introduction-weak-interaction-volume-one

CHAPTER THREE : vCLEAR BETA DECAY .
3 .1 Phenoscnolcg of Beta Bees .
In 1896 Becquerel (1) found an ionizing radiation issuing from uraniu m
salts . In 1899 Giesel (2) demonstrated that it was deflected by a magneti c
field, and it 1900 the Curies (3) showed that it consisted of a stream of
negatively-charged particles . In the same year Becquerel (4) obtained a value
for the charge-to-mass ratio, e/a, for the new radiation, and by 1903 mien had
made a first estimate for the magnitude of their charge (5) . Rutherford name d
tt .s radiation 'beta radiation' (6) in 1902, and by 1904 the experiments of
Kaufmann (7), Bestelmeyer (8) and Bucherer (9) had confirmed that it consiste d
of a stream of high-energy electrons whose mass varied in the manner prescribe d
by relativity.
At first, absorption experiments seemed to indicate that the electron s
emitted in the beta decay of nuclei were monoenergetic, but in 1909 Wilson (10 )
showed that they did, in fact, have a wide range of velocities . Von Baeyer ,
Hann and iH .eitner (11) found that there existed seversl distinct velocity group s
in the beta rays emitted from mesothorium two . The much more accurate method
of the Dempster mass spectrograph was used by Rutherford and Robinson (12) i n
1913, she found a number of well-defined lines in the beta ray spectrum o f
radium B and C . The principle of the mass spectrograph was to place th e
radioactive source to be analysed, in the forum of a thin wire, at the edge o f
a uniform magnetic field, so that the emitted beta rays would be bent roun d
in semicircles whose radii were inversely proportional to their velocity . The
distribution of the deflected beta rays was recorded by means of a photographi c
plate . In 1914, Chadwick (13) showed that two types of beta ray spectra
existed, continuous spectra and line spectra. The line spectra were explained
as being due to the removal of extranuclear atomic electrons from their shell s
by high-energy gamma rays from the disintegrating nucleus .
However, the existence of continuous beta ray spectra posed a problem .
If the nuclei in the initial and final states in the decay had unique characteristic