RADIO-ACTIVITY 



646O 



RADIO-ACTIVITY 



RADIO-ACTIVITY AND THE ATOM 



J. M NUM. ill. Assist. Director, Physics Laboratory. Manchester Univ. 



For related information see Atom ; Crookes ; Curie ; Matter ; 



Radium ; Uranium ; X-Rays 



The term radio-active is usually 

 understood to apply to those sub- 

 stances such as uranium, radium, 

 thorium, actinium, and their com- 

 pounds, which have the special 

 property of spontaneously emitting 

 radiations possessing the following 

 characteristics : 



1. They will penetrate sub- 

 stances opaque to ordinary light, 

 e.g. they penetrate sheets of metal. 



2. They will affect a photo- 

 graphic plate in a dark room. 



3. They will produce lumin- 

 escence and phosphorescence in 

 certain substances placed near 

 them, notably in barium platino- 

 cyanide, and in the minerals zinc- 

 blende and willemite. 



4. They have the power of 

 " ionising " any gas through which 

 they pass, that is, render it into a 

 conductor of electricity, and hence 

 these radiations will discharge 

 electrified bodies. 



X-rays and Radio-activity 

 It is important to note that these 

 same properties are also exhibited 

 by other radiations, e.g. by Rontgen 

 or X-rays, but the essential differ- 

 ence between X-rays and radio- 

 active radiations is that the former 

 have to be excited by some ex- 

 ternal agency, whereas the latter 

 are spontaneously emitted from 

 radio-active substances, and are 

 entirely independent of any ex- 

 ternal exciting cause. In other 

 words, they are due to the breaking- 

 up or disintegration of the actual 

 atoms of the radio-element. We 

 may regard the latter as being in an 

 unstable state, and continuously 

 disintegrating, giving rise to new 

 atoms which differ entirely in 

 chemical properties from the 

 parent element. It is these contin- 

 uous explosions of the radio-active 

 atoms that give rise to the char- 

 acteristic radiations. This in- 

 stability is only found in the heavi- 

 est known elements the radio- 

 elements are all heavier than lead. 

 Historically the impetus for the 

 discovery of radio-activity was 

 provided by the discovery of X-rays 

 in 1895, by Rontgen. In the follow- 

 ing year Becquerel examined a 

 number of phosphorescent and 

 fluorescent substances, to see if 

 they emitted photographically 

 active rays, and found that salts of 

 uranium were effective, independ- 

 ent of previous exposure to light 

 Later, other substances were found 

 to give off rays similar to those 

 from uranium. 



Special methods of measure- 

 ments were devised for studying 



the radio-activity of these elements, 

 the most useful being based on the 

 ionising property of the rays. The 

 " ionisation current," through a 

 layer of gas exposed to the rays, 

 can be measured by the rate at 

 which it discharges a gold-leaf 

 electroscope, and this gives a com- 

 parative measure of the activity 

 of the substance used. 



The next advance made was the 

 discovery of the nature and classi- 

 fication of the types of rays given 

 off by radio-active elements. Ex- 

 perimentally it is observed that 

 their penetrating powers are widely 

 different; some, for instance, are 

 stopped by a sheet of paper, 

 others will pass through a few 

 inches of lead. Secondly, it is found 

 that some of the rays are bent by a 

 magnetic or an electric field, prov- 

 ing them to consist of minute 

 electrically charged particles mov- 

 ing at high 

 speed; the 

 direction of 

 the deviation 

 gives the na- 

 t u r e of the 

 charge carried 

 by the rays, 

 that is, decides 

 Radio-activity. Dia- whether they 

 gram illustrating are positively 

 effect of magnetic or negatively 

 field in separating electrified, 

 radiations. See text , . i , , ' 



amount of the deviation gives 

 an estimate of the speed and 

 mass of the particles. Using these 

 methods of measurement, Ruther- 

 ford showed conclusively that the 

 radiations from radio-active sub- 

 s tances were of three distinct types, 

 which he called the alpha, beta, 

 and gamma rays (a, ft, 7). 

 Nature of X-rays 



The a rays are positively charged 

 particles, each of mass four times 

 that of the hydrogen atom. They 

 are emitted at high speeds, about 

 the order of 10,000 miles a second, 

 but are very easily stopped, e.g. a 

 sheet of ordinary writing paper 

 will stop the swiftest known a 

 rays. They travel up to about 3 

 inches in air at ordinary pressures, 

 but they ionise the air very in- 

 tensely indeed in their short range. 

 They will also produce very strong 

 photographic and phosphorescent 

 effects. Finally they have definitely 

 been proved to be electrically 

 charged atoms of helium. 



The ft rays are also corpuscular 

 in nature, and consist of very high 

 speed, negatively-charged particles 

 of small mass (about two thousand 



times less than mass of the hydro- 

 gen atom). That is to say, ft rays 

 are electrons moving with high 

 velocity. Their range is from 10 to 

 20 times greater than that of the 

 a rays, and they can pass quite 

 easily through thin sheets of alu- 

 minium. The fastest known ft rays 

 are emitted with velocity approach- 

 ing that of light The ionising 

 power of the ft rays, however, is 

 only small about 100 times less 

 than that of the a rays. 



The mass of the ft rays being so 

 exceedingly small, although their 

 average velocity is great, their 

 momentum is small, and hence 

 they are easily deflected or bent by 

 magnetic or electric fields, the 

 direction of the deflection being 

 opposite to that of the a rays. In 

 character the ft rays are "identical 

 with the " cathode rays " produced 

 when an electric discharge passes 

 through a rarefied gas. 



Electro-magnetic 7 rays 



The 7 rays are not corpuscular 

 in nature, but consist of electro- 

 magnetic impulses of very short 

 wave-lengths. Hence in character 

 they are analogous to X-rays. The 

 7 rays, bearing no electric charges 

 of any kind, are therefore not de- 

 flected in magnetic or electric 

 fields, and are far more penetrating 

 than the a or ft rays they will 

 pass through several inches of 

 aluminium. The ionising power of 

 these rays is still smaller than the 

 ft rays, and about 1,000 times less 

 than that of the a rays, and their 

 photographic and phosphorescent 

 actions are correspondingly small. 



The effect of a magnetic field in 

 separating out the radiations is 

 shown in the figure. A beam of the 

 rays emitted by a small amount of 

 radium at R is subjected to a 

 magnetic field at right angles to 

 the plane of the paper, and di- 

 rected downwards. The ft rays are 

 bent to the right, showing them to 

 be negatively charged particles, 

 and the amount of the deflection is 

 large, proving them to be of small 

 mass. The a rays are bent to the 

 left, and are therefore positively 

 charged particles, whilst the extent 

 of the deflection is very small 

 (compared with the ft rays), show- 

 ing them to be of relatively large 

 mass. In the diagram, the bending 

 of the a rays is greatly exaggerated. 

 Lastly, the 7 rays go straight on 

 without any deflection, proving 

 them to be electrically uncharged. 



In order to account for the phe- 

 nomena of radio-activity, Ruther- 

 ford and Soddy put forward the 

 transformation theory, according 

 to which the atoms of a radio- 

 element are undergoing a process 

 of spontaneous disintegration, that 

 is continually breaking up, with 



