Doppler Correction

Doppler broadening is the spread in measured energy caused when gamma rays from moving nuclei are observed using a stationary detector whose opening angle is large enough to accept varying angles of incidence. The effect gets worse as the velocity of the nucleus increases and as the detector size (and hence opening angle seen by the projectile nucleus) is increased. Doppler broadening can be corrected, but to do so we must know the angle between the origin of the gamma rays and the detection point. We can use pulse shape analysis to identifying the gamma ray's angle of incidence on the detector, giving a single correction angle for the angle of emission rather than a range of possibilities across the whole detector opening angle. However, this assumes that we know the gamma rays point of origin perfectly. In fact there is an uncertainty in the speed of the nucleus (delta-v) and an uncertainty about the direction in which it is travelling (delta-theta). The effectiveness of the Doppler correction is proportional to the precision of the hit position determination but is limited by delta-v and delta-theta.

As an example of the Doppler correction improvement we consider the study of mass 100 nuclei using 72Kr+32S, V/C=6.3%. The overall resolution in a segmented clover detector is 12.6keV, but by measuring the radius of interaction in the detector in 6 sectors at radius increments of 5mm we can improve the resolution by a factor of 2 to 5.9keV. The improvement is limited by the uncertainty as to the velocity and direction of the nucleus that emitted the gamma-ray as well as the radial increment size.

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