Determination of Absolute Neutron Fluence to sub-0.1% uncertainty (and better) Andrew Yue University of Maryland / NIST for the Alpha-Gamma Collaboration
NIST-ILL-Sussex neutron lifetime experiments Neutron monitor Proton trap Each result was limited by the determination of the absolute detection efficiency of a thin neutron monitor
6 Li-based fluence monitor (FM) detection efficiency 6 Li deposit Neutron beam ( ) Detected + t ( ) wavelength Absorbed neutrons Neutron absorption probability, t detection probability
CALCULATED from measured detector solid angle ( ), measured foil areal density ( ), and evaluated thermal neutron cross section ( ) of target material: -Method used for each published lifetime, best achieved uncertainty is 0.3% OR MEASURED with a second, totally absorbing neutron detector used on a monochromatic oc o atc beamline e -Alpha-Gamma (AG) device (completed, achieved 0.06% uncertainty) - 3 He gas scintillation chamber (device under construction) -Liquid 3 He target radiometer (device under construction)
FM ( 6 LiF target) signals - n + 6 Li + t ~3 s -1 per detector
Slide courtesy of F. Wietfeldt
Neutron Radiometer Slide courtesy of J. Nico Measurement in 2002 using LiMg target but concern about solid state effects. Measurement in 2004 with LHe-3 target but limited around 2%. Z. Chowdhuri Investigation into an improved measurement using LHe-3 (T. Chupp, M. Snow) R.G.H. Robertson and P.E. Koehler, NIM A 37, 251 (1986) Z. Chowdhuri et al., Rev. Sci. Instrum. 74, 4280 (2003)
The Alpha-Gamma device HPGe detector Totally absorbing 10 B target foil Neutron fluence monitor PIPS detector with aperture Alpha-Gamma device HPGe detector
Experiment ran on NCNR beamline NG-6m Shield wall Pyrolitic graphite monochromator Collimator 2 (8.38mm) Alpha-Gamma device Be filter He-filled beam tube Neutron fluence monitor Collimator 1 (15mm) Motorized LiF plastic flag
Calibration of Alpha-Gamma as a black detector 1. Measure the absolute activity i of an alpha source 2. Use this source to determine solid angle of alpha detector 3. Use an (n, ) reaction to transfer the calibration to the (, ) gamma detectors
Calibrate the -source 1 PIPS detector Diamond-turned copper aperture 239 Pu -source measured in stack of known solid angle - source activity determined from measured -rate and known stack stack Scatter-suppressing precision spacer Pu source spot
2 Calibrate the -detector with -source Source loaded into AG vacuum chamber and counted - known source activity gives detector 239 Pu
3 Calibrate the -detectors 239 Pu replaced with thin 10 B foil, beam on -n + 10 B 7 Li + + (b = 93.70(1)%) -Observed gamma rate and neutron rate (determined from alpha rate) give gamma efficiency
4 Measure neutron rate Thin foil replaced with thick 10 B foil - all neutrons absorbed -observed gamma rate and established gamma efficiency determine incident neutron rate To calibrate the FM, step 3 (calibrating the gamma detectors) and step 4 (measuring neutron rate) are repeated many times with the FM upstream
Or, more rigorously 239 Thick target Pu counting 1/ /FM R Pu Thin target 1/ / Wavelength R In practice, and Pu are measured infrequently Every efficiency measurement has its own measurements of /FM and /
Wavelength measurements - R
Wavelength measurements - R
Alpha-Gamma Current progress Plutonium measurements - R Pu
Thin 10 B target spectra - n + 10 B 7 Li + + ~20 s -1
Thin 10 B target spectra - n + 10 B 7 Li + + ~8 s -1 per detector
Thick 10 B target spectra - n + 10 B 7 Li + + ~200 s -1 per detector
Statistical accumulation Three sets of data with different beam sizes yields three efficiency measurements with different systematics Uncorrecte ed 8.4 mm 10.5 mm 7.2 mm
Corrections Beam-size e dependent e corrections o Thick target -detection dead time / pile-up pulser-based correction FM & AG charged particle detection dysprosium foil beam images Common corrections Neutron abs. & scattering in FM foil substrate add Si wafers and measure effects Neutron scattering in thick target calculation + measurement on SPINS apparatus -production in thin target substrate dedicated runs with blank Si target -scattering in thin target stack additional Si wafers and extrapolate -scattering in thick target measure for thick target #1, #2, and #1 + #2
Typical correction size
Statistical combination
Uncertainty budget
The path to 0.01% Sources of uncertainty that must be addressed in a 0.01% 01% calibration: Statistical uncertainty (0.032%) 239 Pu source counting uncertainties (0.031%) Wavelength determination i (0.024%) 024%) Beam spot corrections to solid angle (AG - 0.015%, FM 0.009%) -attenuation in thick (0.023%) and thin (0.012%) targets
Effects that require more measurement time Wavelength measurement (0.024%) determine cause of point spread, G.L. Hansen measurement achieved 0.006% uncertainty Beam spot corrections to solid angle (AG 0 015%, FM 0 009%) Beam spot corrections to solid angle (AG 0.015%, FM 0.009%) more beam images, better control and assessment of image background
Effects that have been resolved 239Pu source counting uncertainties (0.031%) limited by uncertainty in source calibration stack solid angle (0.03%) new stack characterization with better aperture and improved metrology techniques achieved 0.007% uncertainty
Effects addressed in an apparatus rebuild Statistical uncertainty (0.032%) build apparatus that accepts a much larger beam (~35 mm diameter) -attenuation in thick (0.023%) and thin (0.012%) targets new apparatus has only front-facing HPGe detectors
Upgraded experiment setup Beam shaped by 40 mm and 35 mm dia. collimators 60 mm dia. targets on 76 mm dia. substrates 6 LiF target - 5 g/cm 2 areal density Thin 10 B target 30 g/cm 2 areal density
New AG 4x 5% rel. eff. HPGe dt detectorst 10.15 foil-xtal distances Total = 3 x 10-4 4x 0.5 apertures, 3.85 foil-ap. distances = 4.202 x 10-3 New FM 4x 15 mm apertures, 4.25 foil-ap. distances = 4.8 x 10-3
Projections Assuming beam flux of 1.0 x 10 6 n/cm 2 s and a 35 mm diameter beam: 105 s -1 total rate in FM 875 s -1 total -rate in AG 52 s -1 total -rate with thin target 2760 s -1 total t -rate t with thik thick targett Approximately 55 three-day cycles of thin-thick-thin target in Alpha-Gamma to 0.01% 01%
Conclusions The Alpha-Gamma method has been successfully used to determine the absolute detection efficiency of a 6 Li-based fluence monitor to 0.06% 06% uncertainty Calibration of the monitor is statistics-limited and the systematic effects are well understood Statistical uncertainty can be addressed with off the shelf items and well Statistical uncertainty can be addressed with off-the-shelf items and welldeveloped techniques no R & D required
Sample 49Si-3-3 (Half-life 24000 years)
Thank you
Extra slides
Updated fluence monitor 60 mm diameter targets on 76 mm diameter substrates (from 38 / 50 mm) Identical target-to-detector to distance and aperture size ( ) Areal density reduced to 5 g/cm 2 (from 39.3 g/cm 2 )
Updated Alpha-Gamma Target is held perpendicular to the beam accepts much larger beam spot Four, front-facing facing PIPS detectors for detection Four, front facing HPGe detectors for detection
Measured for each configuration
Apparatus close-up (structure hidden) 4x 20% rel. eff. HPGe detectors t 10.15 foil to crystal distances Total = 1.3 x 10-3 0.5 apertures, 3.85 foil to aperture distances = 4.202 x 10-3 4x 0.75 apertures, 4.25 foil to aperture distances = 7.7 x 10-3