Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment
Thin YAP:Ce and LaBr3:Ce scintillators as proton detectors of a thin-film proton recoil neutron spectrometer for fusion and spallation sources applications
Introduction
Neutron spectrometers for measurements in the MeV range have played important roles in spallation sources and fusion plasma devices in recent years [1], [2], [3], [4], [5], [6], [7]. The instrumentation used in both cases is of dedicated design that depends on the specific diagnostic needs of each experiment. For example, in fusion plasma applications at JET, a Magnetic Proton Recoil (MPR) spectrometer has been used for 14 MeV neutron measurements at 5% resolution, providing information of unprecedented detail on neutron emission from the plasma [8], [9], [10]. The significant dimensions (several tens of meters) and weight (about 80 t) of the instrument, however, do not make the MPR technique particularly suitable for applications where there are space limitations, such as arrays of neutron detectors arranged in a camera system. In this context, a Thin-film Proton Recoil (TPR) spectrometer could be an interesting alternative. The TPR detection principle is based on neutron-to-proton conversion via elastic scattering on hydrogen nuclei at a given angle in a plastic thin foil. The scattered proton energy can be easily measured and converted back to the incoming neutron energy, provided that the recoil angle is known [11]. A preliminary design of a non-magnetic TPR detector for fusion plasma diagnostics has been presented in Ref. [12]. Here it is shown through calculations that TPR could attain an energy resolution close to that of the MPR, combined with an increased efficiency of 2.9 10−4 n cm2 and compact dimensions. The design in Ref. [12] used silicon detectors as proton spectrometers, given their excellent energy resolution and fast signals. In particular, a proton energy resolution better than 2% would be ideal for a TPR system, so that the overall energy resolution of the spectrometer, that gains contributions also from the finite aperture of the recoil solid angle and the thickness of the scattering foil, could still be about 5%.
In this paper we demonstrate that such requirement for the proton energy resolution could be also achieved using fast inorganic scintillators as alternatives to silicon detectors. Their main advantages are the resistance to neutron irradiation and cost effectiveness. In particular LaBr3(Ce) and YAP(Ce) are the proposed scintillator crystals, the latter being the most cost effective one. Besides, the fast scintillation time constants of these crystals would enable their use at high count rates up to few MHz. A prototype of a TPR spectrometer of such design was tested at the ISIS neutron source of the Rutherford-Appleton Laboratory (UK) in a proof-of-principle measurement using YAP scintillators, presented in Ref.[13]. In this experiment neutron spectroscopy in the energy range 30 to 80 MeV was demonstrated and advantage was taken from the fast scintillation time of the crystal, which is needed to cope with the high instantaneous count rate provided by the pulsed nature of the ISIS neutron source. However, for this application no strict requirement was set on the overall energy resolution.
In this paper, we report on measurements aimed at the determination of the energy resolution of YAP and LaBr3 to protons in the energy range 4 to 8 MeV. The experiment, performed at the Uppsala Tandem accelerator at low counting rates (a few kHz), is presented in the next section. The results on the energy resolution are then illustrated and compared to laboratory calibrations using γ-ray sources.
Section snippets
Experimental setup
Two thin inorganic scintillators based on YAP and LaBr3 crystals (1 in. diameter × 0.1 in. height) have been coupled to two eight dynode Photo Multiplier Tubes (PMTs), model R6231 by Hamamatsu [14]. Special care was taken in the case of LaBr3 which, being hygroscopic, was encapsulated on all sides, with a thin (125 μm) Be entrance window. This is where the proton beam was impinging in the experiment and was needed to minimize energy loss, which would otherwise not be tolerable in the thick
Detector characterization with laboratory gamma-ray sources
The thickness of the two crystals is optimized to stop protons up to 20 MeV. For this reason the detectors have low efficiency to γ-rays, which are the main background sources during the measurement. Nevertheless, the high density and high effective Z of the crystal allow distinguishing full-energy-peaks when the crystal is irradiated with laboratory γ-ray sources. These measurements are useful to determine the energy resolution of the two crystals to γ-rays in the MeV range, obtained from the
Proton measurements at the Uppsala tandem accelerator
The light yield of a scintillator crystal under proton irradiation is different from that measured with γ-rays of same energy due to quenching effects. For this reason, dedicated measurements of the crystal energy resolution with protons of known energies were undertaken. Fig. 4 compares proton energy spectra, normalized to peak height, measured with the thin LaBr3 and YAP scintillators at the Uppsala tandem accelerator in the Rutherford scattering experiment described in Section 2 and using an
Conclusions
Energy resolution measurements have been performed with thin (1 in.×0.1 in.) LaBr3 and YAP scintillators using protons and γ-rays between 1 and 8 MeV. The measurements show that both crystals are good candidate components for a TPR spectrometer for fusion and spallation source applications as they match the required energy resolution and fast scintillation time. In particular, our measurements show that a proton energy resolution better than 2% can be achieved at 8 MeV. These results extrapolate
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Response of a telescope proton recoil spectrometer based on a YAP: Ce scintillator to 5–80 MeV protons for applications to measurements of the fast neutron spectrum at the ChipIr irradiation facility
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2018, Nuclear Instruments and Methods in Physics Research, Section A: Accelerators, Spectrometers, Detectors and Associated EquipmentCitation Excerpt :2) Beam mapping and profiling is performed with diamond [20] and GEM detectors [21]. ( 3) Spectroscopy with enhanced energy resolution is being investigated using the telescope proton recoil technique [22]. In this paper the unfolding methodology, based on multi-foil activation data, is presented in Section 2.
Light response of YAP:Ce and LaBr<inf>3</inf>:Ce scintillators to 4-30 MeV protons for applications to Telescope Proton Recoil neutron spectrometers
2016, Nuclear Instruments and Methods in Physics Research, Section A: Accelerators, Spectrometers, Detectors and Associated EquipmentCitation Excerpt :These crystals have been chosen for their good resolution (due to the large light yield, which is 63,000 photons-per-MeV for LaBr3 [12] and 20,000 to 25,000 photons-per-MeV for YAP [19]) combined with fast scintillation time, which is 16 ns and 27 ns, respectively, and their resilience to damage at high neutron fluxes, unlike silicon detectors. Energy resolution measurements, carried out at Uppsala Tandem Accelerator in the 4–8 MeV energy range on LaBr3:Ce and YAP:Ce, indicated that a peak energy resolution better than 2% can be achieved when these crystals are irradiated with 8 MeV protons [21]. Concerning spallation sources applications, inorganic scintillators are the natural choice, since Si detectors cannot be made thick enough to stop protons at those energies (several tens of MeV).
Response of LaBr<inf>3</inf>(Ce) scintillators to 14 MeV fusion neutrons
2015, Nuclear Instruments and Methods in Physics Research, Section A: Accelerators, Spectrometers, Detectors and Associated EquipmentCitation Excerpt :Fig. 4 also shows the response to direct 14 MeV neutrons (presented in Fig. 3) and which can be here compared to background. In order to obtain the γ-ray equivalent energy for the direct component, quenching effects were taken into account [23–25]. The finite energy resolution of the detector was included in Fig. 4 by convolution of the simulated spectra with a Gaussian function of energy dependent width.