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Oxide and Semiconductor Scintillators in Scintielelectronic Detectors for Detection of Neutrons V.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 Contact

ABSTRACTS

Results are presented for pulse amplitude spectra of crystals based on complex oxides (CCO) irradiated in mixed fields of neutron and gamma-radiation. These crystals - CdWO₄(CWO), Gd₂Si₂O₅(Ce) (GSO(Ce)) and NaBi(WO₄)₂ - include elements with large radiation capture section of thermal and resonance neutrons. Results of studies of dispersion scintillation detectors based on A^IIB^VI compounds and transparent materials with high neutron trapping cross-section are reported.

I. INTRODUCTION

The method of neutronography is one of the most informative and promising for nondestructive testing. At present, there is a clear tendency to be the use, alongside with gas discharge devices, solid-state scintillation detectors and dosimeters based on materials with high capture cross-section, such as ⁶Li, ^155,157Gd, etc. In recent years STC RI has developed a member of detectors of "scintillator-PMT" "scintillator-photodiode-charge-sensitive preamplifier" types, as well as various instruments on their base. The aim of this work was to continue our studies of different possibilities of oxide and semiconductor scintillators applications for neutron detection.

II. EXPERIMENTAL STADIES

Main problems which arise when thermal neutrons (TN) are detected over radiation capture reaction products are connected with the presence of intense gamma-field of ²³⁹Pu-Be(a ,ng )-neutron source (NS), especially in the low-energy range. The most intense total absorption peaks (TAP) from internal conversion electrons (ICE) of neutron radiation capture by Gd nuclei are within the energy range from 29.1 to 39.0 keV and from 71.5 to 81 keV. The most intense gamma-lines of ²³⁹₉₄Pu in this energy range are Eg =51.6 keV and 98.81 keV. This is also true for the most intense TAP of ICE for neutron radiation capture on ¹¹³₄₈Cd (E_e=69.0 and 91.9 keV) and ¹⁸⁶₇₄W (E_e=46.7 and 76.2 keV) [1].
The value of ²³⁹₉₄Pu gamma-quanta flux (Eg =56.83 and 98.81 keV), which was measured experimentally by us, was 4.0× 10⁶ g -quanta/s with NS of 10⁵ neutrons/s yield; activity of ²⁴¹₉₅Am with the same NS, measured by the procedure [2], was 1.6 MBq.
When TN flux is measured using CCO, either deep passive protection from NS gamma-quanta is used (lead, 5-10 cm thick), or the neutron flux is measured by the method of variation of induced activity of CCO nuclei after the irradiation.
In this work, for suppression of gamma-background we used both passive shielding method and the method of measurements ensuring discrimination of background and noise from the accompanying radionuclides with subsequent account for them in detection channels.
For the first time in this work we used a new instrument developed by us - RK-AG-1 alpha-gamma-radiometer (based on GSO(Ce) scintillator [1]) - for routine measurements of TN flux. The radiometer consists of the detection block (DB) with a passive protection and the data processing block. The software used allows to selectively increase the detection efficiency of ²⁴¹₉₅Am gamma-radiation (Eg =59.6 keV). Method of ²⁴¹₉₅Am specific activity measurement used in this radiometer includes measurement of the background and noise from continuous distribution of radionuclide fission products (^134,137Cs etc.) and contribution from the ¹³⁷₅₅Cs^+137m₅₆Ba line with E_KX=33 keV, calculation of the ²⁴¹₉₅Am signal in the windows and further information procession using the microcomputer as part of the instrument. This ensures high sensitivity and noise protection of the radiometer in the typical activity range of ²⁴¹₉₅Am under activity ratio of ^134,137Cs to ²⁴¹Am of 9.0× 10³ and more.
Irradiation of CCO as part of DB was carried out by NS neutrons (10 ⁵ neutrons/s). NS was placed into a polyethylene sphere 150 mm in diameter. TN yield at the surface of the sphere was 0.09 with respect to NS. Activation of CCO samples was carried out using a similar NS of 5.0× 10⁶ neutrons/s in a paraffin container-moderator. TN flux density at the location of CCO was 10³ neutrons/s. Testing of the TN flux was carried out by activation of ¹²⁷I in NaI(Tl) single crystal.
Measurements of active pulse spectra for CWO and GSO(Ce) samples in the NS mixed field were carried out using an AMA-02-0 installation with 1.6 ms for CWO crystals. Measurements of induced activity gamma-quanta of NBWO crystals were carried out using DB with NaI(Tl) under combined passive protection with wall thickness 150 mm (steel), 50 mm (lead) and 10 mm (copper).
Under deep suppression of gamma-background from NS, which was placed inside the polyethylene sphere near the walls of this protection, spectrograms were measured for CCO placed into DB inside the protection. TN flux density measured under these conditions using ⁶LiI(Eu) crystal was 0.3 neutrons/cm² ×s.
For our studies we used crystals CWO, GSO(Ce) and NBWO, prepared as described in [1]. CWO and GSO(Ce) crystals were of 40x5 mm size, and dimensions of NBWO were 25x25x110 mm³.

III. DISCUSSION OF EXPERIMENTAL RESULTS

In Table 1, data are presented which were obtained in a mixed field of NS with 10⁵ neutrons/s yield. The measurements were carried out using RK-AG-1 radiometer.
TN flux density in the place of location of the detector at the distance of 0.5 m from NS was 17 neutrons/cm^2×s. It follows from the Table 1 that GSO(Ce) can be also used for detection of fast neutrons (if a moderator is used - paraffin, organic glass). The first energetic window (from 41 to 45 keV) corresponds to predominant detection of KX-quanta of gadolinium, the second (from 45 to 87 keV) - of KX-quanta and ICE of gadolinium, the third (from 87 to 375 keV) - of ²³⁹₉₄Pu gamma-quanta (Eg=98.81 and 375.0 keV). For protection from gamma-, KX- and LX-quanta of ²³⁹₉₄Pu (Eg=51.62 and 98.81 keV, E_KX from 94.66 to 11.3 keV and E_LX from 11.62 to 20.03 keV) steel shields were used.

Neutron source	Shield, moderator	r=0.5 m
		n, pulses×s-1 in energy window, keV
		41-45	45-87	87-129
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
	r=0.25 m
	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
	r=0.125 m
		0.594	8.463	17.578
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 [1], on the basis of ZnSe(Te) - in [2].

Crystal	Crystal dimensions, mm	Duration of pulse shaping, ms				Arrangement of scintillators
		1.6/1.6		12.8/12.8
		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
CWO+ GSO(Ce)		205 184		478 160		GSO(Ce) above CWO without OC
CWO+ GSO(Ce)		230 164		393 206		GSO(Ce) above CWO without OC
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 K_abX and L_abX 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 E_ba£20 keV, including tritium, and the use of CdWO₄ - of E_ba³ 20 keV.

Scintillators on the basis of A^IIB^VI 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
6Li	7.42	6L(n,a)T	4.785	936
10B	19.61	10Bn,a)7Li*	2.781	3813
14N	99.63	14N(n,p) 14C	0.626	1.77
33S	0.76	33S(n,a) 30Si		7.10
35Cl	75.53	35Cl(n,p) 35S	0.615	0.3
39K	93.10	39K(n,a)39 Ar	0.218	3.8
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 [3]. 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, C_H is hydrogen content, t is decay time, V_ac and V_g - pulse amplitude from alpha-particles and gamma-quanta, respectively.

Material	Purpose	CH, (“1022sm-3)	t, ns		Vg	a/b- ratio	Reference
BC408	scintillator	5,3	5	1,0	-	0,09	[3]
NaI(Tl)	-"-	0	200	-	1,0	0,35	-"-
NH4Br(Tl)	-"-	6,0	1000	5,5	0,6	0,18	-"-
NH4I(Tl)	-"-	4,2	200	3	0,4	0,15	-"-
(NH4)2SO4(Tl)	-"-	6,5	-	0,4	0,05	0,14	-"-
NH4H2PO4	dispersion medium	5,6	-	-	-	-
KH2PO4		2,1	-	-	-
ZnSe(Tl)	scintillator	-				1,0
CdS(Te)	-"-	-				0,8
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 A^IIB^VI 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 E_p~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).

Crystal	Dimensions,mm	Eg, keV	Rg,%
CsI(Tl)	10“10“10	662	5,9
BGO	10“10“5	662	25,0
CWO	10“10“5	662	14,8
	40“40	4430	R1/2*=25
Table 6: Scintillation parameters of the system scintillator - Si-PIN-PD -PA.

The output signal value is sufficient to allow measurements using the hangover output window to increase the sensitivity.

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.

IV.CONCLUSION

Possibility of operation was shown in principle for the neutron detection block based on GSO, CWO and DSD and Si-PIN-PD-PA.

V. REFERENCE

1. V.D.Ryzhikov, S.F.Burachas, V.G.Volkov et al. Neutron flux measurements using "scintillator-photodiode- preamplifier system and new types scintillators. - Procceeding of International Conference "Neutrons in Research and Industry", 9-15 June 1996, Crete, Greece, P.P. 586-595.
2. V.D.Ryzhikov, Yu.A.Borodenko, V.G.Volkov et al. Studies of dispersion scintillation detectors based on semiconductor compounds A_IIB_IV and transparent materials with hish neutrons trapping cross-section. Proceedings of the International Conference on Inorganic Scintillators and Hoir Applications "SCINT'97", Shanghai, P.R.China, September 22-25, 1997, CAS, Shanshai Branch Press, pp.51-54.
3. J.Bart Czirr, Manuel Berrondo. Inorganic Hydrogenous Scintillators for Neutron Detection. - Proc. of the Int. Conference on Inorganic scintillators and Application "SCINT95", Delft, The Netherlands, August 28-September 1, 1995, p.p. 462-464.

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