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238 U Neutron Capture with the TAC

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238U Neutron Capture with the TAC

Toby Wright

University Of Manchester

n_TOF Analysis Meeting

CERN

October, 2011

- 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%
- Currently being measured at nTOF

- 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)
- ‘Bonbon’ casing with ~60 microns of Al, ~75 microns Kapton

- 40 detectors going into 5 different streams
- Each stream contains 2 digitizers, each with its own clock
- Clocks have an accuracy to 2ppm therefore each detector must be calibrated to one reference
- Done using the two gamma rays of Y88

Delta t (ns)

TOF (ns)

TAC Calibrations - Time

- Three calibration sources used:
- 137Cs (661.7 keV)
- 88Y ( 0.898 and 1.836 MeV)
- AmBe (4.44 MeV)

- Gaussians are fitted after the background has been subtracted
- Routines for the 137Cs and 88Y have been rewritten, but are not completely finished

Counts

Channel Number

Counts

Channel Number

- Resolution = 2.35*FWHM/Energy
- In this case, we have used channel number instead of Energy
- Individual detectors resolution determined at the peak of 137Cs (661.7 keV)

2004

- 26 days more running time (20 days completed)
- Everything seems to be going well!!

- As of this morning, we have :
- 268e15protons for 238U (normal nominal and parasitic) (~2e5 events)
- 21299 ‘events’for beam off
- 52.8e15protons for sample out (~4e4 events)
- 14.8e15protons at low intensity (~1.4e4 events)
- 12.45e16protons at high intensity (~7e3 events)

Second Resonance

Third Resonance

First Resonance

- Aim to have at least 1000 counts per resonance with reasonable binning
- We already have this for the larger resonances
- We are well on track for reaching this goal

Plot for runs 12983-13008 (4000 bins between 1 and 2 keV)

Approximately 7% of total statistics

- Measurement is going smoothly
- Preliminary analysis shows promising results
- The TAC should be able to be used to measure at higher energies (100keV?) than in the past