238 U Neutron Capture with the Total Absorption Calorimeter

1. 238U Neutron Capture with the Total Absorption Calorimeter Toby Wright University Of Manchester

2. Outline • Introduction • Previous work • The PKUP problem • Background contributions • Pile up effect • *Preliminary* yield calculation

3. 238U (n,γ) TAC measurement • Featured on the NEA high priority list • High accuracy (n,γ) 238U cross-section measurement • Joint future analysis with the same measurement using C6D6 detectors at nTOF and GEEL • Aim of reducing the uncertainty in the cross-section from ~5% to ~2%

4. JEFF 3.1.1 238U (n,γ) cross-section 20keV TAC C6D6

5. The 238U Sample The 238U arrived as a metal, suspended in solution • 6.125 ± 0.002 Grams • Isotopic analysis done in 1984 • <1 ppm 233U • <1ppm 234U • ~11ppm 235U • <1ppm 236U • Wide sample, with perfect alignment it covers ~97% of the beam (53.90 x 30.30 mm) • Encased with ~60 microns of Al, ~75 microns Kapton

6. Other Samples • Carbon and Gold samples were measured to determine the background due to scattered neutrons, and normalisation respectively • Measurements were done with no sample at all present, and also with the 238U sample but no beam to determine the backgrounds present in the measurement The 3 samples measured. From L to R: 238U, Gold, Carbon

7. The Measurement • Lasted around 41 days in total • Used 4 different proton intensities (0.5, 0.9, 3 and 8e12ppp) to investigate the effect of pile up due to the large mass and therefore large counting rate from the sample • A spherical C12H20O4(6Li2) surrounded the samples to reduce the neutron induced background • The whole running went very smoothly • Due to extremely little sample encapsulation, it should be possible to measure up to higher neutron energies than ever before with the TAC

8. Proton Intensity

9. TAC Calibrations – Energy + Time • The two 88Y gamma rays are emitted simultaneously, so can be used to calibrate all 40 detectors in time • Three calibration sources used: • 137Cs (661.7 keV) • 88Y ( 0.898 and 1.836 MeV) • AmBe (4.44 MeV) Fit a second order polynomial to the 4 energy points

10. TAC Calibrations – Energy + Time • The two 88Y gamma rays are emitted simultaneously, so can be used to calibrate all 40 detectors in time • Three calibration sources used: • 137Cs (661.7 keV) • 88Y ( 0.898 and 1.836 MeV) • AmBe (4.44 MeV) Fit a second order polynomial to the 4 energy points

11. Runs Before 13018 • Currently, runs 12973-13018 are not being used (~4 days of running) • BAF2 # 11, and possibly #19 were failing

12. Dst creation Investigation – Detector Resolution • In the past, the pulse shape analysis routine wasn’t working so well • We saw 2 peaks where there should be 1 peak in calibration runs • By changing the ‘slowtau’ variable in the PSA routine, we fixed this problem and also improved the detectors resolution

13. Proton Intensity

14. PKUP Problem • When taking the lowest proton intensity (0.5e12ppp), the PKUP signal was so small that the previous routine was often failing to find the signal, due to oscillations along the baseline • This was often giving a negative tflash value, this was solved by putting a gate on the time the routine looked for the pkup signal

15. PKUP Problem • Now only we always have a positive tflash value • Unfortunately, due to the small size of the signals and the oscillation of the base line, around 40% of the 0.5e12 pulses produce no signal in the PKUP • It’s a future job to find a way to retrieve the signals!

16. Background Contributions

17. Background Contributions

18. Background Subtraction

19. Background Subtraction

20. Background Subtraction

21. Background Subtraction

22. Pile Up Problem • Peak height decreases with increasing Pile Up effects • By looking at the ratio between the resonance peak heights in spectra with different pulse intensities, one can quantify the effect

23. Pile Up Problem • The integral gives a more accurate quantification • If the ratio is close to one, then the pile up correction is easily manageable (assuming there is no pile up in the Lowest pulses..) • 3.0e12 pulses seem to be always highly affected by pileup

24. Counting Rate

25. 238U Preliminary Yield • Scaled by 1/0.516 to first saturated resonance • Beam interception factor is almost one, therefore we have ~52% efficiency • We fit the edges of the first resonance better with a 10% increase in the number of atoms BUT we see a worsening of the fit at higher energies By fitting the resonances in energy, a new flight path of 185.24m was found (It was previously 185.7m)

26. 238U Preliminary Yield • Second saturated resonance doesn’t agree as well • Resonance height – normalisation and/or pile up problems? • Resonance shape – multiple scattering effect? • All fits done with ENDF/B-VII.1

27. 238U Preliminary Yield • Third saturated resonance has similar problems • I will try normalising to all 3 of the saturated resonances separately in the future

28. 238U Preliminary Yield In general the unsaturated resonances are fitted reasonably well

29. 238U Preliminary Yield

30. 238U Preliminary Yield

31. 238U Preliminary Yield The statistics are poorer above 1keV, hopefully we can use the data with the higher pulse intensity (1e12) in this region

32. 238U Preliminary Yield

33. 238U Preliminary Yield The libraries have resonances up to 20keV. If we are to do a resonance analysis this high up in energy with the TAC, we need to make use of more of the statistics

34. Conclusions • Data analysis is on-going • We need to decide the best way to use the data with no PKUP signal • We need to decide which pulse intensities we can use in which neutron energy regions • Calculate the background from neutron scattering • Calculate the Yield for Gold, check the background subtractions etc. • Just using 50% statistics of the 0.5e12 pulses, the preliminary data analysis already looks promising, but the upper energy limit that we can do resonance analysis to needs to be determined