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CCD-introscope with luminescent storage screens for digital radiographyMoskalyov Ju. A., Dmitrieva A. V., Grigoryev S. V.
The main components of the introscope are:
The therrmoluminiscent screen is a metal plate, coated with a luminescent material (thermoluminophor). The thermoluminophor has the property to absorb and accumulate the energy during its exposure to X-rays. After the exposure this energy may be liberated out of the thermoluminophor bulk by heating it to 100¸ 3000C. If a TLS, like an X-ray film, was exposed in conjunction with a test specimen, then a luminescent X-ray pattern of a test specimen appears on the thermoluminophor surface as a TLS undergoes heating. This image from the TLS is registered by a CCD-camera with a cooled target and stored in the computer memory.
The Table 1 gives the characteristics of thermoluminophors that can be used for TLS fabrication. From this table one can see that the CaSO4× Mn,Sm and ZnS× Cu,Co thermoluminophors are the materials of choice for NDT of low density and low thickness specimens because the peak of their spectral sensitivity lies in the range of 20 to 50 keV.
The CdSO4× Mn,Sm thermoluminophors and those with higher density can be used to fabricate a TLS for testing of steel specimens with various thickness because the region of their peak spectral sensitivity is in the range of 80 to 200 keV. The optimum thickness of a TLS phosphor layer is about 1 mm. In contrast to luminescent phosphor screens, operating on the principle of 'translucence' in digital radiography, a thermophosphor layer in a TLS operates on the principle of 'reflection', and the effect of a layer thickness on resolution is negligible. The resolution of a TLS based on CdSO4× Mn,Sm and developed for industrial radiography is 5 line pairs/mm. These TLS were fabricated by the method of casting of suspension of a thermoluminophor powder with a siliconorganic binding. TLS dimensions are 150´ 200 mm. As a substrate a plate of stainless steel was used that performed functions of a both heating element and a back intensifying screen.
A thermophosphor layer was heated by passing of current through a metal substrate. In this case the whole cycle of image luminescence required only 10¸ 15 s during which the temperature of a phosphor layer increased up to 2000C. An image in the form of a luminescent X-ray pattern is visible for 3¸ 8 s, after which the TLS goes out and after cooling down to room temperature it can be used for subsequent exposures.
The dynamic range of a TLS is about 105¸ 106 and is presented by the ratio of a minimum detected radiation dose (~ 10 mRad) to a maximum dose of a linear portion of a characteristic curve (~104 Rad).
|Thermoluminophor||Peak spectral sensitivity,arbitrary units||Region of peak spectral sensitivity, keV||Density g/cm3||Conversion efficiency|
Accumulating properties of thermoluminophors are characterized by a value of conversion efficiency (columns 5 and 6 of table). As it can be seen from presented data this value is of the same order as the one for efficient X-ray phosphors suck as CsI× Tl, Gd2O2S× Tb and CaWO4. So the TLS with these thermoluminophors can be efficiently used in radiography. The capability to conserve stored energy and information after exposure is determined by the technology of thermoluminophor fabrication and activators, used for it.
On the average one may consider that during first 48 hours after exposure termination about 20% of accumulated energy is lost. The diagram of the CCD-in-troscope, based on TLS is given in Fig. 1 (a,b).
|Fig 1a: Exposure of TIP||Fig 1b: Image reading|
According to experimental results the contrast sensitivity of this digital radiography system, based on the TLS, is 1.5¸ 2 times higher, compared with similar systems with the X-ray screen, used as converters. This is conditioned, first of all, by the fact that the TLS is an element for accumulation of X-ray patterns, causing a decrease of a radiation statistical noise. Besides, the TLS allows one to eliminate a radiation noise, resulting from the direct affect of radiation upon the matrix of a CCD-camera, especially in the range of 1¸ 20 MeV. The additional advantage of TLS employment is enhancement of images contrast, especially preferential for testing of products made of light materials e.g. Mg-Al alloys and heat-shielding plates, made of foamed SiO2. Fig.2 represents the process of image contrast enhancement with a radiation spectrum, passing through a test specimen (curve 1) and a radiation spectrum, forming an image in the TLS.
|Fig 2: Radiation spectra on formation of radiation (1) and optical (2) images|
Due to the shift of the spectrum to the region of softer radiation the image contrast on TLS is 2¸ 4 times higher than a radiation contrast. This, in its turn, provides higher contrast sensitivity in testing of objects.
Both experiments and practical application have shown that employment of the TLS in combination with an X-ray unit with the energy up to 300 keV allows one to test steel specimens with thickness up to 40 mm and Al specimens with thickness up to 120 mm with the contrast sensitivity of 0.4¸ 1.0 %.
With a betatron or a linear accelerator as a radiation source the CCD-introscope with TLS provides testing of different specimens with thickness equivalent to 150 mm steel and contrast sensitivity of 0.5¸ 1.0 %. It was established that one unit for image reading and one complete set of 10¸ 20 TLS will allow one to solve the problems of industrial radiography for different enterprises without employment of X-ray films.
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