·Table of Contents
·Methods and Instrumentation
Oxide and Semiconductor Scintillators in Scintielelectronic Detectors for Detection of NeutronsV.Ryzhikov, V.Chernikov, L.Nagornaya, N.Starzhinskiy, E.Lisetskaya, E.Danshin
STC RI of Concern "Institute for Single Crystal"
60 Lenin Ave., Kharkov, 61001, Ukraine
|Neutron source||Shield, moderator||r=0.5 m|
|n, pulses×s-1 in energy window, keV|
|Source without sphere||-||0.094||1.022||1.808|
|organic glass, 40 mm||0.599||0.311||2.066|
|steel,10 mm, CWO, 40 mm||0.215||3.923||8.718|
|paraffin, 40 mm CWO, 40 mm||0.425||4.195||8.838|
|Source inside sphere||r=0.5 m|
|steel, 10 mm||0.284||3.340||8.630|
|steel, 200 mm, CWO, 40 mm||0.153||1.111||1.980|
|Table 1: Measurements of neutron flux using an RK-AG-1 radiometer|
In Table 2 data are presented on resolution of signals using pulse duration of combined detector (CD) based on GSO(Ce) and CWO. Similar CD on the basis of an organic crystal and CWO was described in , on the basis of ZnSe(Te) - in .
|Crystal||Crystal dimensions, mm||Duration of pulse shaping, ms||Arrangement of scintillators|
|Vmax, a.u.||R662,%||Vmax, a.u.||R662,%|
|GSO(Ce)||43x5||370||11,1||318||13.9||on PMT in optical contact (OC)|
|CWO||30x30x5||494||10.9||on PMT in OC|
|GSO(Ce) above CWO |
|GSO(Ce) above CWO|
|Table 2: CD on the basis of GSO and CWO(Ce) under irradiation by gamma-quanta with Eg =662 keV|
Beta-radiometers based on a "scintillator-photodiode" system are used for measurement of specific activity of beta-active nuclides in the energy range of the detected radiation 0.2-1.5 MeV. The detection threshold at low values is determined by thickness of the input window of the detector and by the minimum value of the detectable signal, which is about 50-60 keV for ZnSe(Te). To expand the energy range towards lower energies and to increase sensitivity, we have tried to use a combined detector of "sandwich" type on the basis of detachable metal foils and Si-PIN-PD.
|Fig 1: Spectrometric characteristics of "sandwich"-type detectors a) - calibration by241 Am b),c) - K-LX quanta of Gd and W foils, respectively d) - KX quanta from CdWO4|
X-ray quanta of K- and L- series which appear when beta particles from 90Sr+90Y are decelerated in metal foils are detected in the sensitive layer of an 18x18 mm2 Hamamatsu PIN-PD operating in semiconductor detector mode. A specially designed charge-sensitive preamplifier connected with the photodiode has equivalent noise level of 700 electrons with formation time of 5 ms.
Beta particles from irradiated from neutrons were decelerated in 100 mm thick foils of molybdenum, cadmium, gadolinium, tantalum and tungsten. Spectrograms of KabX and LabX quanta of these elements are presented in Fig.1. For comparison, a spectrogram of X quanta of cadmium and tungsten is presented, i.e., of elements which are present in the CdWO4 scintillation single crystals.
As follows from Fig.1, the use of metal foils as converters of beta radiation allows to detect beta particles with minimum energy of Eba£20 keV, including tritium, and the use of CdWO4 - of Eba³ 20 keV.
Scintillators on the basis of AIIBVI compounds, such as ZnS(Ag), ZnSe(Te) and CdS(Te), have alpha to beta ratio not worse than 1 and can be used for detection of secondary charged particles coming from nuclear reactions in which neutrons interact with target nuclei of atoms present in transparent materials of dispersion scintillation detectors (DSD) matrices. Several exothermic (n,a ) and (n,p)-reactions on stable light and medium-mass nuclei are known which could be used for neutron detection (Table 3). In this Table, c is relative content of the isotope in the natural mixture, Q is reaction energy, s is cross-section of the reaction.
|Target nucleus||c,%||Reaction||Q, MeV||s, barn|
|Table 3: Nuclear-physical constants of (n,a ) and (n,p)-reactions|
We have tested the following scintillation materials and dispersion media for DSD (Table 4). The grain size of the powdered scintillation materials did not exceed 20 mm. This ensured complete absorption of secondary charged particles: alpha-particles, protons, tritons, as well as protons and recoil deutons. At the same time the grain size should be significantly less than the free path length of electrons formed as a result of interaction of high-energy gamma-quanta with the DSD matter.
|Scintillation material||Dispersion medium||Method of introduction of scintillation material into the dispersion medium|
|ZnSe(Te)||Boroglycerol plastic||Plastic was synthesized from glycerol and boric acid. At 60°C powdered scintillation materials were added|
|ZnSe(Te)||Thermoplastic materials: acrylic plastic, polyethylene, polystyrene||Polymerization or hot pressing together with scintillation powders|
|ZnSe(Te)||Polyvinyl alcohol||Pressing of scintillator powders with polyvynil alcohol|
|CdS(Te)||Adhesive compositions based on silicon-organic materials||Polymerization together with scintillation powders|
|Table 4: Composition of dispersion detectors|
Apart from dispersion media based on organic materials and listed in Table 4, hydrogen-containing inorganic materials, like thallium-activated ammonium bromide, iodide and sulphate, can also be used . Characteristics of these materials are presented in Table 5 together with salts of orthophosphoric acid proposed by us as a dispersion medium. In this table, CH is hydrogen content, t is decay time, Vac and Vg - pulse amplitude from alpha-particles and gamma-quanta, respectively.
|Material||Purpose||CH, (´1022sm-3)||t, ns||Vg||a/b- ratio||Reference|
|Table 5: Characteristics of inorganic hydrogenous scintillators for neutron detection|
Complex borohydrides of sodium and lithium were not sufficiently stable and easily reacted with some inorganic scintillators.
|Fig 2: Pulse amplitude spectrum of the DSD-based detection block with Si-PIN-PD-PA in the radiation field of 239Pu-Be (a,ng)-source: a) detection block as a whole; b) DSD separated from Si-PIN-PD by a sheet of black paper; c) difference between a and b; d) 241Am with Si-PIN-SD.|
DSD preparation methods included the use of known hydrogen-containing substances together with binding materials as transparent matrices for finely dispersed scintillators based on AIIBVI compounds. Detectors of fast neutrons were made on the basis of CdS(Te) and ZnSe(Te) and hydrogen-containing compounds, as well as detectors of thermal neutrons on the basis of ZnSe(Te) and boron-containing compounds.
Detectors were tested using a Hamamatsu Si-PIN-PD of S3590-01 with a "fast" charge-sensitive preamplifier (CSPA). Duration of the pulse forefront at the CSPA output under excitation by neutrons of the ZnSe(Te)-based DSD with Si-PIN-SD was ~0.1 ms, under excitation by neutrons of the ZnSe(Te)-based DSD with Si-PIN-PD - several ms. This allowed to easily discern signals coming from low-energy gamma-quanta of 239Pu-Be (a,ng ) source from signals excited by high-energy gamma-quanta and neutrons in the DSD itself.
Fig.2 shows amplitude distribution of pulses for DSD of fast neutrons based on ZnSe(Te) and a hydrogen-containing substance obtained under irradiation by the mixed field of 239Pu-Be (a ,ng )-source of neutrons. One can easily single out the peaks of total absorption (p.t.a.) from gamma-quanta of 241Am, which was accumulated in this source as a result of decay of 241Pu. Signals from recoil protons are below p.t.a. with Eg =26.4keV on Si-PIN-SD itself, for which the lower detection threshold of gamma-quanta is below 5 keV. This corresponds, for the system ZnSe(Te) single crystal + Si-PIN-PD, to approximately 50 keV. Thus, signals from the recoil protons with average energy Ep~2MeV for the given DSD are detected in the 50-250 keV range when the single crystal is irradiated by gamma-quanta.
The detection system based on S3590 type Si-PIN-PD, CsI(Tl), BGO and CWO scintillators together with operational amplifier is sensitive to gamma-radiation and neutrons and can be characterised by the following parameters (see Table 6).
|Table 6: Scintillation parameters of the system scintillator - Si-PIN-PD -PA.|
To protect Si-PIN-PD from low-energy gamma-radiation from 239Pu-Be (a ,ng )-source, BGO and CWO crystals were used as protective light transducers. These transducers, in turn, were used for separate detection of high-energy gamma-radiation and neutrons in the detecting system Si-PIN-PD - complex oxide-based scintillator and DSD of neutrons.
|Fig 3: Pulse amplitude spectrum from thermal neutrons absorbed by the 10B2O3+ZnS(Ag) detector with a Si-PIN-PD of S-3590 type. Thermal neutrons obtained by moderation of fast neutrons 239Pu-Be(a,ng)-source inside the polyethylene sphere 150 mm in diameter. In this source 241Am is accumulated, which is formed as a result of beta-decay of 241Pu (T1/2 = 13 years) present as an impurity in 239Pu. Full absorption peaks from 241Am gamma-quanta (Eg= 26.3 and 59.5 keV) absorbed in Si-PIN-PD are presented.|
Fig.3 shows pulse amplitude spectrum of the combined DSD of thermal neutrons based on 10B2O3 + ZnS(Ag), applied to the organic glass light diode of 16x40 mm size. Irradiation by a flux of thermal neutrons was effected from one side. In the Figure, peaks are observed from alpha-particles of the 10B(n,ag)7Li* reaction, as well as from 241Am gamma-quanta, observed by Si-PIN-PD as SD.
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