Mod-01 Lec-25 Alpha decay Contd..

We were discussing alpha decay, and two important observations I pointed out; one is that if you calculate or measure this Q value of the reaction some A X Z going to A minus 4 Y Z minus 2 and plus 4 H e 2 and then plus energy. We calculate this Q value using semi empirical

mass formula or from the known masses of nuclei you find that for A somewhere around say 150 or so this Q is positive. So, all nuclei having nucleons number of nucleons more than say 150 or 145, the Q value of this reaction is positive. That means, if they decay through a emission of alpha particle and they will be saving energy is rest mass energy. So,

rest mass energy decreases in this and therefore spontaneous alpha decays energetically possible. So, that was one, another thing we saw in that the n verses z diagram that you do have stable or nearly very nearly stable nuclei somewhere up to say 210. So, I beyond that of course, you do not have any stable nuclei confirming to this conclusion, but around

say 150, 160, 170, 180. You do have number of large number of nucleic, which do not seen to go through this spontaneous alpha decay. So, that was one observation we pointed out and the other observation was about lifetime and Q value relationship between these 2. So, we found that typical Q for the known alpha emitters typically, Q varies between

say 4 M e V to 9 M e V and lifetimes if you look at it goes from giga to microsecond are even less. So, there is a huge range of lifetime and it is related with this Q, smaller the Q values smaller the energy available to the self of particle larger is the lifetime. That

means, decay probability the small and as Q increases this lifetime decreases and the points to be seen is that this varies only by a factor of 2. Whereas, this varies by a factor of say 10 to the power 24, 25, 26 and so on. So, very sensitively it depends on Q value, now to understand this we describe started describing