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

Duke/TUNL NCSU/TUNL Gettysburg College JINR ARRITP Direct Investigation Of a nn Association (DIANNA)

nn-Scattering Length  s  a nn 2 as k  0  ¼  s  ¾  t  ¼  s  a nn 2

Charge Symmetry Breaking – 0.5 fm   a CSB  2.5 fm a pp = (-17.3 ± 0.8) fm a nn = (-18.5 ± 0.3) fm a nn = ( ± 0.40) fm Nagels et al. NUCL. PHY B 147 (1979) 189. Howell et al. PHYS LETT B 444 (1998) 252. González Trotter et al. PHYS REV LETT 83 (1999) Huhn et al. PHYS REV C 63 (2001)

YAGUAR Reactor All-Russian Research Institute of Technical Physics Snezhinsk, Russia

YAGUAR Reactor Pulsed reactor with high instantaneous flux Annular design with open through- channel (nn-cavity) 90% enriched 235 U-salt/water solution Energy per pulse – 30 MJ Pulse duration – 900  s Fluency – 1.7x10 15 /cm 2 Flux – 1x10 18 /cm 2 /s Neutron density – 1x10 13 /cm 3

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

The Experiment Collisions take place in reactor through-channel Shielding Reactor with Moderator sleeve To detector 40 cm To absorber 40 cm Through Channel

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

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

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

Detector Count Rates and the Need for Modeling Detector Counts n-Production Rate along z-axis MCNP and Analytic Modeling to determine c avP Spatial, angular, energy, time distributions

MCNP Modeling Modeling of Yaguar reactor core with moderator sleeve Neutron Field Distributions in through-channel

MCNP Modeling Spatial DistributionAngular Distribution* cos(  z/L a ) cos(  ) + A cos 2 (  ); A=0 *Amaldi and Fermi, PHYS REV 50 (1936) <  <  3

MCNP Modeling Energy Distribution Maxwellian (E 0 =26 meV) with epithermal tail (1/E)

Geometry for Analytic Calculations Neutrons from source points Q 1 and Q 2 collide at point field point P

Neutron Density and Collision Rate Dickinson, Lent, Bowman, Report UCRL (Livermore, 1970)

Isotropic scattering in CM-frame P z =2N nn /4  neutrons/steradian) Anisotropic scattering in Lab-frame Production Rate in Direction of Detector  = angle between v cm and z-axis

Production Rate Small r-dependence Small dependence on angular distribution parameter A

Calculation of c avP Yaguar Anisotropic Case Monovelocityc avP =0.78 Maxwellian dist. c avP =0.84 Angular, spatial, energy (Maxwellian only) distributions have been included. Isotropic, monovelocity ideal gas

Neutron Background Sources of background Thermals direct from moderator sleeve Collimation Wall scattered thermals Collimation Backscattered neutrons Absorber Scattering from residual gas Torr  2% background Reactor neutrons…… 40 cm Shielding Reactor with Moderator sleeve To detector 40 cm To absorber

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

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)

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)

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., a nn should be known to ± 0.2 fm…” Four suggestions for further research: “(1) Perform a direct n-n scattering measurement.”

Shielding Modeling Using MCNP with energy-dependent weight windows (WWE) variance reduction Simplified geometry Preliminary Results Fission neutrons with E inital <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.

Shielding Modeling Energy Spectrum at DetectorRadial Distribution of detector events G.P. Gueorguiev, et. al, Accel. App. in a Nucl. Ren., AccApp’03, June 1-3, 2003, San Diego.