1. The γ-flash problem at cross-section measurements by the prompt in beam γ-ray spectroscopy at n-TOF facility at CERN N. Citakovic, S. Jokic, S. Lukic, P. Baumann, G. Rudolf Military Academy Belgrade, Serbia, ncitakovic@eunet.yu ncitakovic@eunet.yu INN Vinca, Belgrade, Serbia GSI Darmstadt, Germany IPHC – Institut Pluridisciplinaire Hubert Curien, Strasbourg, France

2. Introduction  The n-TOF (neutron Time of Flight) international collaboration has as its goal the measurement of various neutron-induced reactions.  In the frame of n-TOF, the GRACE group is in charge of (n,xn) reactions.  Preliminary measurements of (n,n'γ) and (n,2nγ) reactions cross sections on a natural lead target were performed on the GELINA white neutron source in IRMM Geel, Belgium.

3. Method  One of the possible methods to study (n,xn) reactions is to use the in beam γ – ray spectroscopy and neutron time of flight technique.  Neutron induced reactions leave the residual nuclei in highly excited states, which subsequently decay via γ – cascades to ground states.  In this way one actually directly measures (n,xn γ) reaction cross sections.

4.  The idea is to use the existing facility at CERN for these measurements.  However, γ – flash appears at every beam pulse. It often leads to HPGe detector saturation.  This difficulty is the subject of our study.

5. Experiment  The experiment was performed in framework of the n- TOF Collaboration on October 2004. Our aim was to check the possibility of (n,xn) reactions cross sections measurement by this experimental method in spite of γ- flash presence.  Experimental setup consists of HPGe detector, target support and acquisition system. HPGe detector is placed at the tunnel end, at 45 degree with respect to the proton beam.

6.  Since γ-rays are reflected from the collimator and experimental room, they can hit the detector too.  This fact must be taken into consideration in the future experiments with real target.  In order to study possible background for HPGe detector no target has been used in this experiment.

7. Conclusions  On the basis of experimental data (more than 40000 processed events) it has been concluded that the (n,xn) reactions cross sections can be measured because signal was not saturated for 55% of proton beam pulses.  Long measurement time or using a great number of detectors (several tens) is necessary because the beam frequency is low (one proton burst every 2.4 to 12s).  More efforts are needed to adapt the detection technique to the remaining background.