Measurements of the (n,xn) reactions cross sections using new digital methods. Habib Karam Group GRACE

outlook General context - importance of the (n,xn) reactions - gamma prompt spectroscopy method Experiment at GELINA - digital cards - work already achieved by the group GRACE PHD work - adaptation of the method used before - tests at IReS(Strasbourg) and Geel (Belgium) - results

3  General context Increasing need of energy resources worldwide that enhances the value of the nuclear energy. The problem of the radioactive waste is still lacking a good solution. The hybrid system could be the solution for the transmutation of the minor actinides. Need of nuclear data Imprecise cross sections

- Th/U neutron cycle Above 6 MeV the 233 U(n,2n) 232 U and the fission cross sections values are very close but they were never been measured. Cross section values for the 233 U(n,2n) 232 U in three different data bases

 Method :  prompt spectroscopy Reactions studies using the characteristics of the gamma rays emitted by the target while bombarded with a beam. Can be used with « white beams ». In this case the time of flight method should be applied. A very good energy and time resolution is required. Time of flight method : time windows corresponding to neutron energies in order to determine (n,xn) reactions.

HPGe detector (76 % eff. rel.) at 110° Gamma rays spectra : 208 Pb target

Experiment at GELINA Experiment at GELINA GELINA U target ( γ, f ) ( γ, f ) ( γ,n) ( γ,n) Enriched 235 U target (93.17 %) γ « Bremsstrahlung » Planar detector - t 0 : Reference time - t : Detection time - Flight path : 30 m - Flash is detected after μs - 20 MeV neutrons detected after μs - Frequency = 800Hz Flight path t- t 0 = time of flight

(n,2n) Time structure at 200 m Time of flight (s) E n =20 MeV E n = 6 MeVE n = 1 MeV (n,n’) (n,) flash Dead time with conventional electronics Dead time with our digital electronics

Digital card :TNT 2 Digital card :TNT 2 Digital card developed at IReS, Strasbourg 14 bit coders, coding rate100 MHz – 4 inputs channels available. Operated by PC using a specialised software. Data Transfer on PC for storage/treatment.

Work done by the group grace Measurement of the 207 Pb et 208 Pb (n,xn) cross sections successfully done using a flight path of 200 m and 60 MHz digital cards coding rates: - 20 MeV neutrons could be detected even if there is a flash gamma. Sample of 90 g enriched 207 Pb borrowed from Oak Ridge - relatively low activity Big CLOVER detectors - gamma rays energies up to MeV

PHD work PHD work Adaptation of the method for the 235 U (n,xn) cross sections measurements. - Digital cards : faster coding rate (100 MHz) - New code C to read and analyse data. - High activity of the 235 U target - Thinner targets less interactions shorter flight path should be used - No neutrons detected after the flash gamma. - Planar detectors - Low gamma energies.

Test of the TNT2 cards Test of the TNT2 cards Cobalt source Plastic detector Ge detector Experimental setup at IReS to simulate Geel’s conditions. -Testing the digital cards. - Finding the Ge planar time and energy resolution. Scintillator anode CFD NIM input TNT 2 Scintillator Neutron beam t 0 Ge planar detector input 2 and 4 TNT 2 Detection time t in the Ge t – t 0 time of flight

Time resolution Delay fixed at 80 nsFraction fixed at 1/8 ns

- Energy spectrum 1332 KeV 1172 KeV 1172 = 1.89 KeV 1332 = 1.91 KeV k=500 ns k=500 ns m=1000 ns m=1000 ns

Results Code C for reading the files Data analysis using PAW Test run 11/2005 at Geel purpose : testing the experimental setups and optimizing the acquisition parameters 2. Gamma flash rate The gamma flash rate was 23 % (only 9 % due to the target) - The gamma flash rate ( i.e gamma flash hits per second divided by the accelerator frequency) was 23 % (only 9 % due to the target) => Beam’s collimation should be improved Data analysis Data analysis Flash γ γ Induced by neutrons 1. Time spectrum

} X 235 U Th Neutrons between 1 and 6 MeV Th Th 3. Energy spectrum 235 U activity Neutrons (n,n’) 235 U (n,n’) 235 U Time of flight between and μs

152.7 (n,2n) 235 U Neutrons between 6 and 20 MeV 235 U activity Neutrons Time of flight between and μs Energy spectrum

Determine the neutron flux set up a flux monitor (plastic scintillator) Optimization of the scintillator efficiency -> Geant 4 simulations -> test using a neutron source at IReS Upcoming steps Upcoming steps In beam tests - Adapt the TNT2 cards to fast detectors (scintillator) - Testing the scintillator (neutron flux) - Testing the new collimator ( gammas flash rate) - … Prise de données … … … …

On-line energy and time recording

E n [MeV]t 200 m [μs] Δt 200 m [μs] t 30 m [μs] Δt 30 m [μs] Flash

Digital card parameters