Neutron Billiards: Direct measurement of the neutron-neutron scattering length at the YAGUAR reactor...
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Transcript of Neutron Billiards: Direct measurement of the neutron-neutron scattering length at the YAGUAR reactor...
Neutron Billiards: Direct measurement of the neutron-neutron scatteringlength at the YAGUAR reactor
Bret Crawford
March 21, 2007
Direct neutron-neutron scattering measurement of ann (neutron-neutron
scattering length)
• Experimental Goal– Make the first direct measurement of ann (~strength of
attraction between two neutrons) to a precision of 3%
• Motivation– Current indirect results for ann are in conflict
– Current lack of precision in ann does not constrain theory
Scattering Length
2
04lim nns
ka
ka o
knn
sinlim
0
0Defined in terms of the phase shift, o,
or the low-energy cross section
Total cross section is combination of states
but Pauli Exclusion prevents triplet state, so
stsnn 4
1
4
3
4
1
2nnnn a
pp vs. nn
• Is the strength of the strong interaction between two protons the same as between two neutrons?
Experiment says No.Experiment says No.
• How different are they? • Can we test different theories by measuring this effect?
p-p n-n
pp vs. nn: Charge Symmetry Breaking
app = (-17.3 ± 0.8) fm
ann = (-18.5 ± 0.3) fm (-d capture, n-d breakup)
ann = (-16.27 ± 0.40) fm (n-d breakup)
aCSB = (app – ann)
Use aCSB to test theory. But the magnitude and sign of aCSB are uncertain!
We need a direct measurement of aWe need a direct measurement of annnn . .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) 3788. (1999) 3788.
Huhn et al. Huhn et al. PHYS REV C PHYS REV C 6363 (2001) 014003. (2001) 014003.
Neutron Scattering – a way to investigate the strong force
Many processes depend on the nature of the strong force
• Elastic scattering (deflection angle)
• Inelastic scattering (deflection angle, energy loss)
• Neutron capture (gamma ray emission)
• Fission (fission products)
• Reactions (reaction products)
Target nucleiNeutron beam
Cross section
• We measure cross sections and then relate the cross section to more fundamental parameters (like scattering length)
• Units of area, like cross sectional area
• Represents probability of a particular process happening
224 cm1010barn10~ nn
pp vs. nn: Charge Symmetry Breaking
app = (-17.3 ± 0.8) fm
ann = (-18.5 ± 0.3) fm (-d capture, n-d breakup)
ann = (-16.27 ± 0.40) fm (n-d breakup)
aCSB = (app – ann)
Use aCSB to test theory. But the magnitude and sign of aCSB are uncertain!
We need a direct measurement of aWe need a direct measurement of annnn . .
But there are NO neutron targets!!But there are NO neutron targets!!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) 3788. (1999) 3788.
Huhn et al. Huhn et al. PHYS REV C PHYS REV C 6363 (2001) 014003. (2001) 014003.
No Beam on Targetso
Beam on Beam
Chances of collision are very small unless we have very, very intense beams
(flux~1018 neutrons/cm2/s).
Where can you find lots of free neutrons?
Where can you find lots of free neutrons?
Where can you find lots of free neutrons?
Where can you find lots of free neutrons? …how about a secret city in Russia?
Snezhinsk (Chelyabinsk-70) has got ‘em!
YAGUAR ReactorAll-Russian Research Institute of Technical Physics
Snezhinsk, Russia
ISINNInternational Seminar on Interactions of Neutrons with
Nuclei
Frank Laboratory of Neutron Physics Joint Institute for Nuclear Research
Dubna, Russia
Experiment at YAGUAR first proposed at ISINN-8 (2000)
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 – 680s• Fluency – 1.7x1015 /cm2
• Flux – 1x1018 /cm2/s• Neutron density – 1x1013 /cm3
The Experiment• Polyethylene moderator
inserted in to through channel (thermal neutrons)
• ann can be found by relating the number of collisions to the number of neutrons in the cavity during the pulse
• Goal – count the number of n-n collisions by counting ONLY scattered neutrons
• All other neutrons MUST be stopped before the detector– Collimators, absorbers,
sheilding
Shielding
detector
absorber
Reactor with Moderator sleeve
2n
nnnn
n
Na
Shielding
detector
absorber
Reactor with Moderator sleeve
Count = 1
Shielding
detector
absorber
Reactor with Moderator sleeve
Count = 0
Shielding
detector
absorber
Reactor with Moderator sleeve
Count = 0
Shielding
detector
absorber
Reactor with Moderator sleeve
Count = 1
Shielding
detector
absorber
Reactor with Moderator sleeve
Count = 1
Shielding
detector
absorber
Reactor with Moderator sleeve
Count = 1
Shielding
detector
absorber
Reactor with Moderator sleeve
Count = 1
Shielding
detector
absorber
Reactor with Moderator sleeve
Count = 1
Shielding
detector
absorber
Reactor with Moderator sleeve
Count = 1
Shielding
detector
absorber
Reactor with Moderator sleeve
Count = 1
Shielding
detector
absorber
Reactor with Moderator sleeve
Count = 2
Shielding
detector
absorber
Reactor with Moderator sleeve
Count = 3
Shielding
detector
absorber
Reactor with Moderator sleeve
Count = 3
Shielding
detector
absorber
Reactor with Moderator sleeve
Count = 3
Shielding
detector
absorber
Reactor with Moderator sleeve
Count = 3
Shielding
detector
absorber
Reactor with Moderator sleeve
Count = 4
Shielding
detector
absorber
Reactor with Moderator sleeve
Count = 4
Shielding
detector
absorber
Reactor with Moderator sleeve
Count = 4
Shielding
detector
absorber
Reactor with Moderator sleeve
Count = 5
Shielding
detector
absorber
Reactor with Moderator sleeve
Count = 5
Shielding
detector
absorber
Reactor with Moderator sleeve
Count = 5
Shielding
detector
absorber
Reactor with Moderator sleeve
Count = 5
Shielding
detector
absorber
Reactor with Moderator sleeve
Count = 6
Shielding
detector
absorber
Reactor with Moderator sleeve
Count = 6
Shielding
detector
absorber
Reactor with Moderator sleeve
Count = 6
Shielding
detector
absorber
Reactor with Moderator sleeve
Count = 7
Shielding
detector
absorber
Reactor with Moderator sleeve
Count = 7
Shielding
detector
absorber
Reactor with Moderator sleeve
Count = 7
Shielding
detector
absorber
Reactor with Moderator sleeve
Count = 7
Shielding
detector
absorber
Reactor with Moderator sleeve
Count = 8
Shielding
detector
absorber
Reactor with Moderator sleeve
Count = 8
Shielding
detector
absorber
Reactor with Moderator sleeve
Count = 8
Shielding
detector
absorber
Reactor with Moderator sleeve
Count = 9
Shielding
detector
absorber
Reactor with Moderator sleeve
Count = 9
Shielding
detector
absorber
Reactor with Moderator sleeve
Count = 10
The Experiment• Neutron collisions take place in
reactor through-channel• Neutrons are detected 12 m
below reactor• Time of flight determines
neutron energy • nn determined from detector
counts and measured average neutron density
• Expect ~150 counts/pulse• ~30 days of pulses should
achieve required statistics
TVvfnN relannD2
To Do…
• Vacuum system, shielding, collimation (JINR, ARRITP, TUNL)
• Neutron detectors (JINR)• Data acquisition electronics (JINR, ARRITP,
TUNL)• Computer modeling
– Characteristics of neutron field (GC)
– Detector count rate sensitivity to neutron field characteristics (GC)
– Neutron background (JINR, ARRITP)
• Background Test Experiment (JINR, ARRITP)
Neutron Background
Sources of background • Thermals direct from moderator sleeve
– Collimation, neutron absorbers
• Wall scattered thermals – Collimation, neutron absorbers
• Backscattered neutrons– Long reverse flight path, 10B absorber
• Scattering from residual gas– Pressure <10-6 Torr
• Initial fast neutrons– TOF and thick shielding
• Delayed fast neutrons– Thick Shielding
Fast vs. Thermal TOF spectra
“Back Wall” Background
MCNP modeling of neutron background
Neutron speed
Source of background
Number of neutrons per pulse
Fast (>0.5eV)
Initial and delayed
~10
Thermal
(<0.5eV)
Back wall ~10
Collimators/walls <10
Residual gas P(H2)~10-7 <1
P(N2)~10-6 <1
Total 20—40
A. Yu. Muzichka, et al., NIM-A 2007 (accepted for publication).
Background Modeling Tests
• Neutron flux measured as a function of depth in underground channel.
• Neutron flux modeled with MCNP
• Thermal neutron flux agrees with model (3He ionization detectors)
• Fast neutron flux also agrees with modeling
Thermal Neutron vs. depth
Open circles = measuredClosed circles = modeled
Fast Neutrons vs. depth
Open circles = measuredClosed circles = modeled
Detector Count Rates and the Need for Modeling
• Detector Counts
avg density
anisotropy factor
avg relative velocity
effective solid angle
• MCNP and Analytic modelingSpatial, angular, energy, time distributions
TVvfnN relannD2
n
af
relv
modeled & measured modeledmodeledmodeled & measured
eff
MCNP Modeling of Neutron Field• Model YAGUAR reactor core with moderator sleeve
• Determine Neutron Field Distributions in through-channel
Reactor geometry for MCNP
Iron vessel
235U solution
Polyethylenemoderator
Side view
Top view
MCNP Modeling of Neutron Field
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 of Neutron Field
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
• Calculate neutron density, collision rate, detector count rate
Collision Rate Expression
Nine-dimensional integral that depends on geometry and velocity distribution of neutrons
21
21
22
02
21
21
12
01
02
01
2
000 L
zdd
L
zdddvdvddsinrdrR
R
col
22
21
212211
2
23
6oo vv
orelvv
o evv)cos,v,v(vevvAR
LS
LzcosRd
cosAcosLzcos
Rd
cosAcos22
2
2212
1
11 11
No analytic solution…numerically integrate on computer.
Numerical Calculation – PZSIM.f90
• Choose collision point, source points, velocities• Calculate differential collision rate (big integral)• Simulate isotropic scattering in CM frame• Transform neutron velocities back to LAB frame• Follow neutron trajectories• Sum differential collision rate for 4 and detected
neutrons• Sum differential collision rate in velocity and time
bins to create spectra
Collision Rate vs. position
Velocity Spectrum
•How does velocity spectrum change after collisions?
Detector Time-of-Flight Spectrum
t (ms)
rD nn(t
)/ nn
(105 /s
/cm
2 /ms)
dete
ctor
coun
ts/m
s
0 2 4 6 8 100
0.25
0.5
0.75
1
1.25
1.5
1.75
2
2.25
0
10
20
30
40
50
60
70MT detMT 4 scaled
•~1012 simulated collisions
•4 spectrum scaled by eff
•Very little effect from scattering anisotropy
Modeling Results
• Anisotropy factor
• Effective Solid Angle
of 4
oorel v1.60vv
22• Average Relative Velocity
•Ideal Max., isotropic gas
•YAGUAR pure Max.
•YAGUAR Max.+Tail
orel v.v 731
orel 1.84vv
980.fa
610474 .eff
Status• Shaft holes in floor
and ceiling completed
• Vacuum pipes tested
• Building collimation system
• Preparing for run this calendar year
Vacuum testing of upper section of neutron channel.
Summary
• nn-scattering experiment at the YAGUAR reactor is in final construction phase.
• Background modeling indicates possibility of keeping background from all sources to ~20%.
• Measurements of neutrons in the underground channel confirm models for thermal neutron background.
• Modeling of neutron field and nn-scattering kinematics allows accurate extraction of scattering cross section from detector counts.
• Time dependence of nn-scattering kinematics has been studied already in more detail this summer.
Geometry for Analytic Calculations
• Neutrons from source points Q1 and Q2 collide at point field point P
• Calculate neutron density, collision rate, detector count rate
Time-of-Flight SpectrumPure Maxwellian flux vs. realistic flux (Maxwellian plus epithermal tail)
before and after scattering
t (ms)
r nn(t
)/ nn
(1010
/s/c
m2 /m
s)
dete
ctor
coun
ts/m
s
0 2 4 6 8 100
0.25
0.5
0.75
1
1.25
1.5
1.75
2
2.25
0
10
20
30
40
50
60
70
MT afterM afterMT beforeM before
Duke/TUNLNCSU/TUNLGettysburg College
JINR (Dubna)
ARRITP (Snezhinsk)
Direct Investigation
Of ann
Association(DIANNA)
The Atom
• Electrostatic attraction between electrons and protons holds electrons in orbit
• Electrostatic repulsion between protons tries to push apart nucleus (several pounds of force!!)
• Nuclear Strong Force binds protons and neutrons to form stable nucleus
Electrons (~10-30 kg, negative charge) in orbits far from nucleus
Nucleus contains protons (~10-27 kg, positive charge) and neutrons (~10-27 kg, no charge)
Cross section• We measure cross sections and then relate the cross section to
more fundamental parameters (like scattering length)
• Units of area, like cross sectional area
• Represents probability of a particular process happening
• Example: Attenuation of beam through target material
224 cm1010barn10~ nn
xno
toteNxN )(x
target
n = target # density (1/cm3) = cross section (cm2)x = distance (cm)