1. 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)

2. 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.

3. UCN loss in Superfluid 4He
UCN density:
(Limited by loss)
The figure of merit: 3

4. 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

5. 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) 5

6. UCN loss in Solid D2 Storage bottle Solid D2
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

7. Los Alamos s-D2 UCN Prototype Source
C. Morris et al., Phy. Rev. Lett.
89, (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)
PSI, NCSU-Pulstar, FRM, etc..
are building solid D2 based UCN source.
Best vacuum: 104 atoms/c.c.

8. Solid Oxygen as a UCN Source
Electronic spin S=1 in O2 molecules.
Nuclear spin = 0 in 8O
Anti-ferromagnetic ordering
-phase, T < 24K.
UCN Production in S-O2
Produce UCN through magnon excitations.
Magnetic scattering length ~ 5.4 fm.
Null incoherent scattering length.
Small nuclear absorption probability.
P.W. Stephens and C.F. Majkrzak, Phys. Rev. B 33, 1 (1986)
A very large source possible.

9. UCN production in Solid Oxygen
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 ~
Compared with S-D2,
Gain > 1000 is possible !

10. Some Recent Results of UCN Production in Solid O2
PSI-SINQ (2005)
CN = (4.51.0)107/cm2-s-mA
No superthermal temperature dependence.
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?)

11. UCN Production in D2 and CD4
PSI, 2005
From D2 and CD4.
Signature temperature dependence of a superthermal source.

12. Cold Neutron Transmission (TOF)
PSI-SINQ
CN = (4.51.0)107/cm2-s-mA
Flight path =2.83m.
Neutron Chopper.
Scattering probability
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.)

13. UCN Production vs. CN Transmission
Material: solid O2

14. 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.

15. 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.
Optimize UCN production.
Definitive demonstration of the magnon mechanism.
An unique feature of oxygen!

16. UCN Source Cryostat at IU

17. Superconducting Solenoid & Solid O2 Target Cryostat
5.5T with 90 Amp
SC solenoid Cryostat
SC Solenoid Power
Supply
Flow He Cryostat
for O2 target

18. O2 Gas Handling System (all VCR)
Optical cell
beta-gamma phase
transition
(slow cool-down
~0.017K/min)
beta phase
(slow vapor deposition)
beta phase
(slow cool down)
O2 Gas Handling System (all VCR)

19. 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

20. 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.