A first look to capture with fission tagging (TAC+MGAS)

1. A first look to capture with fission tagging (TAC+MGAS) C. Guerrero (CERN) and E. Berthoumieux (CEA) on behalf of The n_TOF Collaboration

2. The Letter or Intent submitted to INTC (Nov. 2009) • The experimental set-up combines the use of the TAC (for capture) with a total of three MGAS (for fission) detectors loaded with 235U samples. • The detectors and samples were already used in 2009 (for monitoring purposes) • The long MGAS chamber has been designed and constructed at CERN. TAC HV (drift) MGAS Signal+Mesh 3 MGAS detectors each equipped with a 1 mg 235U sample

3. Design and construction of the fission tagging chamber The chamber has been designed and constructed at CERN in collaboration with Damien Grenier and Vincent Barozier. Gas entrance/exit Vacuum valve Connectors He at atmospheric pressure Kapton windows

4. Design and construction of the fission tagging chamber The chamber has been designed and constructed at CERN in collaboration with Damien Grenier and Vincent Barozier. Separators Sample assembly

5. Design and construction of the fission tagging chamber The samples are mounted in the Class-A lab of ISOLDE (B.179) The SAFETY FILE describing the detectors, the samples , risks and measures, usage procedures , etc. is available in EDMS: https://edms.cern.ch/document/1097338/1

6. Experimental set-up • The experiment was carried out after the 241Am with the TAC, thus the set-up for the BaF2 is that of the 241Am measurement (250 MSamples/s) • The MESH signals from the three MGAS detectors were preamplifier, amplified and the plugged into the DAQ (100 MSamples/s) named FTMG #1, #2 and #3.

7. Experimental set-up • The experiment was carried out after the 241Am with the TAC, thus the set-up for the BaF2 is that of the 241Am measurement (250 MSamples/s) • The MESH signals from the three MGAS detectors were preamplifier, amplified and the plugged into the DAQ (250 MSamples/s) named FTMG #1, #2 and #3. New/improved preamplifier (INFN) ready for next year!

8. Analysis of TAC and MGAS Time and energy calibration of the TAC modules using a 88Y source. Coincidence (Dcoinc=20 ns ) in the TAC to convert “signals” into “events” . Create ROOT files with “nt_baf2” and “nt_ftmg” for each run. Loop over each MGAS detector looking for coincidences (Dcoinc=50 ns ) in the TAC

9. Analysis of TAC and MGAS Time and energy calibration of the TAC modules using a 88Y source. Coincidence (Dcoinc=20 ns ) in the TAC to convert “signals” into “events” . Create ROOT files with “nt_baf2” and “nt_ftmg” for each run. Loop over each MGAS detector looking for coincidences (Dcoinc=50 ns ) in the TAC 25 ns Random coincidences?

10. Analysis of TAC and MGAS Time and energy calibration of the TAC modules using a 88Y source. Coincidence (Dcoinc=20 ns ) in the TAC to convert “signals” into “events” . Create ROOT files with “nt_baf2” and “nt_ftmg” for each run. Loop over each MGAS detector looking for coincidences (Dcoinc=50 ns ) in the TAC Questions at this stage (just 2 months after the experiment): 1. There is a coincidence in the TAC for ~80% of the MGAS fission signals (Esum>1 MeV and mcr>1). The study of high multiplicity events shows that 20% of fission event are not marked as “in concidence” with FTMG. Revision of the coincidence algorithm 2. It is known that ~7% of the prompt fission radiation is emitted with a delay of 20 to 100 ns. How can we take this into account ? (if DT large: background events could trigger the coincidence) 3. Shall we worry about the background from neutron emission?

11. Preliminary results from TAC+MGAS: Deposited Energy

12. Preliminary results from TAC+MGAS: Deposited Energy

13. Preliminary results from TAC+MGAS: Neutron Energy

14. Preliminary results from TAC+MGAS: Neutron Energy PRELIMINARY counting rate: lower than expected (equivalent to a misalignment of ~7mm)

15. Preliminary results from TAC+MGAS: Neutron Energy 63,65Cu Very large neutron scattering background: where is it coming from? x2.5 x1.7 x3 Ba resonances

16. The reasons behind the high neutron scattering background • Inner diameter of present chamber = 35-42 mm • Total (>99.9%) diameter of the beam = 40 mm • Accuracy of the alignment = ±???? 3% of neutrons beyond 3.5 cm diameter 55 50 35! 42! The new chamber should have all inner diameters exceeding 45-50 mm!! Outer diameter ~65 mm (need to enlarge the n-abs diameter)

17. Conclusions • We have measured for the first time at n_TOF simultaneous neutron capture and fission. • The use of thin target has allowed to measure in “veto” mode, thus obtaining nice “fission-clean“ spectra in the TAC. • The knowledge acquired (ongoing) and the modifications foreseen should allow us to propose a cross section measurement in the near term. Capture dominated resonances High fission contribution

18. Conclusions • We have measured for the first time at n_TOF simultaneous neutron capture and fission: • The use of thin target has allowed to measure in “veto” mode, thus obtaining nice “fission-clean“ spectra in the TAC. • The knowledge acquired (ongoing) and the modification foreseen should allow us to propose a cross section measurement in the near term. • Following a very preliminary analysis, there are several open questions: • There is a coincidence in the TAC for ~80% of the MGAS fission signals: • some fission signal are escaping to the coincidence analysis algorithm! • 2) It is known that ~7% of the prompt fission radiation is emitted with a delay of 20 to 100 ns. • How do we take this into account ? (if DT large: background events could trigger the coincidence) • 3) Shall we worry about the background from neutron emission? • Do we really know the cause of the high (n,n) background? • (alignment, MGAS assembly, tubes, gas, backings, vacuum windows, etc.) • 5) Veto vs. tagging (more tomorrow during the proposals session) • 6) […]

19. Thank you!