RevSciInstrum_84_013304.pdf

Performance of the Los Alamos National Laboratory spallation-driven solid-

deuterium ultra-cold neutron source

A. Saunders, M. Makela, Y. Bagdasarova, H. O. Back, J. Boissevain et al.

Citation: Rev. Sci. Instrum. 84, 013304 (2013); doi: 10.1063/1.4770063

View online: http://dx.doi.org/10.1063/1.4770063

View Table of Contents: http://rsi.aip.org/resource/1/RSINAK/v84/i1

Published by the American Institute of Physics.

IBIS: An inverse geometry Brillouin inelastic neutron spectrometer for the SNS

Rev. Sci. Instrum. 84, 025113 (2013)

A laser-induced repetitive fast neutron source applied for gold activation analysis

Rev. Sci. Instrum. 83, 123504 (2012)

The neutron imaging diagnostic at NIF (invited)

Rev. Sci. Instrum. 83, 10D317 (2012)

A novel method for modeling the neutron time of flight detector response in current mode to inertial confinement

fusion experiments (invited)

Rev. Sci. Instrum. 83, 10D915 (2012)

Microtron MT 25 as a source of neutrons

Rev. Sci. Instrum. 83, 083502 (2012)

Additional information on Rev. Sci. Instrum.

Journal Homepage: http://rsi.aip.org

Journal Information: http://rsi.aip.org/about/about_the_journal

Top downloads: http://rsi.aip.org/features/most_downloaded

Information for Authors: http://rsi.aip.org/authors

Downloaded 14 Mar 2013 to 131.215.71.79. Redistribution subject to AIP license or copyright; see http://rsi.aip.org/about/rights_and_permissions

REVIEW OF SCIENTIFIC INSTRUMENTS

84

, 013304 (2013)

Performance of the Los Alamos National Laboratory spallation-driven

solid-deuterium ultra-cold neutron source

A. Saunders,

M. Makela,

Y. Bagdasarova,

H. O. Back,

J. Boissevain,

L. J. Broussard,

T. J. Bowles,

R. Carr,

S. A. Currie,

B. Filippone,

A. García,

P. Geltenbort,

K. P. Hickerson,

R. E. Hill,

J. Hoagland,

S. Hoedl,

A. T. Holley,

G. Hogan,

T. M. Ito,

1,3

Steve Lamoreaux,

1,6

Chen-Yu Liu,

J. Liu,

3,8

R. R. Mammei,

J. Martin,

3,10

D. Melconian,

M. P. Mendenhall,

C. L. Morris,

R. N. Mortensen,

R. W. Pattie, Jr.,

M. Pitt,

B. Plaster,

J. Ramsey,

R. Rios,

A. Sallaska,

S. J. Seestrom,

E. I. Sharapov,

S. Sjue,

1,4

W. E. Sondheim,

W. Teasdale,

A. R. Young,

B. VornDick,

R. B. Vogelaar,

Z. Wang,

and Yanping Xu

Los Alamos National Laboratory, Los Alamos, New Mexico 87544, USA

Department of Physics, North Carolina State University, Raleigh, North Carolina 27695, USA

Kellogg Radiation Laboratory, California Institute of Technology, Pasadena, California 91125, USA

Department of Physics, University of Washington, Seattle, Washington 98195, USA

Institut Laue-Langevin, 38042 Grenoble Cedex 9, France

Department of Physics, Yale University, P.O. Box 208120, New Haven, Connecticut 06520-8120, USA

Department of Physics, University of Indiana, 727 E. Third St., Bloomington, Indiana 47405-7105, USA

Department of Physics, Shanghai Jiao Tong University, Shanghai 200240, China

Department of Physics, Virginia Tech, Blacksburg, Virginia 24061, USA

University of Winnipeg, Winnipeg, Manitoba R3B 2E9, Canada

Cyclotron Institute, Texas AM University, College Station, Texas 77843, USA

Department of Physics and Astronomy, University of Kentucky, Lexington, Kentucky 40506, USA

Department of Physics, Idaho State University, Pocatello, Idaho 83209, USA

Joint Institute for Nuclear Research, 141980 Dubna, Russia

(Received 25 July 2012; accepted 14 November 2012; published online 14 January 2013)

In this paper, we describe the performance of the Los Alamos spallation-driven solid-deuterium ultra-

cold neutron (UCN) source. Measurements of the cold neutron flux, the very low energy neutron

production rate, and the UCN rates and density at the exit from the biological shield are presented

and compared to Monte Carlo predictions. The cold neutron rates compare well with predictions

from the Monte Carlo code MCNPX and the UCN rates agree with our custom UCN Monte Carlo

code. The source is shown to perform as modeled. The maximum delivered UCN density at the

exit from the biological shield is 52(9) UCN/cc with a solid deuterium volume of

∼

1500 cm

http://dx.doi.org/10.1063/1.4770063

]

I. INTRODUCTION

Ultra-cold neutrons (UCN) are defined as neutrons that

can be trapped in material bottles and guides because their

kinetic energies are less than the effective potential

(the

volume average of the Fermi potential),

–

Na,

where

is the number density,

is the material’s neutron

scattering length, and

is the neutron mass.

Ni exhibits one

of the largest potentials of available materials, 342 neV. Neu-

trons with kinetic energies below this potential (i.e., velocities

below 8.09 m/s) can be trapped in a

Ni bottle. Two books

and a recent review highlight the wide variety of physics that

can be performed using trapped UCN.

–

In the past decade, experiments have demonstrated

several advantages of using ultra-cold neutrons for beta

decay experiments. Measuring the decay of neutrons trapped

in material bottles eliminates systematic errors associated

with defining the volume and flux that have plagued neutron

lifetime experiments with cold neutron beams

–

and there-

fore allows more precise measurements.

–

More recently,

results obtained with the Los Alamos Neutron Science Center

(LANSCE) UCN source

have demonstrated that the high

polarizations and low backgrounds that can be obtained with

spallation-driven, pulsed UCN sources can reduce the system-

atic errors in measuring the spin dependence of neutron beta

decay

relative to cold neutron beam-based experiments.

The idea that solid deuterium (SD

) can used as a su-

perthermal UCN source goes back to a paper by Golub

and Boening.

Pokotilovski pointed out the advantages of

UCN production in SD

at pulsed neutron sources;

the use

of spallation as a pulsed source for neutrons was suggested

by the Gatchina group.

–

New UCN sources, using su-

perthermal production

with either superfluid helium or solid

deuterium as a converter material, are being built at several

facilities.

–

Recently, a very encouraging result has been

published by Masuda for a prototype source based on produc-

tion in superfluid helium.

The lifetimes of UCN in solid ortho-deuterium are lim-

ited by the neutron’s decay time, nuclear absorption of

UCNs on the deuterium, nuclear absorption on contami-

nating hydrogen, thermal upscattering from the deuterium,

0034-6748/2013/84(1)/013304/10/$30.00

, 013304-1

Downloaded 14 Mar 2013 to 131.215.71.79. Redistribution subject to AIP license or copyright; see http://rsi.aip.org/about/rights_and_permissions

013304-2 Saunders

et al.

Rev. Sci. Instrum.

, 013304 (2013)

and upscattering caused by spin conversion of contaminat-

ing para-deuterium. These lifetimes, measured in solid ortho-

deuterium at temperatures below 12 K,

are long enough to

yield high densities in a spallation driven source, in agreement

with theoretical calculations,

and correspondingly high den-

sities were measured in a small scale prototype UCN source.

UCN production rates in solid ortho-deuterium measured in

experiments at PSI

and at LANSCE

have verified the ear-

lier work in more quantitative and controlled experiments.

Here, we report on results that have been obtained with

a spallation-driven pulsed UCN source, driven with protons

provided by the 800 MeV Los Alamos Neutron Science Cen-

ter linear accelerator at the Los Alamos National Laboratory.

The source moderates and converts spallation neutrons, pro-

duced in a tungsten target, to UCN in a windowless solid

deuterium volume inside of a UCN guide system. The neu-

trons are transported through a stainless steel guide system

to an experimental area where they can be used for experi-

ments. In this paper, we describe the design of the source and

measurements and predictions of the cold neutron and UCN

production rates. We compare Monte Carlo predictions of the

UCN transport times and flux through the guide system to

their measured values, and use the validated Monte Carlo re-

sults to predict the densities available for experiments.

II. DESIGN

The details of the source are shown in the schematic

drawing in Figure

. Protons from the LANSCE 800 MeV

accelerator are delivered to a 12 cm long helium gas-cooled

tungsten alloy-spallation target. Recently (2010), the target

shape has been changed from a right circular cylinder (2 cm

diameter) to a target with an approximate square cross section

with rounded corners with 2.5 cm sides in order to increase

the amount of tungsten in the path of the proton beam. This

has improved the produced UCN rates by

∼

50%. The target

is surrounded by a room temperature beryllium reflector, in

which a volume of SD

is embedded. The SD

volume, a

vertical right circular cylinder 19.7 cm diameter and 5.7 cm

high, is contained in a liquid-2 cm thick liquid helium-cooled

FIG. 1. Cutaway view of the source. The graphite cube is 1.8 m on a side.

This entire assembly is surrounded by the biological shield, consisting of at

least 3 m of steel and 2 m of concrete in all directions.

aluminum cryostat that is coated with

Ni to reflect and con-

tain the UCN. A set of fins (apparent in the inset in Figure

)

are machined in the bottom of the aluminum cryostat to

improve the surface contact between the liquid helium and

the solid deuterium. This entire assembly is surrounded by

approximately 1 m of reactor grade graphite. Between the

solid deuterium volume and the beryllium reflector is a 1 cm

thick layer of polyethylene beads with an effective density of

0.5–0.6 g/cm

, cooled by some of the boil off gas from the

liquid helium that cools the SD

. The temperature of the

polyethylene beads depended on the proton current delivered

to the W-target and was around 150 K at an average current

of 5.8

Temperatures were typically measured using cernox sen-

sors clamped to the aluminum outer wall of the guide vacuum

chamber. These sensors provided reproducible and reliable

temperatures with roughly a 1 K uncertainty. In the case of

the polyethylene, the sensor was mounted to the outside of

the aluminum can containing the polyethylene beads, and

operational conditions (He vapor flow through the polyethy-

lene) could vary significantly. This introduces a difficult to

quantify but significant uncertainty into these temperature

measurements. Measurements and MCNPX Monte Carlo

calculations indicated only a weak dependence of the UCN

production rate on the polyethylene temperature, because a

significant amount of neutron moderation was provided by

the SD

The aim of this design was to couple the solid deuterium

volume as closely as possible to the spallation target in or-

der to take advantage of the high neutron densities possible in

spallation neutron production. Since only

∼

20 MeV of ther-

mal energy is deposited in the target assembly for each spal-

lation neutron produced, compared to

∼

200 MeV per neutron

in a reactor, the cold neutron density in a spallation-driven

source can be higher for a given cooling capability. However,

because of the small spallation source volume, the cold neu-

tron density in the SD

would fall quickly if it were moved

away from the spallation target. The technical challenge was

to provide enough cooling to the solid deuterium to keep the

temperature sufficiently low (

∼

10 K) so as not to impact the

UCN lifetimes in the target due to thermal upscatter,

while

the beam was on. The source of heating was both charged

particles and gamma rays produced in proton beam-target in-

teractions.

The lower part of the target was constructed from alu-

minum and an aluminum-beryllium alloy in order to keep the

heat load from the target walls to a minimum. Thermal iso-

lation between the solid deuterium volume and the upper tar-

get region was provided by an explosively bonded aluminum

to stainless steel thermal break. The temperature of the UCN

guide above the thermal break could be controlled by flow-

ing boil-off helium gas from the SD

cooling through a heat

exchanger built into the outer target wall.

Biological shielding was provided by

∼

2 m of steel and

1.8 m of concrete on the sides and top of the source box

with somewhat more shielding on the downstream face. The

shielding is sufficient to allow experimentalists to occupy the

experimental area with up to 10

A of beam with no precau-

tions other than standard dosimetry.

Downloaded 14 Mar 2013 to 131.215.71.79. Redistribution subject to AIP license or copyright; see http://rsi.aip.org/about/rights_and_permissions

013304-3 Saunders

et al.

Rev. Sci. Instrum.

, 013304 (2013)

The inside wall of the cryostat volume and vertical guide

was sputter-coated with 200 nm of

Ni. When the UCN ex-

ited the top surface of the SD

into the vacuum in the guide,

they received a potential boost of 102 neV; gravity was used to

cancel this effect with a 1 m vertical section of 18 cm diam-

eter guide before the stainless steel horizontal guide section

that transported the UCN out of the biological shield wall.

The 342 neV potential of

Ni ensured that all of the neutrons

that could be transported by stainless steel (potential 189 neV)

into the experimental area were also trapped by the walls in

the lower volume of the source.

A butterfly (flapper) valve, attached to the bottom of the

vertical guide above the SD

, is also shown in Figure

.This

was actuated by a rotation shaft that exited from the top of the

system and was driven by a stepping motor. The opening and

closing time of the flapper valve was about 0.1 s.

When in production, the peak proton current from the

accelerator was typically 10 mA of protons, delivered in 5

pulses of length 625

s at 20 Hz, with a gap between groups

of pulses of 5.0 s. The total charge delivered per pulse group

was 30

Cin

∼

0.2 s for an average current of 150

A during

the time when the UCN flapper valve was open. The longer

term average current delivered to the target in this mode was

5.8

Previous design calculations

predicted that the total

number of cold neutrons (CN) through the solid deuterium

volume would be about 3 CN per proton delivered to the

tungsten target (this gave a cold neutron flux density of

CN/cm

/s/

A). The UCN production was predicted

to be 250 UCN/cm

C inside the SD

volume. Because

the source performance fell below these expectations in

early tests, we conducted measurements of the flux from the

UCN source and performed new calculations with the latest

MCNP5/MCNPX codes and their standard data libraries.

–

These are described below.

III. COLD NEUTRON PERFORMANCE

Measurements of the cold neutron flux in the target vol-

ume were accomplished using two methods: (1) argon acti-

vation by the cold neutron flux and (2) direct counting cold

and very cold neutrons with a

He-based neutron detector of

known efficiency in a time of flight experiment.

A. Activated argon cold neutron flux measurement

An argon-activation experiment was performed by freez-

ing 15.2 g of natural isotopic abundance argon into the empty

volume, delivering a small amount of proton charge

through the tungsten target, and then recovering some of the

gas and counting gamma rays produced by the beta decay

Ar. The argon gas was preloaded into an external cali-

brated loading volume of 7.9 l at 1.3

Pa (Figure

This was subsequently opened to the source and UCN guides,

and the temperature and pressure were allowed to equilibrate.

The pressure after equilibration was 6.3

Pa. The vol-

ume of the SD

chamber, vertical guide, and horizontal guide

was 159 l.

FIG. 2. Schematic layout of the argon activation experiment. Parts of the

biological shield are removed in this figure so that the UCN transport is vis-

ible. The calibrated volume was used for loading, retrieving, and measuring

the decay of the argon gas. The location where the vanadium foil experiment

was performed is indicated.

The cryostat was then cooled to 60 K (freezing the argon

into the empty cold SD

volume of the cryostat) and exposed

to the neutron fluence produced by 1.52

protons im-

pacting the tungsten spallation target. The proton fluence was

measured with a Bergoz coil

to a precision of several per-

cent. The polyethylene (cold moderator) temperature for this

irradiation was 155 K. After the irradiation, the cryostat was

warmed until the pressure in the loading volume and UCN

guide system was 4.9

Pa (82% recovery). The cali-

brated loading volume was closed off from the rest of the

system, the remaining argon in the source and guides was

pumped out, and the 1.2 MeV gamma rays from the beta de-

cay of

Ar (t

1/2

126 s) in the calibrated loading volume

were counted. A sample spectrum is shown in Figure

The product of photopeak efficiency and effective solid

angle for the high purity germanium (HPGe) detector was

measured by mounting a

Co source of known activity at the

loading volume location and comparing rates of the 1.17 and

1.33 MeV

-ray peaks to the known decay rate. The finite

source volume of the argon was accounted for by averaging

the solid angle over the argon volume. From these measure-

ments, the

Ar production rate per proton was calculated to

FIG. 3. Spectrum of gamma rays from the irradiated argon (red) and the

background (blue). The main peak consists of the 1.2 MeV

-rays from

argon-41 decay.

Downloaded 14 Mar 2013 to 131.215.71.79. Redistribution subject to AIP license or copyright; see http://rsi.aip.org/about/rights_and_permissions

013304-4 Saunders

et al.

Rev. Sci. Instrum.

, 013304 (2013)

be 6.5

−

integrated over the neutron spectrum. This can

be compared to the MCNP prediction of 7.1

−

. The cold

neutron fluence was calculated from these measurements, us-

ing the thermal neutron capture cross section of

Ar of 0.66

neutron velocity dependence of the cross section,

the assumption of a Maxwellian neutron spectrum centered

at the polyethylene moderator temperature of 155 K, and a

MCNP calculation of the relative contribution of cold neu-

trons to the argon-41 activation of 60%. This gave a cold neu-

tron fluence of 0.84

0.17 neutrons/proton through the SD

source area in the absence of SD

, which corresponded to a

flux density of (1.7

0.3)

CN/cm

/s/

A. The peak

neutron flux was about 1.33

CN/s/cm

. A 20% un-

certainty, which dominated the total uncertainties quoted on

these results, was assigned to account for our lack of knowl-

edge of the exact geometry of the frozen argon in the SD

volume.

B. Time-of-flight (TOF) cold neutron flux

measurement

The time-of-flight experiment to benchmark the MCNP

cold neutron flux was performed with a

He-based neutron

detector mounted on the top of the 2 m long iron and borated-

polyethylene laminated shielding plug that closed the biolog-

ical shield package from the top. The plug had two open 2 cm

diameter vertical channels: one with a 0.75 mm aluminum

window and the other with a 2 mm pyrex window, each 2 m

from the detector. The detector viewed the cold neutron flux

through both channels. The length of the neutron flight path

in this geometry was 3.60 m. The limiting solid angle was the

2 cm diameter aperture immediately below the detector, with

a solid angle of 2.4

−

sr. The calculated efficiency of

the detector for 25-meV neutrons was 0.19. A pulse-height

spectrum from the detector in a typical cold neutron detec-

tionrunisshowninFigure

. For these measurements, pro-

tons were delivered to the tungsten spallation target in 250 ns

long pulses of

∼

1.4

protons at a rate of one pulse per

second. As in the argon-activation measurements described

above, the proton fluence was measured with a Bergoz coil.

The arrival of pulses from the neutron detector (TOF) was

measured as a function of time after the proton pulse. A typi-

cal TOF distribution with the polyethylene at a temperature of

220 K, measured with the source empty (no solid deuterium),

is shown in Figure

Figure

also shows the time-of-flight distribution mea-

sured with 1000 cc of solid deuterium at a temperature of 5 K

in the cryostat and with a polyethylene temperature of 60 K.

The distribution demonstrates a considerable enhancement of

the relative population of neutrons with a time of flight longer

than 3.35 ms, which corresponded to an energy below 6 meV.

The total number of protons delivered to the tungsten target

in this run was 5.7

. Deducing the shape of the cold

neutron energy spectrum from these data required a compli-

cated deconvolution of the TOF-spectrum due to the energy-

dependent moderation time in the source (400–600

s),

which dominated the resolution function. This width was

comparable to the width of the measured spectrum. The inte-

FIG. 4. Pulse-height spectrum from the 3He-based neutron detector used for

the CN and very cold neutron time-of-flight measurements. The measure-

ments can be observed to be very clean.

gral over the spectrum, however, did not depend on the resolu-

tion function; therefore, it could be directly compared with the

MCNP modeling. We performed such a comparison for neu-

tron flux at the detector position after taking account of the

energy-dependent

He detector efficiency. For the first spec-

trum, the integral of neutrons with energy below 100 meV was

1.56

neutrons per

C of protons, while the correspond-

ing MCNP prediction was 1.70

per

C. For the second

spectrum, the integrals below 25 meV were 0.75

per

C of protons measured and 0.86

per

C predicted. In

both cases the agreement is within 20%.

FIG. 5. Time-of-flight spectrum with the source empty and the polyethylene

moderator at 220 K (red) and the source full of SD

(

∼

1.2 l) and the polyethy-

lene at a temperature of 60 K (blue). The time of the proton pulse corresponds

to time

0. The total number of protons delivered through the tungsten tar-

get in this run was 5.2

. The volume of the target was

∼

1000 cc. The

vertical black lines mark neutron energies of 40 meV and 6 meV at times of

1350

s and 3350

s, respectively.

Downloaded 14 Mar 2013 to 131.215.71.79. Redistribution subject to AIP license or copyright; see http://rsi.aip.org/about/rights_and_permissions

013304-5 Saunders

et al.

Rev. Sci. Instrum.

, 013304 (2013)

The MCNP simulation was performed with the codes

MCNP5

and MCNPX.

We used the

(

) option of the

MCNP cross sections for SD

, graphite, and beryllium, and

the Maxwellian MCNP gas model cross sections for other

materials, including polyethylene. The MCNP5 data library

does not have polyethylene kernels for temperatures below

300 K. We therefore were forced to use the MCNP gas model

for temperatures other than ambient. In order to estimate the

systematic effect of making this change, we made compari-

son calculations (with SD

in the cryostat) using the MCNP

gas model for the 150 K polyethylene kernel and MCNP5’s

20 K methane kernel (with the density modified to achieve an

equal concentration of hydrogen atoms in the materials). The

methane calculation showed a 25% increase in the cold neu-

tron flux relative to the 150 K polyethylene calculation, thus

putting an upper limit on the effect of using the 150 K gas

model instead of the desired polyethylene kernel.

We used the geometry for the LANL ultra-cold neutron

source developed by Xu,

which takes into account all essen-

tial features of the setup shown in Figure

. The tight collima-

tion in the cold neutron flux measurement prevented a direct

MCNPX modeling of the absolute detector rates. Instead, a

two stage procedure was applied. First, the cold neutron flux

leaving the SD

surface was modeled by MCNPX in a stan-

dard way with a volume proton source.

Next, the solid angle

of the collimator-detector system was modeled using MCNP5

with a surface cold neutron source.

IV. UCN PRODUCTION

The production of UCN in solid deuterium was measured

in the experimental setup shown in Figure

. The source was

cooled and filled with solid deuterium. A detector was in-

stalled in the vertical UCN guide that led up from the SD

It was located about 165 cm above the bottom of the source.

The detector had two active regions, each 2.2 cm thick sepa-

rated by a thin nickel foil. The active area of the detector was

7.8

0.2

7.8

0.2 cm

. Because of the Fermi potential of

the nickel, the second detector saw no UCN.

FIG. 6. Geometry used in the simulations of the internal measurement of

UCN production in the SD

FIG. 7. Schematic view of the detector. Unlabeled dimensions are in cm.

The anode/cathode spacing was 0.4 mm. the center cathode, blue line, was

constructed of 25

m thick natural nickel foil. The analysis here used the

front detector.

The detector was filled with a gas mixture of 8

of CF

and (1.00

0.01)

Pa of

He. A schematic

diagram of the detector is shown in Figure

. The tungsten

target was irradiated with proton pulses and the arrival time of

pulses from the detector was measured as a function of time

after the proton pulse. The detector response was modeled

by tallying only neutrons that were absorbed in the gas. The

lifetime for absorption was determined by the

He pressure

to be 3.2 ms.

The detector was configured with an internal voltage di-

vider to place the cathode planes at 1/3 of the potential of the

anode planes using a string of 100 M

resistors. The cath-

odes were bypassed to ground through 1 nF capacitors. The

outer cathodes were constructed from electro-formed nickel

grids with 97% open area. The inner cathode was a solid

m thick nickel foil. The anode/cathode spacing was

0.4 mm.

The detector was operated at an anode voltage of

3400 V to provide sufficient anode gain to ensure a good

signal-to-noise ratio while driving the 4.5 m of coaxial cable

needed to get the signal out of the shielding package. The

signals were amplified in a fast (200 MHz) amplifier with a

gain of

∼

20, and then in an Ortec timing filter amplifier. An

integration time of 50 ns and a differentiation time of 500 ns

gave good signal-to-noise performance.

The detector was separated from the internal volume of

the UCN guide by a nickel barrier with a potential of 242 neV,

a lifetime of 3.2

−

s, and a thickness of 25

m. The SD

was modeled with a potential of 108 neV and total lifetime for

UCN of 2.0

−

s. The internal guide was modeled with a

Fermi potential of 342 neV, a non-specularity of 0.025, and a

loss per bounce of 2.8

−

Data were taken with a 625

s long beam gate, deliv-

ered at a rate of one beam gate every 30 s. Within each beam

gate, the protons were delivered in micropulses and separated

(leading edge to leading edge) by 1.788

s. The charge in

each beam pulse was about 0.5

C. The time of arrival distri-

bution in the front detector is shown in Figure

In the calculation, neutrons were chosen from a

distribution up to 200 m/s. Arrival times at the detector were

Downloaded 14 Mar 2013 to 131.215.71.79. Redistribution subject to AIP license or copyright; see http://rsi.aip.org/about/rights_and_permissions

013304-6 Saunders

et al.

Rev. Sci. Instrum.

, 013304 (2013)

FIG. 8. Comparison of data to Monte Carlo calculations of UCN arrival time

at the front detector. These data were taken with an SD

volume of 1500 cm

tallied. The resulting spectrum was normalized to the data,

and the normalization was used to determine the production

rate of neutrons with velocities below 8 m/s. The normalized

distribution is shown along with the data in Figure

.Data

were taken with deuterium in the target and with the target

empty. These are also shown in Figure

The transport of neutrons to the detector was predicted

with a Monte Carlo calculation assuming a Maxwellian en-

ergy distribution in the solid deuterium and was normalized

to the difference between full and empty. The result is shown

in Figure

. The cut off near 0.2 s corresponds to the flight

time expected for 219 nV neutrons at the surface of the SD

the minimum energy needed to overcome the sum of the grav-

itational and the aluminum barrier potential to make it into the

detector. The production rate of UCN (Neutrons with kinetic

energies below (380–109 neV) in the solid deuterium, neu-

trons that are trapped by a 58Ni potential after they receive

a 109 neV boost on exiting from the solid deuterium) can be

obtained by integrating the distribution. The resulting produc-

tion density is 85

10 UCN/

C/cm

On the other hand, relying on our successfully bench-

marked (see Sec.

III

) MCNP5 prediction of cold neutron flux,

we have simulated the cold neutron spectrum for the source.

It is shown in Figure

. The temperature of the polyethylene

in this case is 150 K. A visible structure is produced by the

MCNP data file for solid deuterium at 20 K. The fluence av-

FIG. 9. MCNP simulated cold neutron flux for the LANL ultra-cold neutron

source.

eraged over the SD

volume is 2.0

/cm

C for neu-

tron energies below 25 meV (this is the energy range effec-

tive for the UCN production

). We find good agreement be-

tween this prediction and the result obtained from argon ac-

tivation. With this spectrum, with the deuterium molecular

number density of 3

−

and with the UCN spec-

tral (in the dependence of the cold neutron energy) production

cross sections measured by Atchison,

we have calculated

the UCN production density to be 107

20 UCN/

C/cm

in reasonable agreement with the experimental result of

10 UCN/

C/cm

. With an average proton beam current

of 5.8

A and a peak current of 120

A, the source in princi-

ple can achieve a peak UCN density of 10 000 UCN/cm

.In

fact, since the volume of the SD

only fills 1/3 of the volume

below the flapper valve, the actual peak densities were only

observed to be 3000 UCN/cm

We note also that in the latest in-beam study at

LANSCE,

the UCN effective production cross section was

measured to be 1.27

−

b per molecule for the UCN en-

ergy range of 0–300 neV, in agreement with the cross sec-

tions of Atchison,

when these are integrated over the neu-

tron spectrum from LANSCE flight path FP-12, which is char-

acterized by the neutron temperature of 35 K.

The current results can also be compared with previous

measurements of the UCN production in our prototype source

of 460

90 UCN/

C/cm

a factor of 4.3

1.2 higher than

our measurements of the present source. The prototype source

volume had a diameter of 8 cm, a solid polyethylene

moderator cooled to 4 K, and a beryllium reflector cooled to

77 K. MCNP modeling suggests the smaller volume and dif-

ferent aspect ratio of the prototype SD

volume gives a factor

of two higher density when compared to the present source.

The remaining difference of a factor of two is probably due

to the combined effects of a different design and performance

of the polyethylene moderator, to different absorption of cold

neutrons in the construction materials, and to the larger tung-

sten spallation target used in the prototype.

V. TRANSPORT

Measurements of the UCN flux external to the biological

shield package were used to characterize the overall source

and guide performance. The drawings in Figure

show the

configuration used for these measurements. The flux from the

source could be measured using the main detector by opening

the gate valve (labeled in the figure). For normal operation,

the proton beam was delivered in groups of five 625

s pulses

delivered at 20 Hz with a spacing between groups of 5 s. This

is illustrated in the graph shown in Figure

. The average

current in these measurements, measured using the Bergoz

coil, was limited to about 5.0

A by the accelerator safety

system.

The neutron flux and the lifetime of UCNs in the source

and guides were measured by using UCN detectors

external

to the shield wall. The transport properties of the source were

benchmarked using two measurements. In the first, the gate

valve was closed and protons were delivered to the target at a

rate of 1 group of pulses per 60 s. Measurements were made

Downloaded 14 Mar 2013 to 131.215.71.79. Redistribution subject to AIP license or copyright; see http://rsi.aip.org/about/rights_and_permissions