Table Of ContentPreprinttypesetinJINSTstyle-HYPERVERSION
The µTPC Method: Improving the Position
Resolution of Neutron Detectors Based on MPGDs
Dorothea Pfeiffera,b∗, Filippo Resnatia,b†, Jens Birchc, Richard Hall-Wiltona,d,
Carina Höglunda,c, Lars Hultmand, George Iakovidisb,e, Eraldo Oliverib,
Esko Oksanena, Leszek Ropelewskib, Patrik Thuinerb,f
5
1 aEuropeanSpallationSource(ESSAB)
0 P.O.Box176,SE-22100Lund,Sweden
2
bCERN
n
CH-1211Geneva23,Switzerland
a
J cLinköpingUniversity
0 IFMSE-58183Linköping,Sweden
2 dMid-SwedenUniversity
SE-85170Sundsvall,Sweden
]
t eBrookhavenNationalLaboratory
e
d P.O.Box5000,Upton,11973-5000,USA
- fViennaUniversityofTechnology
s
n 1040Vienna,Austria
i
.
s
c E-mail: [email protected], [email protected]
i
s
y
h ABSTRACT: Due to the 3He crisis, alternatives to the standard neutron detection techniques are
p
becoming urgent. In addition, the instruments of the European Spallation Source (ESS) require
[
advances in the state of the art of neutron detection. The instruments need detectors with excel-
1
v lent neutron detection efficiency, high rate capabilities and unprecedented spatial resolution. The
2
Macromolecular Crystallography instrument (NMX) requires a position resolution in the order of
2
0 200 µm over a wide angular range of incoming neutrons. Solid converters in combination with
5 Micro Pattern Gaseous Detectors (MPGDs) are proposed to meet the new requirements. Charged
0
. particlesrisingfromtheneutroncapturehaveusuallyrangeslargerthanseveralmillimetresingas.
1
0 This is apparently in contrast with the requirements for the position resolution. In this paper, we
5 present an analysis technique, new in the field of neutron detection, based on the Time Projection
1
: Chamber (TPC) concept. Using a standard Single-GEM with the cathode coated with 10B4C, we
v
i extracttheneutroninteractionpointwitharesolutionofbetterthanσ =200 µm.
X
r
a KEYWORDS: Neutrondetection;Solidconverters;10B4C;Gasdetectors;Positionresolution;
TPC.
∗Correspondingauthor.
†Correspondingauthor.
Contents
1. Introduction 1
2. Experimentalsetups 2
2.1 Triple-GEMdetectorsetup 2
2.2 Single-GEMdetectorsetup 2
2.3 DetectorreadoutandDAQ 3
3. TPCanalysis 4
4. Conclusions 8
1. Introduction
TheEuropeanSpallationSource(ESS)[1]inLund/Swedenisforeseentobeoperationalin2019. It
willbecometheworld’smostpowerfulthermalneutronsource,withasignificantlyhigherbright-
ness than existing reactor sources like the Institut Laue-Langevin (ILL) [2] or other spallation
sources like the Spallation Neutron Source (SNS) [3] and the Japan Proton Accelerator Research
Complex (J-PARC) [4]. Currently 22 neutron scattering instruments are planned as the baseline
suiteforthefacility[5]. Efficientthermalneutrondetectorsareacrucialcomponentforeachofthe
instruments[6]. Thedominantdetectortechnologyatneutronscatteringinstrumentssofarhasbeen
gaseous 3He detectors [7]. Due to the 3He crisis [8, 9] however, an extensive international R&D
program is currently under way in order to develop efficient and cost-effective detectors based on
other isotopes [10, 11, 12]. The solid isotope 10B has a significant neutron capture cross section
and might thus be an alternative to 3He as neutron converter. The charged particles created in a
solidneutronconverterhavesubsequentlytobeextractedandelectricallydetected. Forlargearea
detectors,theonlyrealisticchoicesaregaseousdetectorslikegasproportionalcounterssurrounded
bythinfilmsof10B-enrichedboroncarbide[13,14,15].
Some ESS instruments like the Macromolecular Diffractometer (NMX) [5] though do not
necessarilyrequire alargedetectorarea. Whereas macromolecularcrystallographyinstrumentsat
reactorsourcestypicallyuseneutronimageplates[16]withca200µmspatialresolution,spallation
sourceinstrumentsrequiretimeresolutionthattheimageplateslackaltogether. Scintillationbased
detectors [17, 18] are currently limited to ca 1 mm spatial resolution. For these instruments solid
converters in combination with Micro Pattern Gaseous Detectors (MPGDs) [19] are a promising
option to achieve the spatial resolution of the neutron image plate with time resolution, high-rate
capabilities and a good neutron detection efficiency. The position resolution of a MPGD with
a solid neutron converter depends on three factors: The average range and the direction of the
charged particle in the gas, the diffusion of the charge and the pitch of the read-out strips. In the
–1–
case of a 10B layer, a neutron is captured by the 10B nucleus, resulting in an α particle and a 7Li
ionthatareemittedbacktoback:
10B+n→7Li∗+α+γ(0.48MeV), Q=2.3MeV (93%)
10B+n→7Li +α, Q=2.79MeV (7%)
Theα particlehasanenergyofE <1.78MeVwhenitenterstheactivegasvolume. Atthiskinetic
kin
energytheaveragerangeamountstoabout8mminAr/CO (70%/30%)atatmosphericpressure. A
2
reconstructionofthepositionoftheimpingingneutronbasedonthecentroidofthetrackresultsin
anerrorof50%oftheprojectedtracklength. TheµTPCconcepthasbeendevelopedbytheATLAS
collaborationandisusedinparticlephysics[20,21]. Itsapplicationleadstoaclearimprovement
inthereconstructionofthepositionsofmuonsdetectedwithMicromegas. Apositionresolutionof
100µmindependentoftheangleofincidenceisreached. Byapplyingthistypeofanalysisforthe
first time to neutron detectors, it is possible to reliably identify the beginning of all α and 7Li ion
tracksandthustoreconstructthepositionoftheincidentneutronattherequiredresolutionforthe
NMX instrument. This work consists of a description of the setup, the measurement and the data
analysisthatwasperformedinordertoretrievethefullinformationonthepositionoftheneutron
conversion.
2. Experimentalsetups
Thedetectorsusedforthemeasurementswereonestandard10x10cmTriple-GEM[22]andone
10x10cmSingle-GEMdetector.
2.1 Triple-GEMdetectorsetup
TheTriple-GEMsetupissketchedinfigure1. Asolidlayerof10B Cof1 µmthicknesssputtered
4
ontoa25 µmthickaluminiumfoil[13,23]wasusedasneutronconverter. Itcoveredonlyhalfof
theactiveareaofthedetector,andwaskeptatthesamepotentialasthestandarddetectorcathode
installed above. The drift length of the detector was 8 mm, long enough to fully contain the α
particlesand7Liionsfromthe10B(n,α)7Liand10B(n,αγ)7Lireactions. Atroomtemperatureand
atmospheric pressure, the entire detector volume was constantly flushed with a flow of 3 l/h with
Ar/CO 70/30 mixture finally released into the atmosphere. The high voltage was supplied via a
2
resistordivider(seevaluesinfigure1). Thecurrentthroughthedividerwassetto565µAinorder
to obtain an effective gain of about 60. A low gain is sufficient due to the large amount of energy
depositedbytheionsofinterestinthegas. Thedriftfieldof700V/cmwaschosentoavoidelectron
attachment to electronegative impurities and the subsequent loss of primary ionization electrons,
whilekeepingthedriftvelocitysmallerthan2.0cm/µs[24].
2.2 Single-GEMdetectorsetup
A300 µmthickaluminiumsheetcoatedwith1 µmof10B C [13]wasusedasneutronconverter
4
and detector cathode of the Single-GEM. As shown in figure 6a, copper tape of 50 µm thickness
was attached to the 10B C layer to stop the α particles and 7Li ions from leaving the 10B C and
4 4
–2–
241AmBe
HV
PE
1 MΩ
n
1 0 MΩ B C
550 kΩ 4
8 mm
4He or 7Li
1 MΩ
1 0 MΩ 2 mm
500 kΩ
2 mm
2 mm
1 MΩ
… …
1 0 MΩ
450 kΩ
1 MΩ
Figure1: SchematicrepresentationofthesetupforaTriple-GEMdetectorconfiguration.
entering the gas, thus providing a sharp edge to determine the position resolution. For the mea-
surements with the Single-GEM a gain of 40 was used in combination with a drift gap of 8 mm
and a drift field of 1 kV/cm. At this drift field the drift speed of electrons amounts to 3.5 cm/µs
in Ar/CO (70%/30%) [24]. Sufficiently low drift velocities assure the ability to distinguish the
2
inclinationoftheα particleand7Liiontracks,allowingtheirthree-dimensionalreconstruction.
counts/bin500 aLtolip tiaohlnass counts/bin440500
400 350
300
300 250
200
200
150
100 100
50
00 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 0 10000 20000 30000 40000 50000 60000
energy [MeV] total integrated charge (a. u.)
(a)SpectrumsimulatedwithGeant4 (b)SpectrummeasuredwithTriple-GEM
Figure 2: Energy deposited in the drift volume by the charged particles from the 10B C neutron
4
capture.
2.3 DetectorreadoutandDAQ
Thereadoutboardwasacartesianx/ystripreadout[25]with256stripsand400µmpitchinxand
y direction in both setups. Four APV-25 [26] hybrid chips per readout were used to pre-amplify
–3–
the signals. The waveforms were digitized with the Scalable Readout System (SRS) [27] and
acquired with the ALICE DAQ system DATE [28] and the Automatic MOnitoRing Environment
(AMORE) [29] software. Thermal neutrons were obtained by moderating fast neutrons from a
370 MBq 241AmBe source with a 5 cm thick polyethylene block. GEANT4 [30, 31] simulations
show that the 241AmBe source produces isotropically about 2.3×104 n/s. After the moderation,
about 100 thermal neutrons per second reach the 10 x 10 cm active area of the detector. Given
the low conversion efficiency of the 1 µm thick B C layer, the detection rate was less than 5 Hz.
4
Figure 2 shows the simulated and measured spectra of the energy released in the gas volume by
thechargedparticlescreatedduringthe10B Cneutroncapture. Withintheenergyresolutionofthe
4
Triple-GEMdetector,thesimulatedspectrumcouldbereproduced.
3. TPCanalysis
The typical signals due to a neutron conversion are shown in figure 3. The figure shows the raw
waveforms from the x view on different strips, digitized every 25 ns as measured with the Triple-
GEM setup. The signals on different strips are not synchronous because different drift depths
requiredifferentdrifttimes.
1000 1000 1000
800 800 800
600 600 600
400 400 400
200 200 200
0 0 0
0 5 10 15 20 25 30 0 5 10 15 20 25 30 0 5 10 15 20 25 30
(a)strip1 (b)strip2 (c)strip3
1000 1000 1000
800 800 800
600 600 600
400 400 400
200 200 200
0 0 0
0 5 10 15 20 25 30 0 5 10 15 20 25 30 0 5 10 15 20 25 30
(d)strip4 (e)strip5 (f)strip6
1000 1000 1000
800 800 800
600 600 600
400 400 400
200 200 200
0 0 0
0 5 10 15 20 25 30 0 5 10 15 20 25 30 0 5 10 15 20 25 30
(g)strip7 (h)strip8 (i)strip9
Figure 3: Typical time structure of the neutron conversion signal in different strips with the 25 ns
timebinsonthex-axis,andthechargeinarbitraryunitsonthey-axis.
During the drift in electric fields, electrons diffuse as a result of multiple collisions with the
–4–
time bin[25 ns]2350 8100000ge [ADC counts] time bin [25 ns]2350 8100000ge [ADC counts]
20 har 20 har
c c
600
600
15 15
tstart tstart 400
400
10 10
200
5tend 200 5tend
0 start of track end of track 0 0 start of track end of track 0
134 136 138 140 142 144 146 148 170 172 174 176 178 180 182
x strip [0.4 mm] y strip [0.4 mm]
(a)xview (b)yview
Figure4: ChargedparticletrackinthedriftvolumeoftheTriple-GEMdetector.
gas molecules, and the initially localized charge cloud spreads out. The diffusion depends on the
gas mixture, the pressure and the electrical field. At normal temperature and pressure (NTP), the
σ ofthetransversalandlongitudinaldiffusionforbothdetectorsamountedtoaround130µmover
the8mmdrift[32].
An ionizing particle crossing a portion of the active volume creates electrons and ions that
move in opposite directions separated by the electric field. The speed of the α particles and 7Li
ions is much larger than the electron drift velocity, therefore the primary charge can be assumed
to be released instantaneously along their path. The electron cloud moves rigidly at a constant
speed due to the uniform electric field. Neglecting the small amount of diffusion, the electron
cloud preserves its original track shape along the drift. The x and y view of one typical track as
measuredwiththeTriple-GEMsetupisdepictedinfigure4. Knowingtheinstantwhentheprimary
charge is released (t ), one can extract the drift time and consequently the position along the drift
0
oftheionizingevent. Inthepresentconfigurationthisinformationisnotaccessible. Nevertheless,
theelectronsfromtheendofthetrackwillalwaysinducethesignalbeforetheelectronsfromthe
beginning of the track. In fact, since the converter is on the cathode, the beginning of the track
has always the largest drift time. This concept forms the base of the Time Projection Chamber
(TPC)[33],whichallowstheidentificationofthebeginningofthetrackevenwithoutknowingt .
0
ThetrackintheTriple-GEMdetectorinfigure4andthetrackintheSingle-GEMdetectorin
figure 5 have a comparable inclination. But one notices a larger time difference between the start
and the end of the track in the Triple-GEM detector, due to the lower drift speed. Changing the
drift speed via the drift field or the gas mixture in the detector can thus modulate the desired time
difference.
Figure6ashowsthecathodewithcoppergridthatwasusedfortheSingle-GEMmeasurement.
The copper grid was added to stop the α particles and 7Li ions from escaping from the converter
intothegas. Thismakesitpossibletomeasurepositionresolutionwithahighlydivergentsource.
–5–
time bin[25 ns]2350 670000 ge [ADC counts] time bin [25 ns]2350 678000000 ge [ADC counts]
20 har 20 har
500 c 500 c
15 400 15 400
tstart 300 tstart 300
10 10
tend 200 tend 200
5 100 5 100
end of track start of track 0 start of track end of track 0
0 0
128 130 132 134 136 138 140 142 144 202 204 206 208 210 212 214 216
x strip [0.4 mm] y strip [0.4 mm]
(a)xview (b)yview
Figure5: ChargedparticletrackinthedriftvolumeoftheSingle-GEMdetector.
Asaresult,sharpboundariesbetweentheregionswithandwithoutrecordedhitsarevisibleinthe
plot of the hit distribution. To determine the improvement in position resolution due to the µTPC
analysis,regionswithalargestepinthehitdistributionasdisplayedinfigure6bwerestudied. Due
tothesubstantialgammafluxofthe241AmBesource,acertainamountofgammabackgroundwas
present during the measurements as indicated by the hits in areas covered with copper tape. To
excludethegammabackground,whichdepositsalotlesschargethanα particlesand7Liions,an
amplitudethresholdwasappliedtothedata. Subsequentlyasimpleconstantfractiondiscriminator
was used to determine the time at which the signal in each strip reached 50 % of the maximum
amplitude. The strip with the highest threshold crossing time was then chosen as the position of
thestartofthetrack.
Figure7showsasexampletheanalysisofthepositionresolutioninregiony5infigure6b. The
standarddeviationσ ofthepositionresolutionisextractedfromthefitofthecomplementaryerror
function to the data. The reconstructed position obtained with the µTPC concept is shown in red,
the centre-of-mass approach in blue. The σ of the position resolution amounts to 150 µm for the
µTPC concept and 909 µm for the centre-of-mass approach. The results of all analysed regions
canbefoundintable1. Theσ ofthepositionresolutioniseverywherebetterthan270 µmforthe
Single-GEMmeasurementsandthe µTPCtechnique. Theexactvaluedependsontheflatnessand
straightnessoftheappliedtape. Inregionswithstraightandflattape,theσ is200 µmorsmaller.
200µmorhalfthesizeofthestrippitchseemstobealimitationforthisfitmethod. Asexpected,if
theσ getstoosmallwithrespecttothereadoutpitch,thefitcannotconvergeasthesharpdrop-off
occursin<1strip.
The strips of the cartesian x/y readout have a 400 µm pitch with 80 µm strip width in y, and
340 µm strip width in x. Since the x strips are underneath the y strips and electrically separated
from them by 50 µm of Kapton, the different strip sizes have been chosen to have a more equal
charge sharing between y and x. But as the data analysis shows, the y strips on the top get about
–6–
mm]50 102hits
n [40
o
Positi30 x1 x2
Y
20
x3 x4
10 10
0 x5 x6
-10
y2 y4 y6
-20 1
-30 y1 y3 y5
-40
-50
-50 -40 -30 -20 -10 0 10 20 30 40 50
X Position [mm]
(a)10B Ccoatedcathode (b)Hitdistributionwithanalysedpositions
4
Figure6: 10B Ccoatedcathodewithcoppertapeandresultinghitdistributionofthemeasurements
4
withtheSingle-GEM.
60% of the charge, whereas the x strips get around 40%. Due to the larger amount of charge, the
resultsfortheycoordinateareingeneralbetterthanforthexcoordinate. Thisisvalidforboththe
µTPC technique and the centre-of-mass approach. During the Triple-GEM measurements half of
the detector area was covered by a 25 µm thick aluminium foil with a solid layer of 10B C. The
4
edgeofthealuminiumfoilwasnotverysmoothandnotwellalignedwiththestripsofthereadout
board. Theσ oftheobtainedpositionresolutionamountedthereforeto311µm.
–7–
s
nt start-of-track
u
co fit start-of-track (s =150 um +/- 31 um)
500 center-of-mass
fit center-of-mass (s =909 um +/- 64 um)
400
300
200
100
0
-34 -32 -30 -28 -26 -24
y position [mm]
Figure 7: Single-GEM position y5: Distribution of the reconstructed y coordinate using a center-
of-mass-basedtechnique(inblue)andtheTPCanalysis(inred).
4. Conclusions
The µTPC concept is shown for the first time with thermal neutrons using 10B C as neutron con-
4
verterinaSingle-GEMandTriple-GEMdetectorconfiguration. Byusingasimplelaboratorysetup
witharadioactiveneutronsourceandcoppertapeonthecathodetomasktheneutronconverterand
give a sharp edge, it is possible to extract the position resolution. These proof-of-principle mea-
surementsdemonstratethattheresultingpositionresolutionisimprovedbyafactorofca5usinga
µTPCtechniquecomparedtoamoretraditionalcentre-of-massbasedmethod. Theobtainedreso-
lutionsarewith<200µmbetterthanhalfofthereadoutstrippitch. TheµTPCconceptthusfulfils
the required position resolution for NMX. It also presents a perspective for time-resolved neutron
detectorswithspatialresolutionsfarbelow1mmbyapplyingthistechnique. Measurementsunder
better conditions such as a higher flux of thermal neutrons and a lower gamma background are
foreseentofurtherevaluatethepotentialoftheµTPCtechnique.
–8–
start-of-track center-of-mass
Position σ [µm] +/-error[µm] σ [µm] +/-error[µm]
Triple-GEM 311 61 1161 111
Single-GEMx1 <200* - 1350 67
Single-GEMx2 <200* - 1246 71
Single-GEMx3 238 17 1113 69
Single-GEMx4 <200* - 1213 62
Single-GEMx5 268 24 1498 67
Single-GEMx6 <200* - 1263 66
Single-GEMy1 <200* - 593 59
Single-GEMy2 <200* - 942 55
Single-GEMy3 146 26 587 49
Single-GEMy4 116 69 786 56
Single-GEMy5 150 31 909 64
Single-GEMy6 <200* - 841 59
Table1: FitresultsfortheTriple-GEMandthe12Single-GEMregionsof
interestdepictedinfigure6b.
* Limitoffitreached.
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