The present invention relates to method and apparatus for measuring a density of a sample by using a .gamma.-ray and, more particularly, to method and apparatus for measuring a density of a sample by counting the number of .gamma.-rays which were scattered by the sample or the number of .gamma.-ray pulses which are transmitted through the sample per unit time.
Hitherto, for instance, a surface scattering type densitometer shown in FIG. 1 or a transmitting type densitometer shown in FIG. 2 has been known as a densitometer using a .gamma.-ray.
In the surface scattering type densitometer of FIG. 1, a radiation source 10 and a detector 12 are arranged on the surface of a sample 14 so as to be away from each other at a predetermined distance. The .gamma.-ray pulses which are scattered in the sample and are returned to the sample surface in the .gamma.-ray which was irradiated from the radiation source 10 are detected by the detector 12. A detection signal of the detector 12 is output to a measuring apparatus 16. The number N of detection pulses per unit time is counted by the measuring apparatus 16. A correlation curve to obtain a density .rho. from the number N of detection pulses of the .gamma.-ray shown in FIG. 3 has been preset in the measuring apparatus 16. The value of the density .rho. corresponding to the pulse number N which was counted in accordance with the correlation curve is obtained and output and displayed.
In the transmitting type densitometer of FIG. 2, the radiation source 10 is arranged in the sample 14, the .gamma.-ray pulses which are transmitted in the sample from the radiation source 10 are detected by the detector 12 attached onto the surface of the sample, and the density .rho. can be obtained from the number N of detection pulses per unit time by the measuring apparatus 16 in a manner similar to the surface scattering type densitometer.
In the conventional densitometer using the .gamma.-ray, the correlation curve shown in FIG. 3 which is used to obtain the density .rho. from the count number N of the .gamma.-ray pulses is formed from the experimental data obtained under the ideal measuring conditions. However, in the actual measurement, the correlation between the count value N and the density .rho. according to the correlation curve is not always obtained due to a change in characteristics of the measuring apparatus depending on a temperature, an influence by a half value period of the radiation source, and the like. Further, there is a problem such that the stability and reliability of the density measurement are insufficient.