Lifetimes of the molecules in the excited states are often measured by using pulsed radiation source of duration nearly in the nano second range. If the radiation source has the duration of 2 ns and the number of photons emitted during the pulse source is `2.5 xx 10^15`, calculate the energy of the source.

The isotopes "_Co^57 decays by electrons capture to "^57Fe with a half - life 272d. The "^57Fe nucleus is produced in an excited state, and it almost instantaneously emits gamma rays. If the activity of a radiation source "^57Co is 2.0 muCi now, how many "_Co^57 nuclei does the sorce contains?

Calculate the energy radiated in 30 seconds by perfectly black sphere of radius5cm maintained at 127^@C . (Stefan's constant s = 5.7 xx 10^-8 J//m^2sk^4)

Monochromatic radiation of wavelength 640.2 nm (1 nm = 10^-9 m) from a neon-lamp irradiates photosensitive material made of caesium on tungsten. The stopping voltage is measured to be 0.54 V. The source is replaced by an iron source and its 427.2 nm line irradiates the same photo-cell. Predict the new stopping voltage

A source contains two species of phosphorous nuclei, "_15 ^32 P (T_1//2=14.3d) and "_15 ^33 p (T_1//2=25.3d). at time t= 0 , 10% of the decays are from "_15^33P. How much time has to elapse for only 90% of the decays?