IONIZATION AND DETECTION 107 



as X rays. The nuclei set in motion by bombardment by fast neutrons (0.1 

 to 15 Mev) have a high LET and leave a wake of intense ionization. Slow 

 (thermal) neutrons are ultimately captured by nuclei; the product is nor- 

 mally unstable, and, for light atoms, usually emits a gamma ray. A good 

 billiard player will attest that maximum energy transfer can take place be- 

 tween two neutral "particles" if they have the same weight. Therefore, 

 neutrons are slowed down, or "moderated" best by materials containing 

 much hydrogen — water, paraffin, etc. Thus, penetration into these ma- 

 terials is slight, or in other words, the absorption coefficient is high. 



Neutrons are by-products of nuclear fission, or of proton- or deuteron- 

 bombardment of light nuclei; they have a half-life of the order of 20 min, can 

 be quite destructive of living tissue, and are difficult to detect. The damage 

 is caused by charged nuclei set in motion by the impact of the neutron, or 

 from artificial radioactivity induced by capture of the neutron by the nucleus 

 (Figure 5-1). 



Defection 



Ionizing radiation is detected by any one of four basic methods: 



(1) Exposure of a Photographic Plate: i.e., reduction of silver halides to silver 

 along the path of the photon or particle. If the plate is placed in contact with 

 a section of tissue containing a radioactive tracer, the plate will be exposed 

 where the activity is. This method of mapping is now known as "autoradi- 

 ography." 



Microradiography is another interesting technique in which a large 

 shadow of a small object is allowed to fall on a photographic plate. This 

 technique has been used for years with X rays as the source, and recently it 

 has been demonstrated to be feasible and useful using alpha rays as the 

 source. Figure 5-2 shows a micro X radiograph of a section of bone — the 

 mineral content is clearly visible — and an alpha radiograph taken of the 

 organic part after the mineral had been removed. 



(2) Ionization of a Gas Contained Between Two Electrodes: As the photon or 

 particle passes through the gas it leaves a wake of ion-pairs. If there is no 

 potential difference between the electrodes, the ions will recombine. If a 

 potential difference is applied (Figure 5-3 (a)), each ion will migrate toward 

 an oppositely charged plate. Those which reach the plate before recombin- 

 ing will be discharged and produce pulses of current in the external circuit. 

 The higher the potential difference, the less is the recombination. Thus at 

 an electric field strength of about 10^/cm almost all the ions produced are 

 "collected" at the electrodes. This is called the "saturation" condition, and 

 most ionization chamber systems operate in this region. 



If the electric field strength is increased still further, the primary ions are 

 given sufficient energy to produce secondary ionization of the s*as molecules, 

 resulting in a multiplication of the original ionization. This is known as an 



