Operation of a DD-fusion neutron generator in building 15 R-018for detector R&D in the ArDM project (RE18)

C. Regenfus, C. Amsler, V. Boccone, W. Creus, P. OtyugovaUniversity of Zurich/CERNContact: regenfus@cern.ch

1 AbstractIn this report we describe the modalities and the safety aspects for operation of a tabletop monochro-matic (2.45 MeV) neutron generator from the company NSD-Fusion in our laboratory 15 R-018. Thegenerator is designed to run at an intensity of up to 107 n/s and serves as the replacement for an existingAmBe source (370 MBq, 2·104 n/s). The system does not contain significant amounts of radioactivematerial and the reaction rate is fully controlled by the applied DC high voltage (max. 120 kV, 15 mA).It is guaranteed to run stable down to 104 n/s. We envisage operation at 106 n/s with an average dutytime of 100 hours per month. Unshielded operation at 107 n/s would create an equivalent radiationdose of 120µSv/h in 1 m distance. A massive but still movable cylindrical radiation shield (diam. 1 m)and a lockable enclosure (3.5 x 3.5m2) brings the radiation level down to well below 0.6µSv/h atplaces of human access during operation. Operational interlocks are provided by a neutron dosemonitor, door switches and the global slow control (temperatures, voltages, currents).

2 IntroductionThe R&D project described here is part of the ArDM experiment [1] which was recently approved asCERN Recognised Experiment RE18. ArDM aims for the operation of a ton-scale liquid argon targetfor direct detection of dark matter particles which produce recoiling argon nuclei in their scatteringprocess. A high sensitivity of roughly 30 keV is projected requiring an efficient detection of both,scintillation light and ionisation charge, produced by the particle interaction with the liquid argon.

LiquidLiquid

GasGas

Drifting

Charge

Scintillation

light (128nm)

Charge read out

Charge extrac-tion, amplifica-tion, imaging

WLS onreflection foilWLS onreflection foil

PMTs

E -

fie

ld ≈ 4

kV

/cm

12

0 c

m Field shaping HV multiplier

80 cm

WIMP

Light read out

430nm

-4kV

0kV

-500kV

HV

multiplier

PEbars

LAr recirc.

Services

LEM(incl. passive

electronics)

PMT array(incl. passive

electronics)

Figure 1: Left: Schematic principle of the ArDM detector with a particle interacting in its centre, scintillationlight and ionisation charge is produced; Right: 3D sketch, showing the light detection system with the PMTarray in the liquid argon in the bottom, and the charge detection system with the Large Electron Multiplier(LEM) in the gaseous phase on top.

1

Many components of the ArDM experiment, like dewars, liquid argon recirculation and cleaningunits, vacuum components and services are currently assembled in Building 182 at CERN, togetherwith part of the light readout and HV system. The development of the light readout and charge detec-tion (LEM) is meanwhile ongoing in our laboratories at CERN.

2.1 Why do we need monochromatic neutronsNeutrons produce recoiling argon nuclei with the same signature as the expected signal and withknown energy if the incoming neutron energy and the scattering angle is known (see chapter 3.2for details). A neutron source hence serves for exploring the phasespace of interesting events, aswell as to study the suppression power of the detector for background neutrons. Those originate incosmic radiation or close by radioactive disintegrations during data taking. Neutron interactions areusually seen as the most difficult background for dark matter detectors since photons or muons pro-duce recoiling electrons, which can be discerned by their characteristic ratio of produced charge andscintillation light 1.

A deep understanding of neutron interactions with liquid argon is hence crucial for the project and isthe subject of the research program presented here. Main research items are:

• Calibration of the energy scale for nuclear recoils (quenching)

• Optimisation of the detector performance, e.g. the light yield

• Determination of the mutiplicity distribution for neutron interactions

• Measurement of the pulse shape discrimination power

• Determination of the position dependent detector response by nuclear recoils

At present we operate a 1` liquid argon test cell (Fig. 2) in our laboratory (building 15 R-018) whichwas used to develop the scintillation light read out of the experiment with wavelength shifting foilsand cryogenic PMTs. For this reason the laboratory was equipped with an UHV and ultra cleanArgon-60 gas supply system, as well as various utilities for producing the optical components, e.g. avacuum evaporator, precision balance and so on. A private workshop from our group is located juston the other side of the corridor. Liquid argon is brought-in by means of movable dewars and usedfor cooling only. The liquid argon for measurements is produced by condensing Argon-60 gas.

The cell is operated with radioactive sources, since 2008 also with a 370 MBq AmBe neutronsource, which was kindly provided by CERN under the supervision of the radio protection groupfrom CERN. The storage and operation of this source required the installation of a fire detectionsystem in the laboratory and the declaration of the room as radiation controlled area.

1In the ArDM experiment we also use the time structure of the scintillation light to discriminate background events

2

Figure 2: Some of the vacuum system (left) and the liquid argon test cell (ca. 1`) set up in our laboratory forlight collection studies. The test cell is illuminated with UV light to demonstrate the working principle of theside reflectors coated with wavelength shifter.

2.2 Previous measurements with AmBe neutrons in 15 R-018Figure 3 illustrates the setup using the above described AmBe source, which we embed in a Poly-ethylene shield for measurements. A drawback in the use of this source is the high photon back-ground, produced in the α-decay and the α-Be capture processes, and the wide spread in the energyspectrum (see fig. 4 left). A large NaI(Tl) crystal can be used to trigger on these photons, by means of

External DewarCooling LAr

Internal VesselPure LAr

Reflector/WL Shifter

PhotomultipliersActive Volume

Polyethylene shieldingNaI(Tl) detector

370 MBq Am-Be sourceγ: (Am) 60keV 34%n: 104 n/s in 4� (390±10) mm

(250±10) mmd=(74±1) mmh=(78±1) mm

d

h

Figure 3: Setup used for measuring neutron recoils.

detecting the high energy photon from the α-Be reaction. This reduces however the event rate signi-ficantly. Figure 4 right shows a preliminary result demonstrating the detection of neutrons by theircharacteristic light pulse shape, expressed on the vertical scale in the ratio of fast to slow scintillationlight emission.

3

Neutron Energy [MeV]0 2 4 6 8 10 12 14

Rela

tive

Yiel

d [A

.U.]

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

/ ndf 2χ 0.041 / 29p0 0.031± 1.7 p1 0.066± 3.3 p2 0.047± 1.1 p3 0.087± 1.2 p4 0.044± 5.3 p5 0.049± 0.74 p6 0.022± 0.92 p7 0.057± 7.7 p8 0.049± 0.97

/ ndf 2χ 0.041 / 29p0 0.031± 1.7 p1 0.066± 3.3 p2 0.047± 1.1 p3 0.087± 1.2 p4 0.044± 5.3 p5 0.049± 0.74 p6 0.022± 0.92 p7 0.057± 7.7 p8 0.049± 0.97

AmBe neutron energy (S.T. Park, 2003)

3 Gaussian fit for ToyMC

Neutron Energy Spectrum

Integrated Pulse Height [pe]0 200 400 600 800 1000

TO

T/L

L5

0n

sC

om

po

ne

nt

Ra

tio

0

0.2

0.4

0.6

0.8

1

Neutrons

Photons/Muons

AmBe source

Figure 4: Spectrum of the AmBe source and scintillation light produced by this source in liquid argon.

3 Monochromatic neutrons from DD fusion

3.1 Neutron generator from NSD-FusionThe commercially available neutron generator described here is of the deuterium-deuterium plasmafusion type [2]. The core of the neutron source consists of a grounded cylinder containing gaseousdeuterium at low pressure (10−2 mbar). An internal perforated cylindrical electrode at high voltage(typically 100 kV) induces ionisation and the formation of a glow discharge plasma (DC operation).The ionised deuterons are accelerated towards the inner electrode and accumulate in the central re-gions with typical energies of 15 keV. This is sufficient to overcome the Coulomb barrier and fusionoccurs. The emission zone (plasma) is of 25 mm length emitting neutrons isotropically. The reactionrate can be controlled reliably by the applied HV and current in a range of 104 to 107 n/s. The systemcould later be upgraded by a pulsed power supply for a time-of-flight measurement to suppress back-grounds from scattered or partially absorbed neutrons. Figure 5 shows a photograph of the source andthe power supply/controller. The certified operation time of the source is 25’000 hours, after whichthe cell must be exchanged (done by the company). Detailed information of the overall system can befound at [2] and in a dedicated design report from the producer [3].

Figure 5: NSD-DD source (left) and power units (right).

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3.2 Kinematics and rates of a neutron scattering experiment

θ

Neutron generator(D D fusion)

Scatteredneutron

RecoilnucleusTarget

Neutrondetector(tagging)

n (2.45 MeV)

0 20 40 60 80 100 120 140 160 180

0

50

100

150

200

250

Nu

cle

ar

rec

oil e

ne

rgy

[k

eV

]

Scattering angle [o]

Ar

θ

Figure 6: Setup for a neutron scattering experiment (left) and nuclear recoil energies in argon (right).

Figure 6 depicts the principle configuration for the 2.5 MeV neutron source running at 107 neutrons/s.The energy of the recoil nucleus is given by the following formula, where En is the incident kineticneutron energy and θ the scattering angle (A is the target atomic number):

Erecoil =2En

(1 + A)2

[1 + A− cos2θ − cos θ

√A2 + cos2θ − 1

]∼=

2EnA

(1 + A)2(1− cos θ)

Figure 6 to the right shows the correlation of recoil energy and scattering angle, with a maximumenergy transfer of 233 keV for central collisions in argon. This matches nicely the energy range ofinterest of our experiment.

With 107 neutrons/s and a source to target distance of 100 cm one obtains a flux of 8000 neutrons/sinto our argon test cell which has roughly 100 cm2 of surface (solid angle of 10 msr). Assuming anelastic cross section σT = 2b for 2.5 MeV neutrons on argon [6], we get a fraction R of interactions inthe l = 10 cm thick cell of

R = 1− exp(−NA ρLAr σT

Al) = 0.34 (1)

(with NA = 6×1023 and ρLAr = 1.4 g/cm3) hence about 2700 interactions on argon nuclei per second.

We now estimate the count rate for a scintillating liquid neutron detector (SCIONIX EJ301 ) witha surface of 100 cm2 at a distance of 50 cm from the argon cell (solid angle 40 msr). The neutronenergy decreases with scattering angle and we assume isotropic scattering in the lab. Assuming anaveraged np cross section σT of 3 b [7] one obtains for EJ301 liquid scintillator with a ratio of freeprotons to carbon atoms of 1.2, a density of 0.87 g/cm3, and a thickness of 7.5 cm, an interaction rateclose to one (R = 0.86). The rate in the neutron detector will therefore be around 10 counts/s providedthat all recoil protons lead to detectable signals in the photomultipliers. The results of this calculationare in agreement with preliminary Monte Carlo simulations additionally taking into account effectsfrom surrounding detector materials, such as stainless steel and Teflon.

5

3.3 Radiation safetyGeneral safety aspects including a detailed description of HV and temperature monitoring within thecentral control unit, as well as operational sequences and the web interface are described in detailin the design report mentioned above [3]. This report also describes extensively the interlock safetyfeatures, the mobile radiation shield and the induced radiation field calculation by Monte Carlo tech-niques using the MCNP framework.

In the following sections of this paper we present calculations of residual radiation during op-eration of the source done with GEANT4, extended to the whole volume of the laboratory and itsvicinity.

3.3.1 The mobile shielded enclosure

Water Extended Polyester (WEP), described in [4], was chosen as the main material to construct themovable neutron shield. This hydrogen rich material is hardly inflammable since it contains 50%of water. The water is incorporated in a liquid thermosetting polyester resin which forms a thickemulsion hardening to a material similar to a fine-grained plaster. The droplets of the emulsion arein the range 1-5µm and when cured, the aqueous phase remains trapped in these droplets within therigid polyester matrix. The final product can be easily drilled or machined and will bear substantialloads (total weight of the shield≈1 ton). However photon dose rates from a pure WEP shielding werefound unsatisfying high. The company did an extensive simulation job to find the best compositeof materials (documented in [3], pages 31–43). The table from figure 7 was taken from there undsummarises these efforts. Satisfying results were achieved with an admixture of 25% of the boratemineral Colemanite 2 (CaB3O4(OH)3 H2O). The addition of a small amount of boron proved effectivein mopping up thermal neutrons, and preventing them from producing highly penetrating 2.22 MeVhydrogen capture γ-rays. In this way an equivalent radiation dose of only 4µSv/h is generated at thesurface of the radiation shield running the generator at the maximum rate of 107 neutrons per second.

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Casting of the WEP-Colemanite mixture has been done in the form of disks which are to be gluedtogether with a polyester resin. Picture 8 shows some of these disks after manufacturing and beforebeing glued together.

2Also known as hydrated calcium borate hydroxide

6

Figure 8: Photo of some of the WEP-Colemanite compound shielding disks, resting on 1x1 m2 base pallets.

Figure 9 sketches the beam like setup in the laboratory (to scale) with the neutron generator in itsshielding, the exit collimator, the liquid argon test chamber and the liquid scintillator proton recoilcounter. The generator is oriented towards a 40 cm thick supporting concrete wall at a distance ofabout 2 m.

Reactor

WEP/Colemanite shield

axis

Top view

Wall (40 cm )

Max. height 2 m

LAr chamber

EJ301 counter

Cylindrical holein shield

Collimator

Typical geometric neutron exit cone

R ~50 cm

R 7.5cm

100 cm 100 cm

Figure 9: Setup of neutron generator, test cell and neutron detector (to scale).

3.3.2 GEANT4 simulation for purely polyethylene shielding

In a first approach a GEANT4 model was created of a cylindrical polyethylene (PE) shielding of100 cm diameter and 100 cm length around a isotropic point-like monochromatic (2.5 MeV) neutronemitter. The collimator was described by a cylindrical hole at the side of 10 cm diameter. Figure 10

7

shows the trajectories of neutrons (green) and photons (blue) created by the interaction of about 50neutrons with the polyethylene. Photon trajectories are cut at the surface of the cylindrical shield. Arough estimate of the different particle densities at the surface confirms the result from the MCNPsimulation with WEP only as absorber (first line in the table from Figure 7). It also confirms thenecessity of some thermal neutron absorber to the hydrogen rich base to cut down residual radiationstemming from photons.

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Figure 10: Neutron (green) and photon (blue) trajectories created by 2.5 MeV neutron interaction with PE.

The neutron spectrum

The energy spectrum of neutrons exiting the cylindrical collimator are shown in Figure 11. A colli-mator design as sketched in Figure 9 which suppresses the emission of intermediate energy neutrons(0.5–2.4 MeV) is presently under study.

E_n

Entries 1495Mean 0.764RMS 1.054

Neutron energy (MeV)0 0.5 1 1.5 2 2.5 3

En

trie

s

0

100

200

300

400

500

600

700

800

E_n

Entries 1495Mean 0.764RMS 1.054

Neutron energy distribution

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3.3.3 Overall radiation doses during operation

Main purpose of this report is the realistic estimation of residual radiation in and around CERN labo-ratory 15 R-018 during the operation of the neutron source at the maximum activity of 107 neutrons/s.To avoid accidental access to the neutron beam at the collimator exit (480µSv/h) and the surfaceof the radiation shield (4µSv/h) a protective cage (360x330 cm) with an operational interlock (doorswitch) is installed. In the following we discuss the results from a GEANT4 simulation taking into

Figure 12: Safety cage in lab. 15 R-018.

account the full geometry of the laboratory (720x750 cm) including the correct wall and floor thick-nesses (20 - 40 cm concrete). The cylindric radiation shield was introduced, as well as a 1 cm thickstainless steel container filled with Liquid Argon (LAr test cell). Figure 13 displays the geometric

Door

Fence

Safety cage (closed during operation)

Room 15 R-018

Office space

Doses are maximal valuesof 10 x 10 cm2 areas.

Up Down

Corridor 0.06 µSv/h

Window 0.01 µSv/h

Labo

rato

ry 0

.03

µSv/

h

Floor 0.58 µSv/h

0.33 - 1 µSv/h

Stai

rcas

e 0.

6 µS

v/h

40 cm

25 cm

20 cm 700 cm

1 m

Figure 13: The dose distribution inside and outside of the laboratory.

9

arrangement including some 50 tracks from the simulation, red lines being neutrons and blue linesphotons. Radiation doses are also displayed in figure 13 at various different locations. Through outthis report doses were determined using the following relation:

D = Φ ∗B, (2)

where D is the equivalent radiation dose, Φ the particle (neutron) flux per area and time in cm2 and sandB = 420 pSv cm2 the conversion factor valid for neutrons in the energy range from 1 - 10 MeV [5].

The displayed values for the doses were calculated by averaging over 10x10 cm2 areas but do not takein account residual radiation emerging from the shield. By far the highest doses appear in directionof the neutron emission, as in the staircase after the main concrete wall (40 cm) of the laboratory (upto 0.6µSv/h). Figure 14 shows the 2-dimensional distribution of the radiation doses at this surfacewhich is however in about 3.5 m height due its position over the lower section of the staircase.

450 400 350 300 250Position [cm]

Posi

tion

[cm

]

Dos

e [µ

Sv/h

]

200 150 100 50 0

25

50

75

100

125

150

175

200

225

250

0.1

0.2

0.3

0.4

0.5

0.6

Figure 14: The dose distribution outside the wall at an average height of roughly 3.5 m.

3.4 Neutron detection and monitoringBeside the neutron dose rate monitor LB 6411 from BERTHOLD Technologies measuring the equiv-alent ambient dose3 by moderation and consequent nuclear reaction with 3He, we purchased a liquidscintillator cell from SCIONIX. The latter contains a PMT coupled to a 3”x3” cell filled with a mix-ture of an aromatic hydrocarbon and organic fluors, C6H4(CH3)2, of a high H:C ratio of 1.21. Thedensity of the liquid is 0.9 g/cc with a number of C atoms, H atoms and electrons per cc of 4.0·1022,4.8·1022 and 2.3·1023 respectively.

3With the high sensitivity of 3 counts per nSv and an hardware interlock to the neutron generator system

10

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Figure 15: The BERTHOLD neutron dose detector (left) and the SCIONIX liquid scintillator cell measuringproton recoils in a liquid scintillator (right)

These kind of detectors are applicable for fast Neutrons (> 50 keV) and give information on the neu-tron energy, since the recoiling proton is fully absorbed in the cell. The liquid is optimised for PulseShape Discrimination (PSD) between proton and electron recoils based on their different ionizationdensities. By this means it is possible to separate neutrons from gammas events, needed to tag neu-trons from the DD generator scattered in our LAr test cell. Time of flight techniques cannot be appliedsince the emission time of the neutrons is not known.

3.5 Details and outlook on extended measurementsNeutrons with kinetic energies around some MeV scattering off the detector target exhibit identicalevent signatures as the hypothetic dark matter particles, both particles producing recoil nuclei, butcross sections for WIMPS are more than 18 orders of magnitude smaller. Therefore the even weak butubiquitous neutron background (e.g. from cosmic ray induced spallation in the surrounding material)limits the sensitivity of dark matter detection. However, the signal acceptance and the sensitivity tothe discriminating variables can be studied with neutron interactions. Here we discuss, as furtherinformation to chapter 2.1, our main motivations for measurements with monochromatic neutrons.

Pulse shape discrimination

As explained above, the light-to-charge ratio between electronic and nuclear recoils will be one wayto suppress background. In addition, a measurement of the shape (time constants) of the light pulsewill greatly enhance background suppression. This effect is especially pronounced in liquid argonand is currently under investigation by different groups worldwide. However, the discriminationpower depends strongly on the performance of various detector components and hence has to bedetermined in situ. We are currently investigating pulse shape discrimination in liquid argon betweenfast electrons and α-particles (see fig. 4 above) in our laboratory. The latter exhibit similar ionisationcharacteristics as nuclear recoils. We will repeat these measurements with our test cell (1` LAr,described in fig. 3), but exposed to the neutron source. The final calibration, however, will have to bedone with the full size detector.

Energy calibration of the nuclear recoil band

To determine the properties of the dark matter particles from the measured recoil spectrum the energyscale must be known precisely. Due to complicated luminescence quenching mechanisms (originatingin the nature of the ionisation process and the local evolution of excitation and ionisation the energy

11

scale of the luminescence differs strongly from that of electronic excitation. For electronic excitationthe quenching factor is defined to be equal to unity. For nuclear recoils the quenching factor is stillpoorly known, seeming to converge around 0.3 in liquid argon (fig. 16), while half a year ago valuesaround 0.6 were reported at conferences. The measurement of the quenching factor requires themonochromatic neutron source (setup shown in fig. 6) to determine precisely the recoil energy.

LAr Quenching Measurements

PRELIMINARY

Figure 16: Preliminary measurement of light yield (quenching factor) in liquid argon (from CLEAN [8]).

Neutron interaction multiplicity

The outstanding feature of large mass detectors is its self-shielding from background events. In par-ticular, recoil events from neutron scattering can be observed, but are in principle not distinguishablefrom signal events. However, in large detectors the probability for two or more interactions is large(due to the huge cross sections, see above), large enough to be measured precisely and compared toMonte-Carlo (MC) simulations. On the other hand, WIMP interaction do not lead to multiple scat-tering events. From a fit of the frequency for multiple neutron interactions (≥ 2) to the multiplicityspectrum we will be able to estimate the number of single neutron scatterings in WIMP searchesand thereby reduce the backround contribution. Figure 17 shows a typical MC energy spectrum ofbackground neutrons (left) and the corresponding histogram of interaction multiplicity (right) for theArDM detector. We plan to determine this spectrum later-on by illuminating the ArDM detector withthe neutron source proposed here and then derive the detection power for neutron background in theArDM apparatus.

Response of the full detector to nuclear recoils

Monochromatic neutrons can also be used to monitor the performance of the detector (stability andhomogeneity). All regions of the detector can be illuminated and events are identical to dark matterparticle interactions in respect to pulse shape and energy deposits. The multiple scattering of neutronsis an advantage since scattering angles and hence recoil energies can be determined. An externalneutron scattering detector is not needed with the full detector.

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C. Regenfus, CHIPP PSI, 15.10.07 ArDM 6

Multiple scatters

Single scatter neutrons

(indistinguishable BG)

Large mass features

Good: Self shielding (external BG sources)

1/A

tten

uati

on

Len

gth

(cm

2/g

)

Photon Energy

Photoelectric

Compton

GEANT neutron MC for ArDM

Problem: Self activity of target (e.g. 39Ar, ! Q=565keV )

• To be suppressed (high BG suppression)

-> Trigger rate (selective trigger)

• Deplete target (liquefy well gases)

LAr: X0 = 14cm

Figure 17: Energy (left) and multiplicity (right) spectra of background neutrons in ArDM (MC).

References[1] A. Rubbia (ArDM Collaboration), J. Phys. Conf. Ser. 39 (2006) 129[2] see also http://www.nsd-fusion.com[3] Neutron generator design report for CERN, NSD-Fusion Doc. No. NSD-188[4] G.D. Oliver and E.B. Moore. The neutron-shielding qualities of water-extended-polyesters.

Health Phys, 19:578580, 1970.[5] Private communication with Thomas Otto, CERN[6] For cross sections see http://atom.kaeri.re.kr/cgi-bin/endfform.pl[7] K. Kleinknecht: Detektoren fur Teilchenstrahlung, Teubner (1984) p. 121[8] mckinseygroup.physics.yale.edu/Publications/McKinsey DSU

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