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

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

2. Not perfect detectors We measure electron energies, not ’s! Photo-electric effect Compton scattering Pair production Radiation from environment (Pb) Threshold

3. The problem (I) incomming N Unfolded u u1 u2 E Raw r N measured e E e

4. The problem (II) Easy to fold Difficult to unfold known = r known unknown

5. The response matrix

6. The solution? Channel by channel Inverting response matrix R But large fluctuations in u

7. Alternative solution Exploit that we know how to fold: Control by iteration:

8. Iterations

9. Compton subtraction method We define the Compton spectrum c = r - v, where photo-peaks (=u) + s.e. + d.e. + 511 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.

10. Examples of unfolding

11. Comparison Raw Old New Old smoothed

12. Summary Consistent unfolding and folding Reduced fluctuations with Compton subtraction method See M. Guttormsen et al., NIM A 374 (1996) 371-376