide range of angles and energies is being developed. Measurements of neutron elastic and inelastic scattering as well as neutron energy continua are possible. Time-of-flight is not used for measuring outgoing neutron energy, and so for continuum measurements this system has some distinct advantages over conventional neutron detectors. Neutron energy measurement is carried out by measuring the energy and angle of the recoil proton produced by the neutron in a CH₂ converter. Spectra from ⁷Li(p, n)⁷Be at 62 MeV and ⁴⁰Ca(n, n′χ) at 65 MeV are presented.     

Application of CLTD’s for High Resolution Mass Identification and for Stopping Power Measurements of Heavy Ions

An array of calorimetric low temperature detectors (CLTD’s) for energy sensitive detection of heavy ions was combined with time-of-flight (TOF) detectors to obtain a detector system for high resolution mass identification of low energy heavy ions. In addition the same setup was used to prove the ability of CLTD’s to be used in electronic stopping power measurements for heavy ions in matter. Experiments with 50?MeV ⁶³Cu and ⁶⁵Cu ions at the tandem accelerator at the MPI at Heidelberg, and with 25 to 250?MeV ²³⁸U ions at the UNILAC accelerator at GSI at Darmstadt have been performed. For ^63,65Cu at 50?MeV a mass resolution of Δm(FWHM)=0.9?amu, and for ²³⁸U in an energy range of 65 to 150?MeV a resolution of Δm(FWHM)=1.28?amu, was obtained. The results for stopping powers of ²³⁸U in carbon and gold are presented and compared with theoretical predictions and data from the literature.     

Test of a micromegas detector with helium-based gas mixtures for active target time projection chambers utilizing radioactive isotope beams

A Micromegas detector was tested in a helium-carbon dioxide mixture at atmospheric pressure. A stable operation of the detector was confirmed at a gain of over 10³ for a standard alpha source of ²⁴¹Am. In order to realize a position-dependent gain, an anode strip was biased while keeping the other strips at the ground potential. The gain of the biased strip was reduced by one order of magnitude without affecting the gains of the neighboring strips. The energy spectra at 3.7 MeV, the energy deposited by Am alpha particles in the sensitive region of the Micromegas, were obtained from the total charge and the charge deposit profile along the track. The energy resolutions for helium:carbon dioxide 90:10 mixture at a gas gain of about 100 were 11% FWHM for the former and 4% FWHM for the latter. The present results highlight the Micromegas as a promising electron amplifier of the forthcoming active target time projection chambers that will be dedicated to nuclear reactions with radioactive isotope beams.     

Crystal growth and characterization of europium doped KCaI3, a high light yield scintillator

The presented study reports on the spectroscopic characteristics of a new high performance scintillation material KCaI₃:Eu. The growth of ∅ 17 mm boules using the Bridgman–Stockbarger method in fused silica ampoules is demonstrated to produce yellow tinted, yet transparent single crystals suitable for use in spectroscopic applications due to very promising performance. Scintillation light yield of 72,000 ± 3000 ph/MeV and energy resolution of 3% (FWHM) at 662 keV and 6.1% at 122 keV was obtained from small single crystals of approximately 15 mm³. For a much larger 3.8 cm³ detector, 4.4% and 7.3% for the same energy. Proportionality of the scintillation response to the energy of ionizing radiation is within 96% of the ideal response over an energy range of 14–662 keV. The high light yield and energy resolution of KCaI₃:Eu make it suitable for potential use in domestic security applications requiring radionuclide identification.     

High-resolution short range ion detectors based on 4H-SiC films

The energy resolution of SiC detectors has been studied in application to the spectrometry of α particles with 5.1–5.5 MeV energies. The Schottky barrier structure of the detector was based on a CVD-grown epitaxial n-4H-SiC film with a thickness of 26 μm and an uncompensated donor concentration of (1–2)×10¹⁵ cm^?3. An energy resolution of 0.5% achieved for the first time with SiC detectors allows fine structure of the α particle spectrum to be revealed. The average energy of the electron-hole pair formation in 4H-SiC is estimated at 7.71 eV.     

Effects of proton escape on detection efficiency in thin scintillator elements and its consequences for optimization of fast-neutron imaging

Plastic scintillators are commonly used for neutron detection in the MeV energy range, based on n-p scattering and the subsequent deposition of recoil proton's kinetic energy in the detector material. This detection procedure gives a quasi-rectangular energy deposition distribution for mono-energetic neutrons, extending from zero to the neutron energy. However, if the detector sensitive element (DSE) is small, the energy deposition may be incomplete due to the recoil proton escape.In the application of neutron imaging, here exemplified by fast-neutron tomography, two conflicting requirements have been identified: (1) thin DSEs are required to obtain high spatial resolution and (2) energy discrimination may be required to reduce the influence of neutrons being scattered into the DSEs, which generally occurs at lower energies. However, at small DSE widths, the reduction of energy deposition due to recoil proton escape may cause a significant decrease in detection efficiency when energy discrimination is applied.In this work, energy deposition distributions in small-size DSEs have been simulated for Deuterium-Deuterium (DD; 2.5 MeV) and Deuterium-Tritium (DT; 14.1 MeV) fusion neutrons. The intrinsic efficiency has been analyzed as a function of energy discrimination level for various detector widths. The investigations show that proton recoil escape causes a significant drop in intrinsic detection efficiency for thin DSEs. For DT neutrons, the drop is 10% at a width of 3.2 mm and 50% at a width of 0.6 mm, assuming an energy threshold at half the incident neutron energy. The corresponding widths for a DD detector are 0.17 and 0.03 mm, respectively.Finally, implications of the proton escape effect on the design of a fast-neutron tomography device for void distribution measurements at Uppsala University are presented. It is shown that the selection of DSE width strongly affects the instrument design when optimizing for image unsharpness.     

Application of a compound nucleus residue detector

A gas filled avalanche detector, capable of operating at high count rates, has been developed in order to detect recoiling ions. The detector has been used to identify the signal of evaporation residue channels in the presence of strong fission competition. The performance of the detector has been investigated for the reaction of 83 and 95 MeV ¹⁸O ions on a ²⁰⁸Pb target.     

A multiwire chamber system for measurements of charged particle spectra

A large solid angle, high counting rate detection system for p, d t identification and energy measurement in the range of 15 to 65 MeV has been developed. The detector consists of two position-sensitive delay-line multiwire chambers, a thin plastic scintillator ΔE, and two large area NaI E detectors. Measurements of energy spectra of protons from ¹²C(n, p) are presented.     

Response of a BaF2 scintillation detector to quasi-monoenergetic fast neutrons in the range of 45 to 198 MeV

We have studied the neutron response of a scintillation detector consisting of a 14 cm long, hexagonal-shaped BaF₂-crystal with an inner diameter of 8.75 cm coupled to an EMI9821QB photomultiplier tube. The detector was exposed to calibrated quasi-monoenergetic neutron fields obtained from ⁷Li(p,n)⁷Be reactions. The measurements were performed at neutron energies of 45, 60, 96, 147 and 198 MeV as given by the energies of the incident protons. The experimental pulse-height spectra of the BaF₂-detector are compared with Monte Carlo simulations using the FLUKA code. The detection efficiency of the BaF₂-detector in the energy range of 45–198 MeV was determined as a function of the discriminator threshold and compared to the literature data. At neutron energies above 100 MeV the detection efficiency of the BaF₂-detector was found to be a factor of two higher than that of an NE213-detector of comparable size.     

Identification of low energy ions (6 ≤ Z ≤ 10) in a plastic track detector CR-39

A method is developed for identifying low energy ions (6 ≤ Z ≤ 10) in a plastic track detector CR-39. After initial etching, rinsing in water and alcohol and final chemical etching of the detector, all in an ultrasound field, the etch rate ratio ($\text{V}{}_{\mathrm{<mtext>T</mtext>}}\text{V}{}_{\mathrm{<mtext>B</mtext>}}$) is directly measured at a defined distance from the end of the track. This method can be used to determine the etch rate ratio at several points along the track. The plastic track detector CR-39 is calibrated using ¹²C, ¹⁶O, ¹⁹F and ²²Ne ion energy in the interval from 2–7 MeV/amu.     

Evaluation of large deuterated scintillators for fast neutron detection (E=0.5-20 MeV) using the D(d,n)He, C(d,n) and Al(d,n) reactions

The response of large deuterated liquid scintillators (up to 10 cm diameter by 15 cm) to neutrons 0.5-20 MeV has been studied using the 2.5 MeV neutron generator at the University of Michigan, and the d(d,n), ¹³C(d,n), ²⁷Al(d,n) and other reactions at the University of Notre Dame FN tandem accelerator. The latter utilize 9 and 16 MeV deuteron beams including a pulsed beam, which permitted time-of-flight measurements. Combining pulse-shape discrimination and time-of-flight allows gating on specific neutron energy groups to determine the detector response to specific neutron energies. This will permit accurate simulation of the detector response functions for applications of these detectors in nuclear research and homeland security applications.     

CdWO4 scintillator as a compact gamma ray spectrometer for planetary lander missions

The objective of this work is to develop a gamma ray spectrometer (GRS) suitable for use on planetary rover missions. The main characteristics of this detector are low weight, small volume low power and resistance to cosmic ray radiation over a long period of time. We describe a 3 cm diameter by 3 cm thick CdWO₄ cylindrical scintillator coupled to a PMT as a GRS for the energy region 0.662–7.64 MeV. Its spectral performance and efficiency are compared to that of a CsI(Tl) scintillator 2.5 cm diameter by 6 cm thick coupled to a 28 mm×28 mm PIN photodiode. The comparison is made experimentally using ¹³⁷Cs, ⁶⁰Co, 6.13 MeV gamma rays from a ¹³C(,γn)O¹⁶* source, 7.64 MeV thermal neutron capture gamma rays emitted from iron bars using a ²⁵²Cf neutron source, and natural radioactivity 1.46 MeV ⁴⁰K and 2.61 MeV ²³²Th gamma rays. We use a Monte Carlo method to calculate the total peak efficiency of these detectors and the full energy, first and second escape peak efficiencies. The experimental and calculated results agree well. We investigated the usefulness of these detectors for a GRS on a Mars lander mission. Although both detectors meet desired specifications, it was found that CdWO₄ has advantages over CsI(Tl) being a more compact detector of higher efficiency. Using a shaping amplifier of 24 ms, CdWO₄ spectrometer exhibited a 6.8% FWHM at 662 keV. At 6.13 MeV, CdWO₄ detector possesses an intrinsic total and full energy peak efficiencies of 16.7% and 6.3%, respectively. These efficiencies are nearly a factor of 1.6 and 4 greater than the corresponding efficiencies of the CsI(Tl) detector.

A proposed gamma ray spectroscopy system to be placed on a rover, consists of a central detector surrounded by a Compton suppressor shield. The central detector is a cylindrical CdWO₄ detector and the Compton suppressor shield is made of segmented CdWO₄, coupled to PIN photodiodes. The shield also prevents thermal neutron activation of the central detector.     

Determination of thicknesses and impurities of targets by measuring backscattered particles from 2 MeV 4He+-bombardment

The measurement of target thicknesses and impurity contents using backscattering, as well as the principle of this technique are briefly described. The targets are irradiated in a beam of 2 MeV ⁴He⁺ ions. The backscattered ions are detected using a Si(Au) solid-state detector. Gold and calcium fluoride on thin carbon backing and selfsupporting silicon foils were determined from peak width measurements. Oxygen and tungsten impurities in a silicon target being prepared by vacuum evaporation from a tungsten boat were determined using the same method. The thickness of a thin aluminium deposit on thick tantalum backing was obtained from the energy shift of the tantalum peak between two spectra recorded with respectively the aluminium and the tantalum facing the ion beam. Copper contamination on the surface on an iron layer electroplated on copper foil was determined by comparison of the copper peak area with the iron peak height in the backscattering spectrum.