Fast Electron Sources for Electron Defectoscopy
Abstract Generated abstract
This paper formulates requirements for fast electron sources used in electron defectoscopy and introscopy, including adjustable and stable energy, narrow energy spread, sufficient beam intensity, low divergence, and uniform density. It relates the required electron energy to inspected material thickness and effective atomic number, discusses how energy and intensity fluctuations limit defect detectability, and estimates practical stability and flux requirements for high sensitivity. The authors argue that betatrons with extracted beams best meet these conditions and describe 6 and 10 MeV units developed for radiographic inspection, reporting electrostatic extraction, beam focusing capabilities, energy stability better than 0.1 percent, intensity stability of about 1 percent, and continuous operation over a working shift.
Full Text
SOURCES OF FAST ELECTRONS FOR ELECTRON DEFECTOSCOPY AND INTROSCOPY
B. A. Kononov, V. V. Evstigneev, B. F. Savel'ev
(Presented by the scientific seminar of the Research Institute of Electron Introscopy)
The requirements for an ideal source of fast electrons used in defectoscopy and introscopy are: smooth adjustment of the electron energy over a wide range of inspected products and materials, monoenergetic electrons, relatively high beam intensity, low background, high stability of the energy and radiation intensity, small angular divergence of the beam, and uniformity of the electron density in the beam.
The required maximum energy of the probing electrons is determined by the mass thickness of the inspected products from the expression
\[ E=\frac{10d+0.9}{\ln \frac{2550}{z+8.5}}, \tag{1} \]
in the range \(3 < E < 10\),
where
\[ \begin{aligned} E &\text{ is the electron energy in MeV;}\\ Z &\text{ is the effective atomic number of the substance;}\\ d &\text{ is the total mass thickness of the inspected substance in } \mathrm{g/cm^2}. \end{aligned} \]
Expression (1) was obtained under the condition that \(d = 0.9 R_{\mathrm{e}}\), since in this case the maximum attainable sensitivity of the method of defectoscopy with fast electrons is realized [1].
To inspect a wide range of materials and products and to obtain a specified sensitivity, it is necessary that the fast-electron source have smooth energy adjustment with a setting accuracy of \(\pm 0.1 R_{\mathrm{e}}\).
In practice, achieving the maximum sensitivity in the inspection of materials and products is hindered by instability of the output parameters of the fast-electron source. One such cause is fluctuations in the energy of the accelerated electrons, which significantly impair the sensitivity of the method, as is evident from the relation valid for the scintillation method near the extrapolated electron range [2]
\[ \left|\frac{\delta I_E}{\delta E}\right| = \left|\frac{\delta I_x}{\delta x}\right|, \tag{2} \]
where
\[ \delta I_E \]
is the relative change in intensity caused by the relative change in energy \(\delta E\), and
\(\delta I_x\) is the relative change in intensity due to the defectiveness of the material or product. For energies greater than the energy obtained from formula (1), the relation
\[ \frac{\delta I_x}{\delta x} > \frac{\delta I_E}{\delta E}. \]
is satisfied.
For the same reason, in defectoscopy the electron energy required for radiography, determined by formula (1), should be increased by 10–15% in order to reduce the dependence of the output signal on energy fluctuations.
An estimate of the required stability of the energy of fast electrons for obtaining a specified sensitivity can be made from expression (2), with the condition that the change in the energy flux beyond the product due to its defectiveness must exceed, by at least a factor of 3, the change in the energy flux due to energy fluctuations. The maximum attainable sensitivity of the inspection method is realized when the energy stability of the electron radiation is 0.1%. The absorption of an electron beam in a substance with different spectral composition is characterized by the difference in the slopes of the curves of the distribution of absorbed electron energy over the depth of the substance. Therefore, the half-width of the energy distribution of the primary electrons must not exceed approximately one percent, in order to increase the slope of the curves of the absorbed-energy distribution and, consequently, to increase the maximum sensitivity of inspection.
The selection and calculation of the necessary electron density in the beam for defectoscopy and introscopy of materials and products is carried out on the basis of the required productivity (inspection rate), according to the magnitude of the statistical error. Thus, for example, to determine defects of less than 2 mm at an inspection rate of about 100 cm/min, the electron flux density must be \(10^9\) electrons/cm\(^2\)·s.
Fluctuations in the intensity of the electron radiation directly affect the detectability of defects. The requirements for obtaining the maximum sensitivity of the method of defectoscopy with fast electrons when differential measurements are used determine the need for stability of the radiation intensity with an accuracy of \(\pm 1\%\).
Of the conditions considered above, the betatron with beam extraction most fully satisfies the required parameters, making it possible to achieve energies of one or several tens of MeV.
To obtain an electron-radiation dose rate of up to 10,000 rad/min, it is most advisable to use fast electrons from a betatron, which is compact, simple to manufacture and service, and reliable in operation.
We have developed special types of betatrons of 6 and 10 MeV with extracted electron beams for electron defectoscopy and introscopy. The electron energy is sufficient for radiography of products and materials with total mass thicknesses of 3 g/cm\(^2\) and 5 g/cm\(^2\), respectively. The developed betatrons have increased access to the accelerating chamber in order to use the maximum intensity of the electron radiation.
Extraction of the beams from the accelerating chamber of the betatron is carried out by an electrostatic method with an efficiency of 60%, with an electron flux density at the output of no less than \(5 \cdot 10^{10}\) electrons per second. The angular divergence of the beam in air is \(1^\circ\) in the vertical plane and \(-6^\circ\) in the horizontal plane. The focusing devices ensure
various geometric dimensions of the electron beam at distances of up to two meters from the betatron. The minimum obtainable beam size is \(5 \times 5\) mm.
The parameter-stabilization systems ensure energy stability with an accuracy better than 0.1%, and smooth regulation of it in the range from 0.5 to the maximum. The stability of the electron-radiation intensity is \(\pm 1\%\). The units provide continuous operation over the course of a working shift.
REFERENCES
- B. A. Kononov, K. A. Dergobuzov, Yu. M. Stepanov. “Defektoskopiya,” No. 4, 77, 1968.
- B. A. Kononov, V. V. Evstigneev, V. M. Zykov. Proceedings of the Second Interuniversity Conference on Nondestructive Testing Methods. Tomsk, 1968 (in press).