238 U Neutron Capture with the Total Absorption Calorimeter

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

Outline • Introduction • Previous work • Quality Checks • The PKUP problem • Background contributions • Pile up effect • Yield calculation

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%

JEFF 3.1.1 238U (n,γ) cross-section 20keV ? TAC

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

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

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

Proton Intensity

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

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

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 temporarily solved by putting a gate on the time the routine looked for the pkup signal

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 45% of the 0.5e12 pulses produce no signal in the PKUP • To increase statistics, it was vital to include these lost pulses!

Quality Checks • Aim to check every detector was working in every run • Look at the ratios between the number of counts in the PKUP, SILI and TAC • They should all be proportional • See if the pkup signal can be recovered by looking at one of the other detectors

PKUP vs Pulse Intensity • Ignore 4 bad runs • LOW = 5.01±0.05 (1%) • MED = 5.05±0.03 (0.6%) • HIGH = 5.04±0.04 (0.7%) The ratio between the PKUP signal, and the PS signal does not go through (0,0) therefore we can’t compare them at different values X Axis – Run Number Y Axis – PKUP/Pulse Intensity

PKUP vs SILI • How to get the SILI counts in each detector? • Select tritium peak • We know this is solely from neutrons • Use a cut in neutron energy to look at the region of interest • (1eV-100keV)

PKUP vs SILI • We find 20 bad runs, but many are from runs with very little statistics, or carbon runs • Ignoring these runs we can work out a mean and standard deviation • DET 1 = 1570±120 (8%) • DET 2 = 1925±155 (8%) • DET 3 = 2130±206(10%) • DET 4 = 1457±125 (9%) PKUP/SILI Counts Run Number

SILI vs TAC • 2 bad runs ignored • U8 LOW = 0.00264±0.00032 (12%) • U8 MED = 0.00285±0.00021 (7.3%) • U8 HIGH = 0.00264±0.0003 (9%) • Au LOW = 0.00270±0.00036 (13%) • Au MED = 0.00281±0.00027 (9.6%) • Au HIGH = 0.00333±0.0003 (10%) SILI/TAC Counts In the case of the 236U TAC measurement, there was an agreement within 2% Run Number

PKUP vs TAC • 11 bad runs ignored • U8 LOW = 5.55±0.068 (1.2%) • U8 MED = 6.14±0.077 (1.3%) • U8 HIGH = 7.52±0.32 (4.4%) • Au LOW = 5.48±0.07 (1.3%) • Au MED = 6.02±0.15 (2.5%) • Au HIGH = 7.35±0.32 (4.4%) PKUP/TAC Counts Run Number

PKUP Data Recovery • We have shown the one can recover the intensity of the PKUP signal within 1% by looking at the signal from the PS (Pulse Intensity) • We still need to recover the tflash value, which gives us our neutron energy • DED Pulses = 6613.64±70 (1%) • PAR Pulses = 6477.24±200 (3%) • With 10000 bins/decade, 200ns won’t affect the neutron energy bin until ~> 100keV

PKUP Recovery • Statistics are improved • Things look nicer End of the PKUP problem…?

Background Contributions

Background Subtraction • Not taking the neutron scattering background into account • Maximum of 5% effect

Background Subtraction

Pile Up Problem Looking at the integrals between the resonances, one can quantify the effect of the pile up as a function of neutron energy Ratio Med/Low > 1keV = 100.72±3.83 Ratio High/Med >3keV = 93.05±6 Including Med pulses above 1keV increases statistics by a factor of 3 Losing High pulses, we miss out on increasing statistics by an extra 1/3

Resonance Ratios

Counting Rate

238U Final Yield Scale the yield using the first 3 resonances. Average value of 0.505(5) [1%]

238U Final Yield First saturated resonance Second saturated resonance Third saturated resonance

238U Final Yield At low energies, the data looks very nice

Upper limit of analysis • Limited not by the gamma flash, and not by sample canning but by statistics! 5keV??

Upper limit of analysis • Limited not by the gamma flash, and not by sample canning but by statistics! 20keV?? Maybe not… BUT we’re in the general correct area

SAMMY analysis, or REFIT? • SAMMY doesn’t fit the second and third resonances very well due to the multiple scattering corrections • REFIT does (And CONRAD??). • But we like SAMMY…. There are 3 possible running modes in REFIT: Basic - calculates the energy loss range for the scattered neutron and averages the cross section over this range and uses that to work out the Y1 and Y2 contributions Int - calculates the energy loss range but then divides it into segments and averages the segment cross section does the same as intermediate but Doppler broadens the scatter neutrons Thanks to Tim Ware for the REFIT calculations

REFIT Comparison • Second resonance has a bigger scattering/capture cross-section ratio • Refit full simulation gives a better shape

REFIT Comparison • Similar situation in the third resonance

Conclusions • We have recovered as much data as is possible from the run, which includes all the low intensity runs, and the medium intensity runs > 1keV • It looks like we should be able to do a useful resonance analysis up to 10keV (Yippee!!) • We should calculate the background from neutron scattering • The backgrounds should be smoothed before being subtracted • The normalisation to the saturated resonances needs to be done correctly • The gold data should be analysed, to check everything is going fine THANKYOU FOR LISTENING!!

238 U Neutron Capture with the Total Absorption Calorimeter