Scintillator-photodiode detecting systems for two-level X-ray inspection systems

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Scintillator-photodiode detecting systems for two-level X-ray inspection systems

V.D. Ryzhikov, N.G. Starzhinskiy, D.N. Kozin, L.P. Gal'chinetskii, V.P. Sochin, N.E.K.Lisetskaya, V.M.Svishch, A.D.Opolonin
STC for Radiiation Instruments, Concern "Institute for Singl Crystals",
Lenin Ave., 60, Kharkov,61001, Ukraine
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Introduction

In two-energy X-ray systems, as a rule detector modules (DM) are used in series, i.e., detectors are arranged sequentially one after another along the path of ionising radiation beam (X-ray source - scanned objecting modules) [1]. Both in low-energy detectors (LED), which at the same time are active filters of X-ray radiation, and in high-energy detectors (HED) the commonly used scintillators are CsI(Tl) crystals. This material, however, has a number of disadvantages: hygroscopicity (requiring special protection), low radiation strength and high afterglow. Another promising material for use in detectors of the above-described systems is a highly efficient scintillator ZnSe(Te) [2-4] parameters of which are presented in Table. However, Z_eff of ZnSe(Te) is low, as well as its transparency to intrinsic radiation [4].
The correct choise of the scintillator for DM is a multi-factor question, determined from their properties and detector design.

Criteria of scintillator choise for DM

In the general case, relative sensitivity (d /d) of DM is determined by the expression

(1)

where A_i - contribution of each X-ray luminescence component of the scintillator; d - o wire diameter, cm; Q_i - exponential X-ray luminescence components of the scintillator; d,d_eq-thickness of the background object and filters, cm; V- speed of the object's movement, cm/s; a (hn ) - dose contribution of the given energy to the X-ray radiation spectrum; m_eq(hn), m(hn) - linear attenuation coefficients of the object with filters and of the background object, respectively; B(hn) - absorption factor of scattered radiation by the object; - threshold contrast of the system; G- contrast function [5]. The contrast function can be expressed as

(2)

where - radiation increment of the detector signal; m_SC - linear attenuation coefficient of the scintillator, cm^-1; g_SC,g_SC- linear coefficients of electron transformation of the scintillator and air, respectively; R_light-coefficient of light collection in the scintillator with photodiode; K_SC - spectral matching coefficient of the scintillator luminescence spectrum and photodiode sensitivity; h _SC - conversion efficiency of the scintillator; h _PD - transformation efficiency of the photodiode.

Thus, it follows from (1), (2) that obtaining maximum sensitivity requires fast response of the scintillator, high conversion efficiency, high efficiency of X-ray absorption in the given ranges, good spectral matching with the photodiode, etc. Another requirement for LED is high transparence to X- ray radiation.

Results and discussion

Comparison of detector characteristics obtained with different scintillators has show that, for the object moving at V=100 cm/s and wire thickness d =0.1 mm, sensitivity decreased in agreement with (1) - by 1.3 - 1.5 times for CsI(Tl), and by 1.1 -1.25 times for ZnSe(Te).
To determine scintillator parameters A_i and Q_i , a specially designed installation was used in combination with PC [4]. Experimental data array on time-dependant signal amplitudes g_n was approximated by an exponentially decreasing function of the form

(3)

A_i

Q_i

(4)

Fig 1: Sensitivity of HED and LED using CsI(Tl) and ZnSe(Te) as function of radiation energy.

Estimates according to (1), (2) using experimental characteristics of scintillators (Table) and detectors have shown that sensitivity for LED with ZnSe(Te) is 1.5 times higher than with CsI(Tl) for energies up to 40 keV (at the optimum crystal thickness of 0.6 mm, Fig. 1). Penetrability for energies E>40 keV is much higher.

Parameter	ZnSe(Te)	CsI(Tl)
Conversion efficiency	19,4	15
Decay time t,ms	5-7	1
Density , g/cm³	5,4	4,5
Effective atomic number Z_eff	33	52
Emission maximum l_m at 300 K, nm	610	550
afterglow ,% (after 10 ms)	0,05	1-8
Attenuation coefficient at l_m, cm^-1	0,1-0,3	0,05
Refractive index for l_m	2,4	1,79
Radiation stability limit to g-radiation, rad	10⁸	10⁴
Coefficient of spectral matching with Si-PD K_SC	0.49	0.3
Table : Characteristics of scintillators used for "scintillator-photodiode" system

For HED with ZnSe(Te), because of low transparence (0.1-0.3 cm^-1) and low atomic number, sensitivity is lower than for HED with CsI(Tl) by 20-30 %. (Typical values for 1.44´ 3´ 4 mm detectors are 80-110 nA×min/(R×cm²) (ZnSe(Te)) and 100-130 nA×min/(R×cm²) (CsI(Tl))). Sensitivities of the detecting circuits of LED(ZnSe(Te)) + HED(CsI(Tl)), LED(ZnSe(Te)) + HED(ZnSe(Te)), LED(CsI(Tl)) + HED(CsI(Tl)) are presented in Fig. 2 and Fig. 3.

Fig 2: Output signal of HED with different scintillators in DM at different voltage U_a on the X-ray tube.

Fig 3: Energy dependence of sensitivity of HED and DM for different combinations of scintillators.

Using such detector modules, objects with different values of m are easily discerned, as it can be seen from Fig. 4 - e.g., sugar and salt (organic and inorganic compounds).

Fig 4: Object images obtained using the two-energy introscope: a)general shadow picture of the object; b)shadow picture with inorganic material singled out; c)shadow image with organic material singled out.

Sensitivity of the system is, with detector arrays of 0.8 mm step, less than 1% at the background of 20 mm steel for X-ray energy 100 keV.
Another important characteristic of scintillators for inspection systems is their radiation strength, which largely determines its total operation time resource. In this respect, semiconductor scintillator ZnSe(Te) is beyond competition, due to peculiar features of its crystal structure. Its radiation strength is ~10⁸ rad, with luminescence intensity decreasing just by 10-15%, while with Csl(Tl) the intensity decrease is 20-30% at 2×10³ rad [5].

Conclusions

Highly sensitive detector modules can be obtained using scintillator crystals - ZnSe(Te) for low-energy detectors (LED) Csl(Tl) for high-energy detectors (HED).
At object movement speed >100cm/s and high dose loads, combined structures LED (ZnSe(Te))+HED(ZnSe(Te)).
Such DM allow to obtain, with penetrating ability at E_p=100 keV more than 20mm (steel), sensitivity not worse than 1%.

References

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