A Scintillation Spectrometer for Studying the Energy and Angular Distributions of X-ray Radiation Behind Various Absorbers

Yu. B. Yankelevich

(Presented by the scientific seminar of the Research Institute of Electronic Introscopy)

The radiation defectoscopy of materials and products that has been developing recently requires the study of the spectral composition of penetrating radiation behind various kinds of absorbers. The foundations of the “counting-spectrometric method in scintillation defectoscopy” [1] presuppose knowledge of the energy spectra of the radiation incident on the scintillation crystal, which makes it possible to choose correctly the optimum parameters of the defectoscopy system.

The spectral composition of X-ray radiation before and after various absorbers is conveniently studied with scintillation spectrometers.

For this purpose the author designed and built a scintillation spectrometer with a NaI(Tl) crystal. The construction of the spectrometer and the geometry are shown in Fig. 1.

Fig. 1.

The X-ray radiation was collimated by two collimators—a lead one \([60\ \mathrm{mm}]\) and a steel one \((80\ \mathrm{mm})\). The diameter of the aperture of the lead collimator was \(15\ \mathrm{mm}\), and that of the steel one \(20\ \mathrm{mm}\). The spectrometer was surrounded by lead shielding \(3\ \mathrm{cm}\) thick, while the front layer and the thickness of the collimator were each \(60\ \mathrm{mm}\). The lead collimator of the spectrometer used in the experiments had an aperture \(5\ \mathrm{mm}\) in diameter. The channel in the collimator was covered with a \(0.5\ \mathrm{mm}\) layer of sheet tin in order to reduce the probability that characteristic lead quanta would strike the scintillation crystal. The scintillation crystal NaI(Tl) was sawn out and polished from a spectrometric crystal

diameter of 70 mm. The crystal was enclosed in an aluminum casing 0.1 mm thick and optically coupled directly to the window of the photomultiplier photocathode by means of silicone oil. The FEU-24 photomultiplier was selected from the 10 available and had the best resolution at the minimum dark current. The voltage divider for feeding the FEU dynode system, located on the FEU panel itself, made it possible to set the optimum voltage on the dynodes. The spectrometer used a USS-1 preamplifier from the “Volna” installation.

The signals from the preamplifier were fed to a cathode follower assembled according to the White circuit, and then through a cable to an AI-100 hundred-channel pulse analyzer.

The energy resolution of the spectrometer was 8.5% at 660 keV and 7% at 1.25 MeV.

The influence of backscattered radiation from the inner walls of the lead shielding was estimated. The intensity of the backscattered radiation amounted to a few percent of the intensity of the radiation incident on the crystal through the collimating aperture.

Using the spectrometer described, the output spectra of the radiation from an industrial RUP-200 X-ray unit (Fig. 2) were recorded at accelerating potentials of 100, 150, and 200 keV. The distance from the source to the crystal was 3 m. By adjusting the tube current, the total counting rate was set within the range 0.5–1 kHz.

Fig. 2. Radiation spectrum of the X-ray tube at accelerating potentials of 100, 150, and 200 keV.

To record the energy spectra of X-ray radiation behind various absorbers, the specimens were arranged according to Fig. 1; the plane of the specimen on the spectrometer side remained strictly fixed, while the absorber thickness was increased from the source side. To record angular spectral distributions, the spectrometer was rotated in the horizontal plane about a vertical axis passing through point “0” by the desired angle (Fig. 1).

Using this method, more than 150 spectra were recorded behind absorbers of various thicknesses made of steel, aluminum, and plastics at various angles \(\Theta\), \(0^\circ \leq \Theta \leq 180^\circ\). Instrumental spectra behind absorbers of various thicknesses made of steel, aluminum, and plastics are shown in Fig. 3a, b, c.

Fig. 3a illustrates the “hardening” of the white spectrum of X-ray radiation as it passes through a steel absorber of varying thickness. Beginning with a thickness of 70 mm, the “accumulation” of multiply scattered photons in the 20–80 keV region becomes noticeable. Estimation

Fig. 3.

of the “accumulation” factor caused by multiple scattering is of great interest for flaw detection of materials, since the conditions of “pure geometry” are very difficult to realize. The sensitivity of a flaw detector, however, depends to a significant extent on the contribution of scattered radiation to the total spectrum of quanta incident on the crystal.

Fig. 4.

Study of the ratio of primary to scattered radiation from experimental data makes it possible to determine accumulation factors for various materials and also makes it possible to choose the optimum discrimination threshold.

Figure 4 presents instrumental spectra of X-ray radiation scattered through various angles. Analysis of these data confirms the possibility of flaw detection in materials by means of Compton and coherent radiation scattered through various angles.

At present the authors are processing the results in order to determine the composition of the radiation field behind various absorbers. The instrumental spectra make it possible to substantiate optimal

parameters of flaw-detection systems, for example, the dimensions of the scintillation crystal, the sensitivity of the photomultiplier photocathode, the discrimination threshold, and the parameters of the electronic circuit.

REFERENCES

1. V. I. Gorbunov, V. B. Kuznetsov, V. K. Kuleshov, Yu. B. Yankelevich. Counting-spectrometric method in scintillation flaw detection (present collection).