chemical elements and their compounds - Sciencemadness Dot Org

Helium
3
the two resulting nuclei (composed respectively of two protons and one
neutron, or of one proton and two neutrons) both weigh 3-0172 13 * 15 ; both
are unstable, though they are relatively long lived.
This 3 He has recently been found to be present in minute quantities in
natural helium; Alvarez and Cornog, 16 ' 17 using a 60-inch cyclotron as a
mass spectrograph, have got evidence that it occurs to an extent of 10~ 7
in atmospheric helium and 10~ 8 in helium from gas wells.
The isotope 6 He is far more unstable. It is formed in several nuclear
reactions; it is produced 18 when 'heavy* paraffin is bombarded with aparticles:
2 4 6 1
D + « ~> He + H.
It can also be made 19 ' 20 along with ordinary helium by bombarding
lithium with swift deuterons:
7 Li + 2 D ~> 4 He + 5 He.
Its mass is 5-0106 21 ; it rapidly decays thus:
0 He has a half-life of 0-85 sec. 22
6 He -> 4 He + %.
The following are the masses of the three isotopes:
3 He
30172
3x10057
4 He
4-00380«
4x1*00097
a = * 4 , 6 = 21
5 He
5-0106*
5x10021
From these figures it is evident that if four hydrogen atoms (4 X 1-00813
-~ 4*03252) could be made to form a 4 He nucleus, they would lose 0*02866
of a unit of mass, and hence emit 0*02866 X 940 = 26-94 m.e.v. of energy;
this is 622 million k.cals. for 4 g. of hydrogen, or 156 millions per gramme.
Bethe has shown 25 that this could be brought about in a star by the following
series of reactions:
1 IC + IK -> "KT; \ 3 N ~-> 1 IC + ^; 1 JC + JH ~> \ 4 N;
1 JN + JH ~> 1 IO; 1 JO -> 1 JN + e + ; X ?N + JH -» 1 JC + 4 He.
u
N. R. Sen and U. B* Burman, Astrophys. J. 1944, 100, 347.
16
T. W. Bonner, Phys. Rev. 1938, 53, 711.
16
L. W. Alvarez and B. Cornog, ib. 1939, 56, 379.
" Ib. 613.
1H
F. Joliot and I. Zlotowski, CR. 1938, 206, 17, 1250.
,e
J. H. Williams, W. G. Shepherd, and B. O. Haxby, Phys. Rev. 1937, 51, 888.
»» H. Siaub and W. E. Stephens, ib. 1939, 55, 845.
ai
P. Joliot and I. Zlotowski, J. Phys. Radium, 1938, [vii] 9, 403.
98
H. S. Sommers and B. Sherr, Phys. Rev. 1946, ii. 69, 21.
» B J. H. Jeans, Nature, 1943, 151, 7.
M
K. T. Bainbridge, Phys. Rev. 1938, 53, 922.
•* H. A, Bathe, ib. 1939, 55, 434.
86
H. N. Standi, J. Franklin ImU 1939, 228, 143.