K.M. PODURETS, VA. SOMENKOV and S.Sh. SHILSTEIN. I.V. Kurchatov Institute of Atomic Energy, Moscow 123182, USSR. A new method of obtaining images ...
Physica B 156 & 157 (1989) 691-693 North-Holland, Amsterdam
NEUTRON RADIOGRAPHY WITH REFRACTION CONTRAST K.M. PODURETS,
VA. SOMENKOV
I.V. Kurchatov Institute of Atomic Energy,
and S.Sh. SHILSTEIN
Moscow 123182, USSR
A new method of obtaining images in neutron radiography is proposed. It is based on separation of refracted and unrefracted beams. The required angular resolution is ensured by consecutive reflection of neutrons from two perfect crystals with the object placed between them. The experiments performed have shown that a high contrast is available even in images of weakly absorbing objects. Neutron images of an inhomogeneous magnetic field are obtained.
In radiography, information on the internal structure of an object is obtained from the radiation (X-, y-rays, neutrons) transmission pattern. The contrast of this pattern is determined by radiation attenuation in different parts of the object. Along with the attenuation, radiation refraction takes place in interfacial areas. The characteristic angular deflections are about seconds of arc for thermal neutrons and X-rays. The aim of the present work is to demonstrate an increase of the contrast due to the refraction process in the case of neutron radiography. The required angular resolution is ensured by a double-crystal spectrometer with perfect crystals. Experimental set-up, shown in figs. la, b, allows direct comparison of transmission patterns with
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Fig. 1. Scheme of experiment under “attenuation” (a) and “attenuation + refraction” (b) conditions: 1 - collimator; 2 and 5- Ge crystals (monochromator and analyzer); 3 object; 4-sample table; 6-detector; 7 -system of data collection and processing; 8 - slit; 0 is Bragg angle.
and without refraction contrast under the condition of the same geometrical resolution. Experiments’with narrow-beam scanning and with immediate visualization of the transmission pattern on the film with a Gd converter were carried out. An aluminium tube with two copper bars inside it (fig. 2a) was chosen as a model object. The maximum contrast is observed in the scanning patterns on the cylinder sides where the glancing angles decrease, and the total reflection must take place in the limit of small angles. For a finite geometrical resolution the contrast remains less than 100% even in detecting the refraction; however, it reaches 80-90%. Comparison of the model object images obtained under conditions of “attenuation” and “attenuation + refraction” (fig. 3a and b) also shows a considerable increase of the contrast in the second case. The thin bar and aluminium tube, hardly discernible in the first pattern, are clearly seen in the patterns with refraction detection. Transmission pattern of a ball point pen may be an example of increasing the contrast of complicated object images. While under conditions of “attenuation” one can see ink inside a refill and a thick metallic case (fig. 4a), in the case of “attenuation + refraction” all the details of a writing assembly, a spring and a thin-walled refill tube with ink are clearly discernible (fig. 4b).
0921-4526/89/$03.50 @ Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
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K.M.
Podurets et al. I Neutron radiography
with refraction contrast
b
a
Fig. 3. Model object radiogram under “attenuation” (a) and “attenuation + refraction” (b) conditions; exposure is 5 hours; A = 1.5 A (tube size is diminished compared with fig. 2). Photograph magnification 2.4~.
Fig. 2. Results of model object, (a) scanning by a slit with 0.2 mm width, (b, c) at A = 2.26 A under “attenuation” (b) and “attenuation + refraction” (c) conditions; contrast is K(x) = (I,, - Z(x))l(Z, - Z,).
In neutron radiography there exists a specific possibility to obtain a purely refraction contrast completely without the absorption contrast, due to neutron magnetic refraction on magnetic inhomogeneities. Fig. 5 presents the results of an experiment in which a neutron beam passes through a gap between two permanent magnets. The field near the magnet edges increases from 0 to 6 kOe in a region of 2 mm width. In this
region the intensity of transmission without refraction is decreased (the deflection for the path length of 4cm at a gradient of lo4 Oe/cm for A = 2.26 8, amounts to 1.5”, which yields the decrease of the crystal-analyzer reflection intensity by 20-30% at a reflection curve width of 4.5”). At the same time, the contrast is, naturally, absent under “attenuation” conditions.
K.M.
Podurets et al. I Neutron radiography
with refraction contrast
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Fig. 5. Radiogram of permanent magnet gap under “attenuation” (a) and “attenuation + refraction” (b) conditions. Photograph magnification 2.4X.
Fig. 4. Radiogram of ball point pen under “attenuation” (a) and “attenuation + refraction” (b) conditions. Photograph magnification 2.4 X
The proposed principle for increasing the contrast in neutron radiograms is useful for the solution of a broad scope of problems: observation of the internal structure of opaque materials, products and instruments, diagnostics of magnetic systems, study of the internal domain structure of ferromagnets, etc. The principle of refraction contrast can be used in all fields of introscopy with neutron and electromagnetic radiation: in radiography, topography, tomography.
