Sec. 13.12] STANDARDIZATION OF RADIOACTIVE SAMPLES 379 



measurements, it is necessary to achieve the condition in which the ionization 

 in the gas of the chamber is in equilibrium with the ionization within the 

 chamber walls [21,32]. The neutrons exert their biological effects by the 

 ionization produced by the secondary recoil nuclei; hence, it is important 

 that equilibrium is also reached between the primary neutrons and the 

 secondary protons in the material studied. Using 10 mev neutrons, equilib- 

 rium may be reached in'about 3^8 m - °f tissue, whereas with 100-mev neutrons 

 the equilibrium is established only after the neutron beam has crossed some 

 3 in. of tissue. 



The measurement of thermal neutrons may be done in several different 

 ways. Among these is the activation of certain elements, e.g., indium, gold, 

 or manganese, which have large thermal-capture cross sections. The 

 neutron flux may be calculated from the observed disintegration rates of the 

 isotopes produced and from the cross sections for the processes. An inde- 

 pendent method utilizes neutron-induced fission of uranium measured with 

 an ionization chamber. Each time the fission occurs a large burst of ioniza- 

 tion appears in the chamber due to fission recoils, and the number of pulses, 

 together with the cross section for fission, yields the absolute number of 

 primary neutrons. If a neutron beam has thermal as well as fast components, 

 separation of the components is possible to some extent by measuring the 

 difference in radioactivity induced in detecting foils with and without cad- 

 mium shielding. The radioactivity induced in the foil shielded with a strong 

 thermal neutron absorber is due only to faster neutrons. 



Recently protons, deuterons, and alpha particles accelerated in cyclotrons 

 have also been used in biological research. The measurement of a number 

 of particles in such beams may be accomplished by two methods: (1) measure- 

 ment of a total charge carried by the beam by means of a Faraday cage, and 

 (2) by measurement of the ionization produced in air by a fraction of a 

 monoenergetic beam at a given energy. From the known charge carried by 

 each particle and the measured ionization, the number of particles in the 

 beam may be calculated [35,37]. 



REFERENCES FOR CHAP. 13 



1. Curie, I., A. Debierne, A. S. Eve, H. Geiger, O. Hahn, S. C. Lind, St. Meyer, 

 E. Rutherford, and E. Schweidler: Rev. Mod. Phys., 3, 427 (1931). 



2. Kohman, T. P., D. P. Ames, and Sedlet: Atomic Energy Commission Report MDDC- 

 852, Mar. 25, 1947. 



3. Condon, E. U., and L. F. Curtiss: Phys. Rev., 69, 672 (1946). 



4. Kovarik, A. F., and T. Adams: Phys. Rev., 40, 718 (1932); 64, 413 (1938); /. Applied 

 Phys., 12, 296 (1941). 



5. Nier, A. O.-.Phys. Rev., 55, (1939). 



Values recalculated according to Ref. 4. See also D. Williams, and P. Yuster, Atomic 

 Energy Commission Report. LA-203, January, 1945. 



