Modeling the YAGUAR Reactor Neutron Field and Detector Count Rates in the Direct ann

Measurement

Bret Crawford and the DIANNA Collaboration

June 9, 2003

Duke/TUNLNCSU/TUNLGettysburg College

JINR

ARRITP

Direct Investigation

Of ann

Association(DIANNA)

nn-Scattering Length

sann2 as k 0

¼s¾ t¼ sann

2

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..

YAGUAR ReactorAll-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 235U-salt/water solution

• Energy per pulse – 30 MJ

• Pulse duration – 900s

• Fluency – 1.7x1015 /cm2

• Flux – 1x1018 /cm2/s

• Neutron density – 1x1013 /cm3

 

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

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 cavP

VvncP nnoavavPz

2

4

2

Spatial, angular, energy, time distributions

TVv

cN nno

avavPD

2

4

2

TPz

MCNP Modeling

• Modeling of Yaguar reactor core with moderator sleeve

• Neutron Field Distributions in through-channel

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

MCNP Modeling

Energy Distribution

Maxwellian (E0=26 meV) with epithermal tail (1/E)

Geometry for Analytic Calculations

• Neutrons from source points Q1 and Q2 collide at point field point P

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

•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

Production Rate• Small r-dependence• Small dependence on

angular distribution parameter A

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

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

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) 2627-2641.

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.

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.”

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.

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.