Solid Oxygen based Ultra-Cold
Neutron Source
9/13/07
Chen-Yu Liu, Yun Chang Shin, Chris Lavelle,
Josh Long
(Indiana University)
Albert Young (NCSU)
Andy Saunders, Mark Makela, Chris Morris
(LANL)
Klaus Kirch (PSI)
Superthermal Process
R. Golub and J. M. Pendlebury, Phys. Lett, A53, 133 (1975)
• Cold neutrons downscatter in the solid, giving up
almost all their energy, becoming UCN.
• UCN upscattering (the reverse process) is suppressed
by cooling the moderator to low temperatures.
UCN loss in Superfluid 4He
 1

1
1
ρucn = P  τ  σ down 
+ +
+ ... 
σ

σ
σ
β
nucl.ab.
 up

• UCN density:
(Limited by loss)
• The figure of merit:
s
Isotop
2
D
4
He
coh
inc
a
a
s/a
purity
Debye T
5.59
2.04
0.000519 1.47104 99.82
110
1.13
0
0
20

15
N
5.23
0.0005
5
0.000024 2.110
99.9999
80
16
O
4.23
0
0.00010
2.2104
99.95
104
11.7
0
0.00049
2.38104 99.93
105
208
Pb
Dynamics of UCN Production -Defeat thermal equilibrium
●
Extract UCN out of the source before it is thermalized
 Spallation N source +
Separation of the source and the storage by a valve
UCN production in Solid D2
• Incoherent scattering (inc = 2.04 barn)
• The difference of singlet and triplet scattering
• Coherent contribution (  coh= 5.59 barn)
– In a cold neutron flux with a continuous spectrum, more neutrons
could participate in the UCN production.
(1,1.73,0)
(1,1,0)
(1,0,0)
UCN loss in Solid D2
Storage
bottle
Nuclear absorption by S-D2
 ~ 150 msec
Nuclear absorption by Hydrogen
Impurities,  ~ 150 msec/0.2% of H
Solid D2
UCN upscattering by
phonons
 ~ 150 msec at T = 5K
UCN upscattering by para-D2
 ~ 150 msec/1% of para-D2
Los Alamos s-D2 UCN
Prototype Source
C. Morris et al., Phy. Rev. Lett.
89, 272501 (2002)
World record
•
•
Source has para-D2: 4%
Bottled UCN density: 100 UCN/c.c. in a S.S. bottle 1 m away from
the source. (world record)
Best vacuum:
104
atoms/c.c.
PSI, NCSU-Pulstar, FRM, etc..
are building solid D2 based UCN source.
Solid Oxygen as a UCN Source
• Electronic spin S=1 in O2 molecules.
• Nuclear spin = 0 in 8O
• Anti-ferromagnetic ordering
– -phase, T < 24K.
P.W. Stephens and C.F. Majkrzak, Phys. Rev. B 33, 1 (1986)
UCN Production in S-O2
• Produce UCN through magnon
excitations.
– Magnetic scattering length ~ 5.4 fm.
• Null incoherent scattering length.
• Small nuclear absorption probability.
 A very large source
possible.
C.-Y. Liu and A.R. Young
UCN production in Solid Oxygen
•
•
•
Production rate
– P = 2.7  10-8 0 (30K CN in S-O2)
– P = 3.0  10-8 0 (15K CN in S-O2)
– P = 1.5  10-8 0 (30K CN in S-D2)
– Gain ~ 2 relative to S-D2
Lifetime
– 375 ms in S-O2
– 40 ms in S-D2
– Gain ~ 10
Volume gain, (l)n, n= 1-3
– lucn = 380 cm in S-O2
– lucn = 8 cm in S-D2
– Gain ~ 50 - 105
Compared with S-D2,
Gain > 1000 is possible !
Some Recent Results of UCN
Production in Solid O2
•
PSI-SINQ (2005)
•
No superthermal
temperature dependence.
– CN =
(4.51.0)107/cm2-s-mA
– Indicates unknown
source of UCN loss.
•
UCN yield is correlated
with how the crystal is
prepared.
•
The UCN yield (best
number) is ~ 3 times less
than s-D2.
•
A peak in the - phase
transition. (critical
scattering?)
UCN Production in D2 and CD4
• PSI, 2005
• From D2 and CD4.
• Signature temperature dependence of a superthermal
source.
Cold Neutron Transmission
(TOF)
•
PSI-SINQ
•
•
•
Flight path =2.83m.
Neutron Chopper.
Scattering probability
•
– CN =
(4.51.0)107/cm2-smA
– I0(E)-I(E)/I0(E)
Features:
– Less scattering
compared with D2.
– Bragg edges
– Additional Bragg peak
in alpha phase.
(indicate the
presence of a
magnetic
structure.)
UCN Production vs.
CN Transmission
Material: solid O2
Anti-Correlation of UCN production vs CN
scattering
Data from 2005
PSI run (1 week)
UCN production was not effected by temperature or phase.
Something (other than downscattering) is dominating the yield of UCN.
Probe the Magnon Mechanism
using a B field
Spin flop transition
around 7 Tesla.
C. Uyeda at. al., J. Phys. Soc. Jpn. 54, 1107 (1985)
•
An external magnetic field to perturb the magnon dispersion curve
– Change the density of states. An unique feature of oxygen!
– Optimize UCN production.
•
Definitive demonstration of the magnon mechanism.
UCN Source Cryostat at IU
Superconducting Solenoid &
Solid O2 Target Cryostat
SC solenoid Cryostat
• 5.5T with
90 Amp
SC Solenoid Power
Supply
Flow He Cryostat
for O2 target
• Optical cell
beta-gamma phase
transition
(slow cool-down
~0.017K/min)
beta phase
(slow vapor deposition)
O2 Gas Handling
System (all VCR)
beta phase
(slow cool down)
Program of O2 UCN Source
IU: Yunchang Shin (graduate student), Chris Lavelle(postdoc), Chen-Yu Liu
Collaborators from LANL : Andy Saunders, Mark Makela, Chris Morris
NCSU: Albert Young
•
This summer (July – October)
•
Lujan Center (ER2) Flight Path 12
– UCN production under B field
– CN TOF transmission
– UCN gravity spectrometer
•
PHAROS: one week beam time to measure
S(alpha, beta) in solid oxygen under high field.
•
Build an university based UCN Source coupled to LENS at IUCF.
– Cold neutron flux: 3.5e+9 CN/cm2-s
(proton: 13 MeV, 2.5mA(avg), 2 cm away from the 22K moderator,
hTCN=35K)
– UCN density: 95 UCN/cc, UCN fluence: ~ 1e+6 UCN/s
– Gamma heating: 0.003W/gram
Conclusions
•
Magnons in the AF phase of S-O2 offer an additional channel
for inelastic neutron scattering.
–
–
–
–
UCN production rate in S-O2~ (1-2)  in S-D2.
UCN lifetime in S-O2 ~ 10  in S-D2.
Larger source possible. (at least 10  S-D2)
UCN current output from S-O2 (at least) 100  from S-D2
•
UCN Source Program
– LENS provides a unique opportunity to study and develop a S-O2
based UCN source.
– FP12 to study magnon mechanism in solid oxygen.
•
Broader impacts
– A positive result would have a major impact on other UCN sources in
proposal/construction
• PSI, TUM, NCSU Pulstar source, national UCN facility at LANSCE…
– A high UCN flux will open up opportunities to perform several UCN
based fundamental experiments, e.g. a UCN nnbar experiment.