Nuclear Resonant Absorption of Gamma Rays byCa40

Ground-state gamma rays ($E_{\gamma }=10.3\mathrm{}$ Mev) emitted by excited ${\mathrm{Ca}}^{40}$ formed in the reaction ${\mathrm{K}}^{39}(p, \gamma ){\mathrm{Ca}}^{40}$ at $E=2.05\mathrm{}$ Mev have been resonantly absorbed in calcium. Nuclear resonant absorption occurs when the absorber is placed at such an angle that the energy discrepancy upon emission and absorption of the gamma rays is restored by the Doppler shift resulting from the recoil velocity of the excited ${\mathrm{Ca}}^{40}$ nucleus. The gamma radiation is detected by a scintillation detector mounted behind an angle-defining collimator. The nuclear resonant absorption curve is obtained by observing the transmission through the absorber as the collimator is rotated through the resonant angle. The observed width, which is of the order of 0.8 degree, is a function of the angular divergence of the proton beam and the angular opening of the collimator. From the integral of the absorption as a function of angle and from a yield measurement, the radiation width for a transition to the ground state is determined to be 3.6±0.24 ev, while 5.8±1.8 ev is found for the proton width, and 10.3±1.7 ev for the total width. These values together with the measured angular distribution of the ground-state radiation are consistent with a $2^{+}$ assignment to this level.