1. Compton Add-Back Protocols for use with the EXOGAM Array Adam Garnsworthy University of Surrey Methods for improving spectra through reducing the effects of Compton scattering Applied to data from a 8 He radioactive beam experiment

2. The EXOGAM array 16 Clovers of Compton-suppressed segmented germanium crystals Various configurations to maximize specific features for an experiment Images taken from: http://www.ganil.fr/exogam/

3. Compton scattering between crystals Polar plot and equation taken from K.S. Krane, Introductory Nuclear Physics, Wiley, 1988 The Klein-Nishina Formula

4. The idea and aims of Add-back Compton suppression vetoes events – Add-back algorithms reconstruct events Radioactive beam experiments suffer greatly from background Increase peak-to-background ratio of reaction gamma-rays

5. Construction of Algorithms Multiplicity one events – include all gamma-rays Multiplicity two events – apply conditions: Using the centre contact data only: Include hits separately in opposite crystals Add together hits in adjacent crystals Using the increased positional sensitivity of the segmentation data: Hits in different crystals are only added together if adjacent segments fire

6. 8 He radioactive beam experiment July 2004, GANIL Background from decay of beam particles (981 keV) and bremsstrahlung Only 4 Clovers in the ‘close packed’ configuration 208 Pb( 8 He,4n) 212 Po 8 He → 8 Li + β - followed by a 981 keV γ -ray 8 Li → 7 Li + nfollowed by a 478 keV γ -ray

7. 8 He beam spectra No Add-back Crystal Add-back Segmentation Add-back Peak/Background0.354(4)0.385(4)0.396(4) Peak/Total0.274(2)%0.346(3)%0.362(3)% Data for the 2 + → 0 + transition (727 keV):

8. Conclusions and further development Some 981 keV events are only partly reconstructed due to scattering between Clovers Timing conditions still to be optimized Other gating conditions, such as triples, can be used

9. Thank you to my collaborators 1.Department of Physics, University of Surrey, Guildford GU2 7XH, UK 2.Dept. of Nuclear Physics, Australian National University, Canberra, Australia 3.Dept. of Nuclear Physics, The Faculties, Australian National University, Canberra, Australia 4.GANIL, BP 5027, Caen Cedex F-14021, France 5.Department of Physics, Royal Institute of Technology, S-106 91, Stockholm, Sweden 6.School of Engineering, University of Brighton, Brighton BN2 4GJ, UK 7.Department of Physics, University of Liverpool, Liverpool, L69 3BX, UK 8.GSI, Planckstrasse 1, Darmstadt D-64291, Germany 9.CLRC Daresbury Laboratory, Warrington, WA4 4AD, UK 10.Institut de Phsique Nucleaire de Lyon, Lyon, France 11.Niels-Bohr-Institute, DK-2100 Copenhagen, Denmark N.J. Thompson 1, Zs. Podolyák 1, P.M. Walker 1, S.J. Williams 1, G.D. Dracoulis 2, G. de France 4, G.J. Lane 2, K. Andgren 1,5, A.M. Bruce 6, A.P. Byrne 2,3, W.N. Catford 1, B. Cederwall 5, G.A. Jones 1, B. McGuirk 7, S. Mandal 8 E.S. Paul 7, V. Pucknell 9, N. Redon 10, B. Rosse 10, R.J. Senior 2 and G. Sletten 11