IEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL. 43, NO. 5 , OCTOBER 1996 2509

Elongate X-Ray Detectors for Application in Security Inspection Systems

Ludmila B. Zagarij, Eduard L. Vinograd, and Yurij T. Vidaj

Abstract-We have developed, manufactured, and investigated a novel type of long X-ray detector up to 1000 mm long with two exit windows. The sensitive element of the detector is an elongated polished rectangular plate of inorganic scintillator. The detector is highly sensitive to X-rays in the energy region 50-250 keV. The factors influencing the sensitivity and longitudinal uniformity of the detector response have been considered.

I. INTRODUCT~ON ETECTION SYSTEMS using a fast scanning X-ray D beam for imaging of an absorbing object require long X-

ray detectors [1]-[3]. High sensitivity to X-rays in the energy region of 50-200 keV is one of the major requirements for the detectors to be used in X-ray scanning systems. A no less important condition for obtaining a clear image of an object is providing for the slight dependence of the output signal value on the X coordinate of the incidence point.

Production of extended inorganic scintillation crystals up to 1000 mm long having uniform distribution of scintillation properties along the length of the crystal is an extremely difficult task. The problem is more complicated for crystals having a large value of length ( I ) to cross dimension ( d ) ratio ( l / d >> 10) due to strong dependence of coefficient of light collection on the flash point coordinate. In the case of an extended scintillator composed of two or more optically coupled crystals, the local nonuniformities on the joints will play a vital role.

For getting maximum uniformity of response along the detector length, it is advisable to use a single long scintillator having a uniform distribution of scintillation parameters and to provide the best conditions for light collection on the photomultiplier tube (PMT).

In this paper, we describe a novel X-ray detector up to 1000 mm long with two exit windows.

The sensitive element of the detector is a polished rectangu- lar plate of inorganic scintillator (CsI(Tl)), which is optically coupled to a long transparent cylindrical light guide made of nonscintillating plastic.

The specific feature of the detector is that most of the scintillation light propagates through the light guide that has far more transparency and cross-sectional area than those of the scintillator. This provides a highly sensitive detector.

Manuscript received January 18, 1995; revised April 11, 1995 and April 8,

The authors are with the Institute for Single Crystals, 310001 Kharkov,

Publisher Item Identifier S 001 8-9499(96)07759-3.

1996.

Ukraine.

'+ 1 Fig. 1. A scheme of scintillation light propagation. 1-Photomultiplier; 2-Light guide: 3-Scintillator.

11. ANALYSIS OF LIGHT COLLECTION IN DETECTOR Let us consider a scheme of scintillation light propagation

in the case when the wide side of an elongated flat scintillation plate is optically coupled with the long transparent light guide (Fig. 1).

Let us suppose that scintillation light having intensity l o is emitted at the scintillator point A at the distance z from the PMT.

The total light that reaches the PMT is composed of the following fractions:

1) Fraction 11 of scintillation light transfers from the scin- tillator to the light guide directly at the point of flash

1; = l ow , (1)

(2) w = -(1- COSQ)

where w is the solid angle and 0 is the limiting angle for the light on the crystal-light guide boundary [4] For the case of immersion contact between the crystal and the light guide

1 2

121 0 = arcsin - 'II

( 3 )

where n ,n l are the indices of refraction in the CsI crystal and the light guide matter, respectively. Taking

'The light retuming to the scintillator from the light guide was not taken into account since the contribution of this light in total light flux is negligible for the considered system. This is explained by existence (besides the meridional rays) of slanting rays never crossing an axis of the light guide for the case when a light source is placed at the periphery of the light guide. The s h t i n g rays propagate inside the light guide in a broken spiral line and it is these rays that mainly transfer the energy over the light guide [4]. The probability of this light retuming to the scintillator is proportional to the ratio of the scintillator surface area (coupled with the light guide) to the light guide surface area. For the case under consideration this value is insignificant.

0018-9499/96$05.00 0 1996 IEEE

IEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL. 43, NO. 5 , OCTOBER 1996

I1 = 0.22IO. (4) 0.7 f

Taking into account the light absorption in the light guide, we find that the light that reaches the PMT is

I10 = I1 exp(-Kz) (5)

where K is the light absorption coefficient in the light guide. Fraction 1, of light propagates through the scintillator. The elementary evaluation [4] enables one to determine this fraction of light as

Further along, the light, scattering in scintillator, trans- fers to the light guide at some distance (2 - .) from the point of flash. This fraction can be represented as

100 200 MO 400 500 600 Z mm

Fig. 2. Coefficient of light collection (T) as a function of distance between the flash coordinate and the PMT. 1-Total light that reaches PMT; 2-Frac- tion of light that propagates through the light guide [sum of the first and the second terms in (1 6)]; 3-“Direct” light propagating through the scintillator [the third term in (16)]; 4-Experimental result.

I , = 12a (7) The total light flux that reaches the PMT is

where a = crsc/(crabs + oSC) is a coefficient that takes 0 = 110 + IDSO + 1 2 0 . (15)

Taking Oabs = 0.05 cm-l , crsc = 0.15 cm-’, K = 0.02 cm-I and taking into consideration (5 j and (1 3 j-( 15), we find the coefficient r of light collection as a function of distance between the point of flash and PMT

T = 0.02exp(-0.02~) + 0.33

into account the light scattering in the scintillator due to departure from the ideal (geometrical and optical); oabs, osc are light absorption and scattering coefficients, respectively, of the scintillator; and (crabs + crsc) is total light attenuation coefficient of the scintillator. Through the ( 2 - x) layer of scintillator passes the flux

(8) I = 1 2 exP[-(oabs + osc)(z - .)I. x [exp(-0.022) - exp(-O.2~)]

The light attenuation in the dX layer of the scintillator can be presented as

d 1 = 1 2 exp[-(aabs + a s c ) ( z - x)](oabs + asc)d.. (9)

Flux will be transferred from the scintillator to the light guide

dIg = d I a . (10)

Taking into account the light absorption in the light guide we have

dIUr = dI, exp(-Kz). (1 1)

Substituting (6j, (9), and (10) into (ll), we find that

dI,, = 0.39IoasC exp[-(oabs + oSc) ( z - z)] x exp(-Kz)dz. (12)

After integration of (12) over the interval (0 - x) we obtain

o s c 1,o = 0.3910 gabs + c s c - K

X { e x ~ ( - K x ) - exp[-(aabs + o,c)z]}. (13)

A fraction of the light propagates directly through the scintillator (so-called “direct” light) and reaches the PMT, being attenuated in the scintillator as follows:

+ 0.39 exp( -0.22). (16)

Fig. 2 shows the relation between r and the “flash coordi- nate-PMT” distance according to (16) (curve 1) and the contribution of light flux passing through the light guide (sum of the first and second terms of (16)-curve 2) and light flux passing through the scintillator (the third term of (16)-curve 3) in light collection. From Fig. 2, one can see that the main part of scintillation light propagates through the light guide.

It is determined, in the first place, by the greater trans- parency to the scintillation light of the light guide than that of the scintillator and, in the second place, by the greater cross section of the light guide relative to that of the scintillator.

Thus, the light guide is an additional parallel way for the propagation of scintillation light in the considered system.

111. EXPERIMENTAL RESULTS AND DISCUSSION

The examined samples were irradiated by a collimated X- ray beam normal to the longitudinal axis of detector.

The model experiments were performed with a 20 mm x 10 mm x 500 mm polished plate of CsI(Tl), which was placed in the groove of the light guide of 60 mm in diameter. Three sides of the plate were optically coupled to the light guide by means of immersion glue.

In order to obtain the relation between the signal output and the “flash coordinate-PMT” distance, the X-ray tube was automatically moved parallel to the longitudinal axis of the detector. Experimental data obtained for the described samples

1 2 0 = I2 exP[-(aabs + as,)z]. (14) are in agreement with the evaluated ones (Fig. 2, dashed curve

ZAGARIJ et al.: ELONGATE X-RAY DETECTORS FOR APPLICATIONS IN SECURITY SYSTEMS

Flash- photomultipl ier distance, mm 0

2511

plate CsI(T1) optically side reflector side reflector coupled with of white of mylar light guide paper

0.73 1.00 0.75 I 500 I 0.17 I 0.25 I 0.4

4). The experimental values were normalized to the calculation at z = 250 mm.

We have investigated the influence of different factors such as reflector type, treatment of scintillator surface on detector sensitivity and longitudinal uniformity. Table I shows the values of output signals corresponding to excitation by a collimated X-ray beam at the nearest and the most distant scintillator regions relative to the PMT. White paper and mylar foil (aluminized) wrapped round the light guide were used as reflectors. As the data in Table I demonstrate, the mylar reflector provides more uniform distribution of output signal along the longitudinal axis than the diffusion one.

The cited ways of improving longitudinal nonuniformity of response 1.51 by means of combined surface treaiment (roughing and polishing) of different regions of the scintillator were considered unacceptable to scintillators with large values of l f d ratio.

For such crystals, the surface treatment (roughing) of the surface results in an increase of the output signal for the scintillator region nearest to PMT and a decrease for the more distant region. For this reason all sides of scintillator plate must be carefully polished.

We have analyzed the possibility of improving the response nonuniformity. As it follows from Fig. 2 (curve 3), the scin- tillation light propagating directly through the scintillator is mainly responsible for the nonuniform distribution of output signal along detector length. This is due to the relatively low transparency of the scintillator to natural luminescence and relatively little area of scintillator end in contact with the PMT.

It is obvious that for getting the best conditions for response uniformity, the minimization of “direct” light is needed. By placing “mask” foils made of light-reflecting nontransparent material (for example, mylar) on the scintillator ends, we would improve the value of longitudinal uniformity by about 25-30%.

IV. THE DETECTOR On the basis of our investigations we chose the principal

detector design with two exit windows containing a 20 mm x 10 mm x 1000 mm polished plate placed in the rectangular groove of a 1020-mm-long, 60-mm-diameter light guide made of plastic material. It should be noted that the light guide ends are the exit windows. The side surface of the light guide was wrapped with mylar foil (Fig. 3).

The plate of CsI(T1) 1000 mm long was produced by “unbending” a ring-shaped blank cut out from a single crystal ingot -350 mm diameter at increased temperature. High uniformity of scintillation properties is obtained thanks to the

W Fig. 3. (mylar); 4-Aluminum housing.

The detector. I-Scintillator; 2-Light guide; 3-Wrapping material

fact that the ring is made of a flat disk cut out normal to the crystal growth axis.

Using light collection from two opposite detector ends and choosing high voltage for each PMT separately, we have obtained a nonuniformity of response along detector 1000 mm length of about &6%.

V. CONCLUSION

We have developed and investigated a novel type of llong X-ray detector up to 1000 mm length with two exit win- dows. The sensitive element of the detector is an elongated polished rectangular plate of inorganic scintillator, which is optically coupled with a long transparent cylindrical light guide made of nonscintillating plastic. The coefficient of light collection of the system was approximately evaluated taking into account light attenuation in the light guide and scintillator. Experimental data are in a good agreement with calculated ones. The specific feature of considered system is that the scintillation light propagates mainly through the llight guide, which is far more transparent for scintillation llight than the scintillator. Optimal conditions for light collection as well as the longitudinal uniformity were determined. The nonuniformity of response is about 333% along detector I000 mm length when the light is collected from opposite detector ends.

Detector construction is relatively simple and cheap. The detector is highly sensitive to X-rays in the energy region of 50-200 keV and can be used in systems with a fast scanning X-ray beam such as airport X-ray scanners.

ACKNOWLEDGMENT

The authors are grateful to B. G. Zaslavskij for his assistance in the technological process.

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