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On the backscattering of β particles
hr. 3. .ippl. Radiar. hr. Vol. 33. No. 6. p. 563, 1984 E Pergamon Press Ltd 1984. Printed in Grear Britain 0020-708X, 84 S3.00 - 0.00
On the Backscattering /I Particles
of
S. R. THONTADARYA Department of Physics, Kamatak University. Dhatwad-580 003, India (Rereiced 1 September
1983)
As an addenda to the technical note published by Mohammadi!” the saturation thickness (S) in aluminium for /3 particles from five different /I emitters are measured in a 2a geometry, suitable to place both detector and source on the same side of the sample. Results agree. in general, both with Mohammadi‘s and the so called rule of thumb”’ values. The advantages of the method are listed. In his technical note Mohammadi”’ described the utility of the backscattering method in thickness gaging. In his method the incident radiation is collimated and the radiation backscattered at an angle of 56’ to the incident beam direction, is counted. We report in this letter. a much simpler experimental arrangement which detects all the particles backscattered by a material whose thickness is to be gaged. Our experimental arrangement yields almost the same relation between the saturation thickness and the range for /I particles of end-point energy in the range of 0.43-2.28 ,MeV. In our method an end window G-M counter with a window of thickness 2.2 mg.cm-’ and diameter 2.5 cm. is used as a detector. The source, prepared between two thin mylar foils of thickness Zmg.cm-?. is placed next to the window of the detector. The sample foil. that is the backscatterer whose thickness is to be measured, is held just in front of the source. In this arrangement, the particles from the source emitted in the direction of the detector, as well as the particles backscattered in all directions from the sample are counted; the solid angle subtended by the active area of the detector at the source and backscatterer is almost 2~. Each source is prepared by evaporating the liquid isotope onto a thin mylar film of thickness 2 mg’cn-r and diameter 3 cm and sealing it with a similar mylar film. Each source has an active area of less than 3 mm in diameter and an activity of approx. 0.01 FCi. In Table I, columns I, 2 and 3 we give a list of pure fl emitters used with their respective end-point energy and range. The source is kept on the window of a Geiger counter and on the source an alumimum foil of desired thickness is kept. The number of /I particles “backscattered in all directions” from the aluminum foils is determined by subtracting the incident intensity after correcting the count rates for background and deadtime of the counter. The measurement is repeated for different foil thickness for each /I source. Table I. Comparison
Beta emitret @SW ‘?I p’Y xp WY
Fig. 1. Saturation thickness as a function of the range of /3 particles. The saturation thickness S is determined in the usual manner by plotting the backscattered intensity as a function of thickness r of the foils. In Fig. 1 we plot these saturation thicknesses as a function of the range R of fi particles of end-point energy E,. We see in Fig. I that experimentally determined pomts lie close to the straight line passing through the origin yielding a relation as S = d R. connecting the saturation thickness S with the range R, over the range values of O-1200 mg.cm-:. In Table I, columns 4, 5 and 6 we give the experimentally determined values of saturation thickness along with the estimated values according to the rule of thumb”’ S = 0.2R and Mohammadi’s value S, (for WY fl particles). We see that our values are in general agreement both with the estimated values and with Mohammadi’s value. Hence we conclude that by adopting 277geometry, the whole process of measuring the saturation thickness will become simpler and have the following advantages over that of Mohammadi. (I) Source and detector can be placed together so that they can be used as a sort of probe to measure the thickness of the sample. (2) There is no need to collimate the incident and scattered beam. (3) Low activity sources can be used. (4) The method can also be extended to utilize low energy T-rays and x-rays.
References 1. Mohammadi
H. Int. J. Appl. Radiat.
2. Bleuler E. and Goldsmith G. J. Experimenrol p. 83 (Reinhart. New York, 1952).
of saturation thickness values obtained in 2n geometry. and in Mohammadi’s experimental arrangement End-point energy (MeV) 0.43 0.76 I.54 1.7 228
Range (mg,cm-‘)
Saturation S,,
127 279 704 787 I IO5
23 50 II5 I26 184
563
Isot.
32, 524
(1981).
the rule of thumb.
thickness (mg.cm-‘) S = O.ZR & 15 56 141 157 121
I Z-200
Nucleonics
On the Backscattering /I Particles
of
S. R. THONTADARYA Department of Physics, Kamatak University. Dhatwad-580 003, India (Rereiced 1 September
1983)
As an addenda to the technical note published by Mohammadi!” the saturation thickness (S) in aluminium for /3 particles from five different /I emitters are measured in a 2a geometry, suitable to place both detector and source on the same side of the sample. Results agree. in general, both with Mohammadi‘s and the so called rule of thumb”’ values. The advantages of the method are listed. In his technical note Mohammadi”’ described the utility of the backscattering method in thickness gaging. In his method the incident radiation is collimated and the radiation backscattered at an angle of 56’ to the incident beam direction, is counted. We report in this letter. a much simpler experimental arrangement which detects all the particles backscattered by a material whose thickness is to be gaged. Our experimental arrangement yields almost the same relation between the saturation thickness and the range for /I particles of end-point energy in the range of 0.43-2.28 ,MeV. In our method an end window G-M counter with a window of thickness 2.2 mg.cm-’ and diameter 2.5 cm. is used as a detector. The source, prepared between two thin mylar foils of thickness Zmg.cm-?. is placed next to the window of the detector. The sample foil. that is the backscatterer whose thickness is to be measured, is held just in front of the source. In this arrangement, the particles from the source emitted in the direction of the detector, as well as the particles backscattered in all directions from the sample are counted; the solid angle subtended by the active area of the detector at the source and backscatterer is almost 2~. Each source is prepared by evaporating the liquid isotope onto a thin mylar film of thickness 2 mg’cn-r and diameter 3 cm and sealing it with a similar mylar film. Each source has an active area of less than 3 mm in diameter and an activity of approx. 0.01 FCi. In Table I, columns I, 2 and 3 we give a list of pure fl emitters used with their respective end-point energy and range. The source is kept on the window of a Geiger counter and on the source an alumimum foil of desired thickness is kept. The number of /I particles “backscattered in all directions” from the aluminum foils is determined by subtracting the incident intensity after correcting the count rates for background and deadtime of the counter. The measurement is repeated for different foil thickness for each /I source. Table I. Comparison
Beta emitret @SW ‘?I p’Y xp WY
Fig. 1. Saturation thickness as a function of the range of /3 particles. The saturation thickness S is determined in the usual manner by plotting the backscattered intensity as a function of thickness r of the foils. In Fig. 1 we plot these saturation thicknesses as a function of the range R of fi particles of end-point energy E,. We see in Fig. I that experimentally determined pomts lie close to the straight line passing through the origin yielding a relation as S = d R. connecting the saturation thickness S with the range R, over the range values of O-1200 mg.cm-:. In Table I, columns 4, 5 and 6 we give the experimentally determined values of saturation thickness along with the estimated values according to the rule of thumb”’ S = 0.2R and Mohammadi’s value S, (for WY fl particles). We see that our values are in general agreement both with the estimated values and with Mohammadi’s value. Hence we conclude that by adopting 277geometry, the whole process of measuring the saturation thickness will become simpler and have the following advantages over that of Mohammadi. (I) Source and detector can be placed together so that they can be used as a sort of probe to measure the thickness of the sample. (2) There is no need to collimate the incident and scattered beam. (3) Low activity sources can be used. (4) The method can also be extended to utilize low energy T-rays and x-rays.
References 1. Mohammadi
H. Int. J. Appl. Radiat.
2. Bleuler E. and Goldsmith G. J. Experimenrol p. 83 (Reinhart. New York, 1952).
of saturation thickness values obtained in 2n geometry. and in Mohammadi’s experimental arrangement End-point energy (MeV) 0.43 0.76 I.54 1.7 228
Range (mg,cm-‘)
Saturation S,,
127 279 704 787 I IO5
23 50 II5 I26 184
563
Isot.
32, 524
(1981).
the rule of thumb.
thickness (mg.cm-‘) S = O.ZR & 15 56 141 157 121
I Z-200
Nucleonics