Folding and unfolding NaI spectra Magne Guttormsen Department of Physics and SAFE University of Oslo

Nuclear Physics Group seminars, Oslo 15th March 2010 Not perfect detectors We measure electron energies, not ’s! ● Photo-electric effect ● Compton scattering ● Pair production ● Radiation from environment (Pb) ● Threshold

Nuclear Physics Group seminars, Oslo 15th March 2010 The problem (I) incomming measured N E N E e e Unfolded u Raw r u1u1 u2u2

Nuclear Physics Group seminars, Oslo 15th March 2010 The problem (II) ● Easy to fold ● Difficult to unfold known = r known unknown

Nuclear Physics Group seminars, Oslo 15th March 2010 The response matrix

Nuclear Physics Group seminars, Oslo 15th March 2010 The solution? ● Channel by channel ● Inverting response matrix R ● But large fluctuations in u

Nuclear Physics Group seminars, Oslo 15th March 2010 Alternative solution ● Exploit that we know how to fold: ● Control by iteration:

Nuclear Physics Group seminars, Oslo 15th March 2010 Iterations

Nuclear Physics Group seminars, Oslo 15th March 2010 Compton subtraction method We define the Compton spectrum c = r - v, where photo-peaks (=u) + s.e. + d.e keV are subtracted from the raw spectrum. The u- spectrum is obtained from about 30 iterations and has large fluctuations. Next step is to smooth c, and subtract it from the raw spectrum giving an unfolded spectrum with fluctuations comparable with the raw spectrum. For further details, see NIM-paper.

Nuclear Physics Group seminars, Oslo 15th March 2010 Examples of unfolding

Nuclear Physics Group seminars, Oslo 15th March 2010 Comparison Raw Old New Old smoothed

Summary ● Consistent unfolding and folding ● Reduced fluctuations with Compton subtraction method ● See M. Guttormsen et al., NIM A 374 (1996)