modeling the yaguar reactor neutron field and detector count rates in the direct a nn measurement
DESCRIPTION
Modeling the YAGUAR Reactor Neutron Field and Detector Count Rates in the Direct a nn Measurement. Bret Crawford and the DIANNA Collaboration June 9, 2003. Direct Investigation Of a nn Association (DIANNA). Duke/TUNL NCSU/TUNL Gettysburg College. JINR ARRITP. nn-Scattering Length. - PowerPoint PPT PresentationTRANSCRIPT
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Modeling the YAGUAR Reactor Neutron Field and Detector Count Rates in the Direct ann
Measurement
Bret Crawford and the DIANNA Collaboration
June 9, 2003
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Duke/TUNLNCSU/TUNLGettysburg College
JINR
ARRITP
Direct Investigation
Of ann
Association(DIANNA)
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nn-Scattering Length
sann2 as k 0
¼s¾ t¼ sann
2
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Charge Symmetry Breaking
– 0.5 fm aCSB 2.5 fm
app = (-17.3 ± 0.8) fm
ann = (-18.5 ± 0.3) fm
ann = (-16.27 ± 0.40) fm
Nagels et al. Nagels et al. NUCL. PHY BNUCL. PHY B 147147 (1979) 189. (1979) 189.
Howell et al. Howell et al. PHYS LETT BPHYS LETT B 444444 (1998) 252. (1998) 252.
GonzGonzáález Trotter et al. lez Trotter et al. PHYS REV LETT PHYS REV LETT 8383 (1999) (1999) 3788.3788.
Huhn et al. Huhn et al. PHYS REV C PHYS REV C 6363 (2001) 014003 (2001) 014003..
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YAGUAR ReactorAll-Russian Research Institute of Technical Physics
Snezhinsk, Russia
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YAGUAR Reactor
• Pulsed reactor with high instantaneous flux
• Annular design with open through-channel (nn-cavity)
• 90% enriched 235U-salt/water solution
• Energy per pulse – 30 MJ
• Pulse duration – 900s
• Fluency – 1.7x1015 /cm2
• Flux – 1x1018 /cm2/s
• Neutron density – 1x1013 /cm3
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The Experiment
• Neutron collisions take place in reactor through-channel
• Neutrons are detected 12 m below detector
• nn determined from detector counts and known flux
• Expect ~150 counts/pulse • Background (non-collision
neutrons at detector) is an issue
absorber
40 cm
12 m
Reactor
collimators
shielding
detector
Moderator
shielding
TVv
cN nno
avavPD
2
4
2
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The Experiment
• Collisions take place in reactor through-channel
Shielding
Reactor with Moderator sleeve
To detector
40 cm
To absorber
40 cm
Through Channel
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The Experiment
Shielding
Reactor with Moderator sleeve
To detector
40 cm
To absorber
40 cm
• Collisions take place in reactor through-channel
• Absorber prevents backscattered neutrons from reaching detector
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The Experiment
Shielding
Reactor with Moderator sleeve
To detector
40 cm
To absorber
40 cm
• Collisions take place in reactor through-channel
• Absorber prevents backscattered neutrons from reaching detector
• Collimation prevents direct path from moderator to detector and wall scattered neutrons
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The Experiment
Shielding
Reactor with Moderator sleeve
To detector
40 cm
To absorber
40 cm
• Collisions take place in reactor through-channel
• Absorber prevents backscattered neutrons from reaching detector
• Collimation prevents direct path from moderator to detector and wall scattered neutrons
• Shielding absorbs neutrons from reactor
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Detector Count Rates and the Need for Modeling
• Detector Counts
• n-Production Rate along z-axis
• MCNP and Analytic Modeling to determine cavP
VvncP nnoavavPz
2
4
2
Spatial, angular, energy, time distributions
TVv
cN nno
avavPD
2
4
2
TPz
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MCNP Modeling
• Modeling of Yaguar reactor core with moderator sleeve
• Neutron Field Distributions in through-channel
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MCNP Modeling
Spatial Distribution Angular Distribution*
cos( z/La) cos() + A cos2(); A=0
0
0.5
1
1.5
2
0 0.2 0.4 0.6 0.8 1
1 - cos (delta)
Norm
alized tally/particle
y = 2 cos (delta)
*Amaldi and Fermi, PHYS REV 50 (1936) 899-928.
0 < < 3
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MCNP Modeling
Energy Distribution
Maxwellian (E0=26 meV) with epithermal tail (1/E)
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Geometry for Analytic Calculations
• Neutrons from source points Q1 and Q2 collide at point field point P
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Neutron Density and Collision Rate
21
21
12
0 11 23
6),(
L
zdd
AR
LSzrn
aLz
Rd
A12
1
11 coscos1cos
Dickinson, Lent, Bowman, Report UCRL-50848 (Livermore, 1970)
),(),()cos,,(),(),( 22122111 orelo vvzrnvvvzrnvv
21 dvdvdVNnn
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•Isotropic scattering in CM-frame
Pz =2Nnn/4neutrons/steradian)
•Anisotropic scattering in Lab-frame
Production Rate in Direction of Detector
= angle between vcm and z-axis
21 dvdvdVPz ),(),()cos,,(),(),( 22122111 orelo vvzrnvvvzrnvv
))cos(),(cos( 12
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Production Rate• Small r-dependence• Small dependence on
angular distribution parameter A
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Calculation of cavP
• Yaguar Anisotropic Case
Monovelocity cavP=0.78
Maxwellian dist. cavP=0.84
Angular, spatial, energy (Maxwellian only) distributions have been included.
0.802cav
• Isotropic, monovelocity ideal gas
Vvn
P
nnoav
z
2avP
24c
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Neutron Background
Sources of background
• Thermals direct from moderator sleeve Collimation
• Wall scattered thermals
Collimation
• Backscattered neutrons
Absorber
• Scattering from residual gas
10-6 Torr 2% background
• Reactor neutrons……
40 cm
Shielding
Reactor with Moderator sleeve
To detector
40 cm
To absorber
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Neutron Background
Main source is reactor vessel
• Lots of Shielding!! – 12m of concrete, borated water,…
• Early fast neutrons – Time of Flight can separate collided thermals from initial burst of fast neutrons
• Delayed fast neutrons – ToF is of no use, rely on shielding
Vary Flux: Reactor background ~, Neutron signal ~2
Two-fold approach
• Two separate teams are modeling shielding effectiveness
• Experiments in fall ‘03 to understand background characteristics under shielding beneath reactor
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Status and Future†
• Neutron-field and count-rate modeling near completion
• Shielding modeling underway(preliminary modeling of delayed fast neutrons for simplified geometry shows background at the 5% level*)
• Experimental background measurements planned for Fall ’03
• Finalize geometry Winter ’04
*G.P. Gueorguiev, et. al, Accel. App. in a Nucl. Ren., AccApp’03, June 1-3, 2003, San Diego.
†W.I. Furman, et al., J. Phys. G: Nucl. Part. Phys. 28 (2002) 2627-2641.
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DIANNA Collaboration
JINR (Dubna, Russia): W. I. Furman, E. V. Lychagin, A. Yu. Muzichka, G. V. Nekhaev, Yu. V. Safronov, A. V. Strelkov, E. I.
Sharapov, V. N. Shvetsov
ARRITP (Snezhinsk, Russia): B. G. Levakov, V. I. Litvin, A. E. Lyzhin, E. P. Magda
TUNL (Durham, NC): C. R. Howell, G. E. Mitchell, W. Tornow
Gettysburg College (G’burg, PA): B. E. Crawford, S. L. Stephenson
W.I. Furman, et al., J. Phys. G: Nucl. Part. Phys. 28 (2002) 2627-2641.
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Review article by I. Slaus et al., Physics Reports 173 (1989)
“..in order to obtain relevant information on CSB and particularly on explicit quark contributions, it is necessary to improve the accuracy [of effective range parameters], i.e., ann should be known to ± 0.2 fm…”
Four suggestions for further research:Four suggestions for further research:
““(1) Perform a direct n-n scattering (1) Perform a direct n-n scattering measurement.”measurement.”
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Shielding Modeling
• Using MCNP with energy-dependent weight windows (WWE) variance reduction
• Simplified geometry
Preliminary Results
• Fission neutrons with Einital<2.5MeV do not contribute
• Some spatial separation between background and signal neutrons at detector
• Variance reduction techniques are working but are challenging for complicated geometries.
• 5% background from delayed fast neutrons is reasonable
G.P. Gueorguiev, et. al, Accel. App. in a Nucl. Ren., AccApp’03, June 1-3, 2003, San Diego.
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Shielding Modeling
Energy Spectrum at Detector Radial Distribution of detector events
G.P. Gueorguiev, et. al, Accel. App. in a Nucl. Ren., AccApp’03, June 1-3, 2003, San Diego.