238 U Neutron Capture with the Total Absorption Calorimeter

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

2. 238U (n,γ) TAC measurement • Featured on the NEA high priority list • High accuracy (n,γ) 238U cross-section measurement up to 100 keV • 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%

3. JEFF 3.1.1 238U (n,γ) cross-section TAC C6D6

4. The 238U Sample The 238U arrived as a powder, 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

5. 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

6. Experimental Setup • Data is taken at a 250MHz sampling rate, for 32ms allowing measurements to be taken from 0.3eV • The raw data is stored in CERN’s CASTOR facility after zero data suppression

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 (up to ~100keV)

8. TAC Calibrations - Energy • Three calibration sources used: • 137Cs (661.7 keV) • 88Y ( 0.898 and 1.836 MeV) • AmBe (4.44 MeV)

9. 88Y

10. 88Y – Fit Background • Background fitting and peak finding done with TSpectrum class in ROOT

11. 88Y – Fit Gaussian

12. Detector Drift • Very little drift over time • Only need to use one set of calibration values for the whole measurement Channel Number Detector Number

13. 88Y Fits

14. AmBe Fits

15. Dst creation investigation – 60Co Change the value of ‘slowtau’ in the routine. This is related to the decay time of the slow component of the signal

16. Dst creation investigation - AmBe

17. Dst creation investigation – Detector Resolution

18. 88Y Fits (version 10)

19. AmBe Fits (version 10)

20. TAC AmBe Plot • Gaussian fits are not accurate due to the presence of the single escape peak • More accurate calibration when the peak position, as found by the Tspectrumpeakfinder is taken Peak position Gaussian peak position 4.44 MeV

21. Dst creation Investigation

22. Proton Intensity

23. Counting Rate

24. Preliminary Results

25. Preliminary Results

26. First Resonances Second Resonance Third Resonance First Resonance

27. Statistics Shown here is resonances around 1.65keV. This is with approximately 15% of the total statistics acquired during the 238U runs.

28. Effect of the gamma flash on the TAC

29. Effect of the gamma flash on the TAC

30. Conclusions • The experiment was successfully completed • A combination of n_TOF’s high instantaneous neutron flux, the massive sample and the TAC’s efficiency should make this the most accurate (n,γ) 238U cross-section measurement ever done • The preliminary data analysis done looks very promising