238 U Neutron Capture with the Total Absorption Calorimeter
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Transcript of 238 U Neutron Capture with the Total Absorption Calorimeter
n_TOF Annual Meeting, Lisbon, Dec 2011
238U Neutron Capture with the Total Absorption Calorimeter
Toby WrightUniversity Of Manchester
n_TOF Annual Meeting, Lisbon, Dec 2011
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%
n_TOF Annual Meeting, Lisbon, Dec 2011
JEFF 3.1.1 238U (n,γ) cross-section
TACC6D6
n_TOF Annual Meeting, Lisbon, Dec 2011
The 238U Sample
• 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
The 238U arrived as a powder, suspended in solution
n_TOF Annual Meeting, Lisbon, Dec 2011
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
n_TOF Annual Meeting, Lisbon, Dec 2011
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
n_TOF Annual Meeting, Lisbon, Dec 2011
The Measurement
Sample Protons Time238U 4.12e+17 24d 16:15:02Nothing (sample out) 1.05e+17 6d 16:56:14
Gold 6.74e+16 6d 10:30:54
Carbon 7.05e+16 2d 17:49:28
• 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)
n_TOF Annual Meeting, Lisbon, Dec 2011
TAC Calibrations - Energy• Three calibration sources used:
– 137Cs (661.7 keV)– 88Y ( 0.898 and 1.836 MeV)– AmBe (4.44 MeV)
n_TOF Annual Meeting, Lisbon, Dec 2011
88Y
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88Y – Fit Background
• Background fitting and peak finding done with TSpectrum class in ROOT
n_TOF Annual Meeting, Lisbon, Dec 2011
88Y – Fit Gaussian
n_TOF Annual Meeting, Lisbon, Dec 2011
Detector Drift
• Very little drift over time
• Only need to use one set of calibration values for the whole measurement
Detector Number
Chan
nel N
umbe
r
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88Y Fits
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AmBe Fits
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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
n_TOF Annual Meeting, Lisbon, Dec 2011
Dst creation investigation - AmBe
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Dst creation investigation – Detector Resolution
n_TOF Annual Meeting, Lisbon, Dec 2011
88Y Fits (version 10)
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AmBe Fits (version 10)
n_TOF Annual Meeting, Lisbon, Dec 2011
TAC AmBe Plot
Peak positionGaussian peak position
• 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 Tspectrum peakfinder is taken
4.44 MeV
n_TOF Annual Meeting, Lisbon, Dec 2011
Dst creation Investigation
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Proton Intensity
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Counting Rate
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Preliminary Results
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Preliminary Results
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First ResonanceThird Resonance
Second Resonance
First Resonances
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Statistics
Shown here is resonances around 1.65keV. This is with approximately 15% of the total statistics acquired during the 238U runs.
n_TOF Annual Meeting, Lisbon, Dec 2011
Effect of the gamma flash on the TAC
n_TOF Annual Meeting, Lisbon, Dec 2011
Effect of the gamma flash on the TAC
n_TOF Annual Meeting, Lisbon, Dec 2011
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