1. Introduction

Currently, strong efforts are under way worldwide tobuild new sources for ultracold neutrons (UCN)(see

e.g., [1], [2], [3]). All of these new sources will usesolid deuterium (sD2) as neutron moderator. Hand-in-hand with the development of these sources, newmaterials to store and guide the UCN are also required.

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[J. Res. Natl. Inst. Stand. Technol. 110, 279-281 (2005)]

Measurement of the Loss and DepolarizationProbability of UCN on Beryllium and Diamond

Like Carbon FilmsVolume 110 Number 3 May-June 2005

Tomas BrysPaul Scherrer Institut,5232 Villingen PSI,SwitzerlandandETH Zürich,Zurich, Switzerland

Manfred DaumPaul Scherrer Institut,5232 Villingen PSI,Switzerland

Peter FierlingerPaul Scherrer Institut,5232 Villingen PSI,SwitzerlandandUniversitaet Zürich,Zurich, Switzerland

Peter GeltenbortInstitut Laue Lagevin,Grenoble, France

Mukul GuptaPaul Scherrer Institut,5232 Villingen PSI,Switzerland

Reinhold HenneckPaul Scherrer Institut,5232 Villingen PSI,Switzerland

Stefan Heule

Paul Scherrer Institut,5232 Villingen PSI,SwitzerlandandUniversitaet Zürich,Zurich, Switzerland

Klaus KirchPaul Scherrer Institut,5232 Villingen PSI,Switzerland

Mikhail LasakovPNPI Gatchina, Russia

Russel MammeiVirginia Tech,Blacksburg, USA

Mark MakelaVirginia Tech,Blacksburg, USA

Axel PichlmaierPaul Scherrer Institut,5232 Villingen PSI,Switzerland

Anatoli SerebrovPaul Scherrer Institut,5232 Villingen PSI,SwitzerlandandPNPI Gatchina, Russia

Ulrich StraumannUniversitaet Zürich,Zurich, Switzerland

Robert B. VogelaarVirginia Tech,Blacksburg, USA

Cedric Wermelinger

ETH Zürich,Zurich, SwitzerlandandPaul Scherrer Institut,5232 Villingen PSI,SwitzerlandandAlbert YoungNorth Carolina State University,Raleigh, USA

Currently several institutes worldwideare working on the development of anew generation of ultracold neutron(UCN) sources. In parallel with sourcedevelopment, new materials for guidingand storage of UCN are developed.Currently the best results have beenachieved using 58Ni, Be, solid O2 and lowtemperature Fomblin oil (LTF). All ofthese materials have their shortcomingslike cost, toxicity or difficulty of use. Anovel very promising material is diamondlike carbon (DLC). Several techniquesexist to coat surfaces, and industrialapplications (e.g., for extremely hardsurfaces) are already wide spread.Preliminary investigations using neutronreflectometry at PSI and Los Alamosyielded a critical velocity for DLC ofabout 7 m/s thus comparable to Beryllium.A low upper limit of depolarizationprobability for stored polarized UCN hasbeen measured at the PF2 facility of theInstitut Laue-Langevin (ILL) by NorthCarolina State University (NCSU), LosAlamos National Laboratory (LANL),and Petersburg Nuclear Physics Institute(PNPI), thus making it also a goodmaterial for storage and guidance ofpolarized UCN. Still missing is the lossprobability per bounce. We will be able toextract this number and a more stringentvalue for the depolarization from ourexperiment thus proving the suitability ofDLC as a wall material for a wide range ofUCN applications.

Key words: depolarization; diamond likecarbon; spin flip; ultracold neutrons.

Accepted: August 11, 2004

Available online: http://www.nist.gov/jrespeter.fierlinger@psi.chaxel.pichlmaier@psi.ch

The material of choice is Beryllium since it has a highcritical velocity of 6.9 m/s, equal to Nickel and secondto isotopically enriched 58Ni (vcrit = 8.1 m/s). Nickelhowever is magnetic and is not suitable for polarizedUCN. Beryllium on the other hand is toxic and there-fore difficult to machine and handle. One promisingalternative material is diamond-like carbon (DLC)which is non-toxic, hydrophobic, stable, and widelyused in industrial applications. Existing technologycan be used allowing a cost effective production.Experience with DLC already exists [4], [5]. At SINQwe have recently measured the critical velocity of DLCand found it to be (7.0 × 0.3) m/s, similar to Beryllium.The quality of the coatings was further investigated bythermal cycling between 450 K and 77 K and gavestable results; the adhesion to the substrate turned out tobe excellent. Contamination with other elements,especially Boron, is a concern but depends directly onthe quality of the raw material used. It is small and wellunder control.

Scientifically most interesting is a measurement ofthe depolarization probability per bounce. It has beeninvestigated in earlier experiments for DLC [5] and forBe [6] during storage. It was found to be below 3 × 10–6

per bounce for DLC and around 2 × 10–5 to 3 × 10–5 forBeryllium. A more detailed investigation of the spinflip probability as a function of energy and temperature,which we propose here, is important in order to

• clarify whether there is indeed a connectionbetween spin flip and “anomalous losses,” as hasbeen proposed recently ([6], [7]). In this picture,some UCN are spin flipped upon reflection due toincoherent scattering from para/ferro-magneticcenters on the material surface. The fraction ofUCN scattered incoherently into the material couldpossibly explain the observed “anomalous losses.”In order to study this experimentally, one has tomeasure both parameters, the depolarization andthe loss probability per wall collision, simultane-ously in the same experiment;

• to supply information on the polarization stabilityduring storage. Precision UCN experiments relyon the fact that the change of polarization duringstorage does not introduce systematic errors.Obviously, improving the accuracy for the plannedexperiments also places stronger constraints on thecontrol of the polarization. Although the sensitivi-ty to depolarization effects can be inherently low,depolarization aspects need to be studied for futurehigh-sensitivity studies [2].

Contrary to the other depolarization experiments ourset-up has no mechanical slits. Although other experi-ments typically found wall losses in the order of5 × 10–5 per bounce, theory predicts values two ordersof magnitude lower [8]. The experiment thus allowsto pin down both, the loss and the depolarizationprobability per bounce.

We use a vertical storage tube sealed on the topby gravity and on the bottom by a conventional1.5 T magnet, corresponding to a neutron energy of90 neV. The neutrons have no contact to any materialsurface other than the coating (DLC or Be). Sinceearlier experiments have been crippled by poor vacuumconditions, special care is taken to achieve the bestpossible vacuum. We use well established vacuumtechniques like baking and usage of a cryo cooler forhydrogen pumping. We are able to adjust the tempera-ture between 40 K and 450 K.

2. Experimental Set up and ExpectedResults

The experiment is carried out at the ILL-PF2 ultracold neutron installation. It works as follows: UCNfrom the turbine enter the storage volume (1, seeFig. 1) through the feeding guide (4), an Aluminumwindow and a beam switch (3). At this point themagnet (2) is still switched off, but the 10–2 T (100 G)holding field (6) in the storage volume is already on.After equilibrium neutron density is reached, the mag-net is turned on. This can be done in about 3 s. Now thebeam switch is moved such that the storage volume isconnected to the detector (5). Neutrons with the“wrong” spin state drain quickly from the storagevolume while the others are confined by the magneticfield, the coated walls and gravity (see Fig. 2). After acertain holding time the magnet is switched off and thestored neutrons fall into the detector. This procedure isrepeated with two different holding times (t1, t2) withthe number of stored neutrons Nt1 and Nt2, respectively,thus yielding the neutron lifetime in the storage volumeτtot Eq. (1):

(1)

Using τtot and the total number of wall collisions νdetermined by Monte Carlo simulations, the loss prob-ability µ per wall interaction can be calculated Eq. (2).

(2)

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( )1 2

tot1 2

.ln /t t

t tN N

−=τ

tot n

1 1= + +υβ υµ .τ τ

Neutrons that experience a spin flip (Nsp) during thestorage are no longer confined by the magnetic field.They are counted in the detector during the holdingtime. The spinflip probability β can be calculatedaccording to Eq. (3).

(3)

Since the background count rate is in the order ofonly a few ms–1, and since the “wrong” spin state drainswithin seconds from the storage volume, we detect aclear signal from the depolarized neutrons.

Alternatively we fill the storage volume with themagnet on and store only depolarized UCN. This servesas a complementary measurement cycle which helps toexclude systematic uncertainties. It must give the sameresult if all parameters are well under control.

With a loss factor µ = 10–4 and a bottle lifetime of180 s, filling and emptying time of around 2 s can beexpected. This enables us to get a ≅15 % statisticalaccuracy for the storage time τ and ≅20 % for the spinflip probability β at β = 3 × 10–6 in one measurementcycle of about 15 min.

We will test DLC coatings made by two differenttechniques on Quartz and Aluminum substrates. Forreference we will also measure Beryllium coatedsurfaces.

Acknowledgments

We would like to thank Karel Kohlik and the PSIworkshops for their competent and fast help buildingthe experiment. At the ILL we like to thank ThomasBrenner for the local support.

3. References[1] http://www.lanl.gov/orgs/p/progrpt 99 00/research/nuc_ucn.pdf

(accessed September 22, 2003).[2] http://ucn.web.psi.ch/ (accessed May 24, 2004).[3] http://www.frm2.tu-muenchen.de/frm2/secsources/de/ucn.html

(accessed May 24, 2004).[4] M. G. D. van der Grinten et al., Nucl. Instr. and Meth. A423,

421 (1999).[5] Proposal to ILL experiment 3-14-150 and 3-14-163.[6] A. Serebrov et al., Nucl. Instr. and Meth. A440, 717 (2000).[7] A. Serebrov et al., Phys. Lett. A. 313, 373-379 (2003).[8] R. Golub and J. M. Pendlebury, Rep. Prog. Phys. 42, 439

(1979).

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Fig. 1. Set up of the experiment. 1 — sample tube, 2 — magnet,3 — beam switch, 4 — feeding guide, 5 — UCN detector,6 — holding field coil.

Fig. 2. Ramping of the magnetic field during filling and spectralcleaning. The field is ramped up to 90 % of the maximum valuewhile the neutrons are filled into the storage volume. Neutrons with“wrong” spin state or too high energy are effectively removed duringthe cleaning time. The magnet is then ramped to maximum field andthe storage measurement starts.

( )sp 1 2 tott tN N N= − ⋅ ⋅ ⋅τ υ β .