The Performance of the Muon Veto of the GERDA Experiment

1.7 MB·

by  K. Freund

Eur. Phys. J. C manuscript No.
(will be inserted by the editor)
Gerda
The Performance of the Muon Veto of the Experiment
K. Freund1,a, R. Falkenstein1, P. Grabmayr1,d, A. Hegai1, J. Jochum1, M.
Knapp1,b, B. Lubsandorzhiev1,2, F. Ritter1,c, C. Schmitt1, A.-K. Schütz1,
I. Jitnikov3, E. Shevchik3, M. Shirchenko3, D. Zinatulina3
1PhysikalischesInstitut,EberhardKarlsUniversität Tübingen,Tübingen,Germany
2InstituteforNuclearResearchoftheRussianAcademyofSciences,Moscow,Russia
3JointInstituteforNuclearResearch,Dubna,Russia
6
1
0
2 Received:date/Accepted: date

n
a
J Abstract Low background experiments need a sup- tal signature of the 0νββ decay is a peak at Q , the
ββ
2 pressionofcosmogenicallyinducedevents.TheGerda Q value of the decay.
2 experiment located at Lngs is searching for the 0νββ Gerdawasconstructedintheundergroundlabora-

] decay of 76Ge. It is equipped with an active muon veto tory of Laboratori Nazionali del Gran Sasso (Lngs) of
et the main part of which is a water Cherenkov veto with InfninItaly,whichoffersanoverburdenof3500meter
d 66 PMTs in the water tank surrounding the Gerda water equivalent (m.w.e.) of rock and hence a reduc-
s- cryostat. With this system 806 live days have been tion of the muon flux by a factor of ∼106 to a rate of
n recorded, 491 days were combined muon-germanium ∼3.4×10−4/(s·m2). This remaining muon flux however
i data. A muon detection efficiency of ε = (99.935±
. µd issufficienttocauseanon-negligiblebackgroundinthe
s
0.015)% was found in a Monte Carlo simulation for
c region of interest around Q =2039 keV when increas-
ββ
si the muons depositing energy in the germanium detec- ing the sensitivity beyond T10/ν2>1025 yr or when re-
y tors.Byexaminingcoincidentmuon-germaniumevents questing a background index BI<10−2 cts/(keV·kg·yr).
h a rejection efficiency of ε = (99.2+0.3)% was found.
µr −0.4 However, also other analyses profit from the reduced
p
Withoutvetoconditionthemuonsbythemselveswould
[ backgrounds (see e.g. Ref. [3,4]).
1 cctasu/s(ekeaVb·akcgk·ygrr)ouantdQind.exofBIµ =(3.16±0.85)×10−3 The origin of muons at Lngs for Phase I of Gerda
ββ
v wastwofold.Firstly,themajorityofthedetectablemu-
5 Keywords cosmic muons · water Cherenkov detec- ons are produced cosmogenically. Spectrum and angu-
3
tors · scintillation detectors · data analysis lar distribution of the muons are both altered by the
9
5 profile of the rock overburden and have been measured
PACS 95.85.Ry · 29.40.Ka · 29.40.Mc · 29.85.Fj
0 for Lngs with high precision [5]. These muons have an
1. average energy of (cid:104)Eµ(cid:105) = 270 GeV. Secondly, a source
0 for muons was the Cngs neutrino beam from Cern [6]
6 1 Introduction which created muons via e.g. ν +d → µ− +u reac-
µ
1
tions in the vicinity of the detector. This contribution
:
v Muonsmaycauseasubstantialbackgroundtorareevent amountedto2.2%ofthetotalmuonfluxinGerda.As
Xi searches like Gerda (Germanium Detector Array) by theCngswasshutdownafter2012thefuturePhaseII
generating counts in the region of interest (ROI) ei- of Gerda will be unaffected. In order to reduce muon
r
a ther through direct energy deposition in the detectors inducedbackground,amuonvetocomprisedofawater
or through e.g. decay radiation of spallation products. Cherenkovvetoandascintillatorvetowasimplemented
TheGerdaexperimentissearchingfortheneutrinoless totagmuonsandtouseitsresponseasavetosignalin
doublebeta(0νββ)decayof76Ge[1,2].Theexperimen- the 0νββ analysis.
This paper describes the hardware and setup of the
apresent address: maxmentGmbH,Germany
vetoandtheDAQsystem.Theperformanceoftheveto
bpresent address: Areva,France
cpresent address: BoschGmbH,Germany will be presented and compared to Monte Carlo simu-
dCorrespondence,email:[email protected] lations. Detection and rejection efficiencies for the veto
2
plastic scintillator veto low-Z materials are used in order to reduce cosmogenic
activation [1,12].
The Gerda muon veto consists of two independent
twin lock
parts which are read out by the same DAQ. The first
and main part is a water Cherenkov veto that detects
theCherenkovradiationofthetraversingmuons.Thus,
glove box the water tank is instrumented with 66 encapsulated
photomultipliers(PMTs).Thesecondpartofthemuon
clean room
veto is comprised of plastic scintillator panels. Since
chimney shutter neck theCherenkovvetooffersreducedtaggingcapabilityat
or around the neck of the cryostat, plastic panels were
590 m 3 > 0.17 M Ω m heat placed on top of the clean room in order to close this
exchanger weak spot in the Cherenkov veto.
water tank
radon
shroud
2.2 The Cherenkov veto
Ge
Below the water level there are 66 PMTs (8” size) of
detector
type 9354KB/9350KB by ETL [13] installed. In order
array
to protect the PMTs from the surrounding water, each
copper
PMT is housed in a stainless-steel encapsulation which
shield
cryostat is further developed from the design of the Borexino
man
3
64 m LAr hole capsules[14].Thecapsuleoflowradioactivitystainless-
66 PMT
Cherenkov steel [15] is closed with a custom made PET cap and is
veto filledwithIRspectroscopyoil[16].Theoilkeepstheop-
2m tical transition between PET cap and PMT as smooth
5m as possible. Thus, efficiency losses by total internal re-
Fig. 1 Asketchofthe Gerdaexperiment. flections are minimized. The bottom of the base of the
PMT is encased in polyurethane [17] and sealed with
silicon gel [18]. The lower part of the PMT, i.e. the
will be discussed. Parts of this work have been pub- dynode structure, is protected from magnetic fields by
lished during the respective PhD periods [7–11]. a cone of µ-metal [19]. Each capsule is equipped with
an optical fibre for optical calibration pulses from out-
side the water tank. To keep the number of cables and
2 Instrumentation connectorswithinthecleanwaterataminimumanun-
derwater high-voltage cable [20] connects the PMT to
Here, a technical description of the apparatus is given. a signal splitter outside of the water tank. This band-
Purposeandfunctionofbothpartsoftheveto,Cheren- pass separates the signal from the high voltage (HV).
kov and scintillator panels, is introduced. The trigger The former is digitized and stored on disk upon a trig-
logic, calibration and data acquisition are summarized. ger (sec. 2.4). The HV is supplied by a HV multichan-
nelsystembyCAEN[21]equippedwithsix12-channel
boards [22]. A sketch of the capsule is shown in Fig. 2
2.1 Gerda and images of the components of the Cherenkov veto
are shown in Fig. 3.
In Gerda an array of bare germanium detectors is op- The water in the Gerda water tank was provided
eratedinacryostatthatcontains64m3 ofliquidargon initiallly by the Borexino water plant. The purifica-
(LAr) as seen in Fig. 1. The cryostat is surrounded by tion system which consists of filters, de-ionizers and an
a water tank with a diameter of 10 m and a height of osmosisunitisrunningwith2.4m3/hourandkeepsthe
9.4mthatisfilledwith590tultra-purewater;itswalls water at the ultra-pure level of of >0.17 MΩ m [23].
are covered with a reflective foil. The cryostat has a The PMTs are arranged in seven rings in the water
connection (neck) through the water tank to the clean tank, their distribution is shown in Fig. 4. One ring of
room above from which the germanium detectors are six PMTs (101-106) is pointing inside the separate vol-
loweredintotheLAr.Bothwatertankandcryostatare ume under the cryostat (commonly referred to as “pill-
partoftheinnovativeshieldingdesignof Gerdawhere box”), two rings ofeightPMTs (201-208)and 12 PMTs
3
Fig. 3 Images of the components of the Cherenkov veto during the installation. Top, left: a capsule mounted on the floor;
top, right: a capsule on the wall; bottom, left: a diffuser ball; bottom, right: PMTs and Daylighting Film in “pillbox” below
thecryostat
PET-window
flanges
chimney
μ-metal
6.5m
710 709 708 707 706 705 704 703 702 701
steel capsule 610 609 608 607 606 605 604 603 602 601 5m
3.5m
510 509 508 507 506 505 504 503 502 501
oil filling tubes fibre holder 2m
410 409 408 407 406 405 404 403 402 401
PMT base cable feedthrough 307 305
306
308 204 304
205 203
309 104103 303
105102
206 106101 202
Fig. 2 Parts ofaPMTcapsule 310207 201302
208
311 301
312
Fig. 4 AsketchofthePMTdistributioninsidetheGerda
(301-312) respectively are placed on the bottom of the
watertank.Thevioletareaonthebottomplatesignifiesthe
water tank looking upwards and four rings of 10 PMTs circumferenceofthecryostatandthegapsshowtheposition
are placed on the wall (401-710), pointing horizontally of manholes into the pillbox. The other three violet areas
showthelocationofthemanholeintothecryostat.Allcables
towards the cryostat. The number of PMTs and their
andfibresareleadoutofthewatertankthroughthree50cm
placement was chosen after an extensive Monte Carlo
flangesmountedina“chimney” abovethewaterlevel(Fig.1)
study [7]. The PMTs closest to the cryostat, i.e those
4
] 30 ]
% .
u
[ .
a
cy 3 [
n 20 ns
e
o
ci t
ffi 2 o
h
e p
m 10 #
u 1
t
n
a
u
q
100 300 500 700[nm]
Fig. 5 Efficiency (red curve) of a 9354KB type PMT
adapted from Ref. [13] and simulated photon spectrum with
wave-lengthshiftingeffect(bluehistogram)
fromthepillboxandtheinnerringonthebottom,were
Fig. 6 SchematicdrawingofadiffuserballafterRef.[11]
selectedaccordingtotheirperformanceandradioactiv-
ity (type 9354KB is a low activity module). However,
due to the distance to the germanium detectors and
thelowoverallmasstheradioactivityofthemuonveto areworkingverywell,thecalibrationfibresattachedto
is negligible compared to the one of the stainless steel every PMT are currently not in use.
cryostat [7].
The floor and the walls of the water tank and of
the cryostat are covered with the reflective Daylighting 2.3 Scintillator Veto
Film DF2000MA (commonly known as “VM2000”) [24]
which offers a reflectivity of >99% in order to increase Muons passing through the neck of the cryostat may
the light yield of each event. This comes at the cost either traverse a too short distance in the “pillbox” or
of a reduction in tracking capabilities of each individ- in the water tank to be detected. In order to keep the
ual muon. The reflective foil does not only increase the muon rejection efficiency as high as possible, a veto of
collectedlightyieldofeachmuon,butitactsasawave- plastic scintillator panels was conceived and installed.
lengthshifteraswell.Itshiftsthepredominantlyultra- Each scintillator panel contains a 200×50×3 cm3
violet Cherenkov photons to around 400 nm where the sheet of plastic scintillator based on polystyrol with an
PMTs are most efficient. The efficiency of a 9354KB addition of PTP (2%) and POPOP (0.03%) [26]. In
type PMT and the wave-length shifting effect imple- addition the panels contain optical fibres [27] on the
mentedintheGeant4simulationsareshowninFig.5. narrow sides as light-guides, an electronics board with
ThePMTsarecalibratedregularlywithasetoffive a trigger and shaper and a PMT. Half of the panels
custom made diffuser balls shown in Fig. 6 which are areequippedwithPMTsbyHamamatsuPhotonics[28]
constructed to provide a light source as isotropic as and the other half with PMT-085 by Kvadrotech. The
possible.Theseareglassballswithadiameterof4.5cm PMT-085 are powered by the same HV supply as the
thatarefilledwithamixtureofsilicongel[18]andglass muon veto PMTs however connecting 3 PMTs to one
pearls[25]withadiameterof∼50µm.Anopticalfibre HV output. The front-end electronics, the Hamamatsu
is glued into a small vial inside the ball with a higher PMTs and all other components are powered by a cus-
content of glass pearls. The cladding at the end of the tom made power source with +12 V and ±6 V.
fibre is removed and the fibre is roughened in order to The panels are arranged in three layers covering an
obtainadiffuselightemission.Ultra-fastLEDsoutside area of 4×3 m2 centered over the neck of the cryostat.
of the water tank can be pulsed to illuminate the five It was aimed to keep both trigger rate and thus data
diffuser balls [11]. Four of these balls are distributed volume of the veto as low as possible. Therefore, the
in the main water tank and a fifth is placed inside the triggerisatriplecoincidenceofallthreelayerssincethis
“pillbox”. With an appropriate setting of the LEDs it option offers a high discrimination against non-muonic
is possible to illuminate all PMTs simultaneously with background events in the panels such as γ rays from
single photons and thus record single-photon responses environmental radioactivity. Thus, the scintillator veto
of every PMT at the same time. As the diffuser balls recordsanalmostpuremuonsample.Individually,each
5
120 scintillator panels
ax panel 2 [mV] 100 events 1110002310 20 40 60 8G0ERD1A0 105/04 670000 ligHhPVt M pduTal sisseiyg nchaalin muslitgipnleaxler pulser
m 80 max [mV] 500 difbfuasller LED driver
splitter PMT
signal
RC
400
FADCs
60 muon PMT & controller
HV supply
Ge-array
300
40 cryostat CAEN HV
200 water tank
remote PC,
20 PMT data storage
100
Fig. 8 Schematic drawing of the data acquisition (DAQ)
setup
0 0
0 20 40 60 80 100 120
max panel 1 [mV]
Fig.7 Scatterplotofthepulseheightmaximaoftwopanels
inatriplestackwithaproductcut(redline)asgiveninEq.1. events in which PMTs next to each other have fired.
Theinsetshowsthepulseheightdistributionofonepaneland
The final trigger condition is set such that five FADCs
aLandau-fit(red)
must trigger on at least 0.5 photo-electrons (p.e.) each
within 60 ns. That signal is realized by a FPGA and
it starts the readout of all traces covering a period of
panelshowsapulse-heightdistributionwhichtakesthe
4 µs. These are stored together with the time stamp,
form of a Landau peak. Despite the triple coincidence,
lately from a GPS clock. A schematic drawing of the
small γ contamination at low pulse heights remains.
entire muon veto and its data flow is shown in Fig. 8.
These events can be discarded with a product cut of
Thetriggersignalisfurthermoresenttothegermanium
the form
DAQ recording it as a redundant but immediate veto
(x −ρ )(x −ρ )<c (1) information for the germanium data stream.
1 1 2 2
The scintillator panels are arranged in three layers
for the pulse heights x of any pair of panels in the
1,2 of12panelseach.ThreeadditionalFADCsofthesame
stack and constants ρ and c. The 2-D pulse height
1,2 type digitize the signal of the scintillator veto. The sig-
distributionoftwopanelsinastack,theappliedcutand
nals of two non-neighboring panels within a layer are
a spectrum of a single panel with a fit to the Landau
multiplexedontooneFADCchannelusingcustommade
peak is shown in Fig. 7.
reflection-free modules with an amplification of -6 dB.
Thusthe36panelsoccupyonly18FADCchannelssuch
that each layer is read out by one FADC module. The
2.4 Data acquisition
panel stack which was hit can be determined by the
uniquecombinationoffiredchannels.Thesametrigger
The entire apparatus is read out and operated by a
logicasfortheCherenkovvetoisappliedforthepanels,
VME data acquisition system (DAQ) which is almost
albeitthetriggerwindowislargerinordertoaccommo-
identicaltotheoneofthegermaniumDAQ(seeRef.[1]
datethemuchlongeroutputsignalsofthepanelPMTs
for details). Ten Flash-ADCs [29] with 8 channels each
because they are shaped with a larger time constant.
digitize the input signals of the Cherenkov veto with
For a panel event, all three FADCs (i.e. all three pan-
100 MHz. The signals in each channel are processed
els in a stack) need to have fired. Both types of trigger
by a trapezoidal filter and if the height exceeds the
signals are accepted during data taking.
threshold set to 0.5 photo-electrons (p.e.) an internal
triggerisgenerated.EachFADCmodulehasonetrigger For a calibration run of the PMTs, the standard
output which is the logic OR of the internal triggers of datatakingisstopped.Theultra-fastcalibrationLEDs
itseightchannels.Thus,thePMTsignalsareconnected are activated with a pulser and the LED luminosity is
to the input channels in such a way over the FADCs, controlled by a current source in form of a digital-to-
thatneighboringPMTsarealwaysreadoutbydifferent analog converter [30]. A separate FADC reads out the
FADCs. This allows the proper detection of clustered pulsersignalandtriggerstheentirevetoforeachpulse.
6
nts [cts] 103 high iSlluPmPi n=a=t io1n p:.eG.ERDA 15/04 T response [a.u.]1105////// PPPMMMTTT 576000111 GERDA 15/04//////1105
ve SPP @ 1.2 p.e. PM // PMT 402 //
e102 DPP @ 2.2 p.e. 5// PMT 301 //5
// PMT 201 //
PMT 101
0 0
Apr/11 Jul/11 Oct/11 Jan/12 Apr/12 Jul/12 Oct/12 Dec/12 Apr/13
10
Fig. 11 Summary of the stability of the light output of
selectedPMTs;notethearbitraryoffsets.Thehatchedareas
indicatemaintenanceperiods
1
0 100 200 300 400 500
FADC channel July 2013, 805.6 live days have been recorded and 491
Fig.9 CherenkovPMTresponsewithdifferentforwardvolt- days of combined muon-germanium data. The duty cy-
ages of the calibration LEDs, i.e. luminosities. With a low
cle is shown in Fig. 10 together with the accumulated
luminosity, only the single photon peak (SPP, broken green
livetime(redline).DuringPhaseIthemuonDAQwas
line) is visible, if the luminosity is too high, the double pho-
tonpeak(DPP,blue)emergesaswell.Thepedestalisshown onlystoppedduringbreaksofthegermaniumdatatak-
inred ing in order to perform short maintenance work and to
calibrate the PMTs by adjusting the HV of each PMT
so that each module shows the same response to sin-
TwocalibrationspectracanbeseeninFig.9.Oneis gle photons. The PMTs were very stable and since the
showingtheconventionallyrecordedsingle-photonpeak beginning only few PMTs needed to be readjusted. As
(SPP) set to channel 100, the other comes from a too example the daily light output per day is shown for
brightLEDsetting.Thiscausesacontaminationofthe selected seven PMTs in Fig. 11. The offsets are for dis-
SPP by the double photon peak (DPP) and a shift of play only. For e.g. PMT101 there was no readjustment
theamplitudesofbothpeakstohighervalues.TheSPP necessary since May 2011, while PMT301 needed sev-
emergesverywellinmostPMTsandpeak-to-valleyra- eral tunings of the HV. However, between the breaks
tios between 1.2 and 3.0 are observed. thelightoutputremainsstable.Duringthegermanium
datatakingthemuonvetowasalwaysfullyoperational.
During Phase I two PMTs were lost due to implo-
3 Veto performance sion of the tube (PMTs 401 & 604). The implosion was
mostly contained by the encapsulation and no other
The Gerda muon veto was installed in 2009 and its PMTsinthevicinitywereharmed.Theimplosionshap-
operationstartedinNovember2010.Duringthegerma- pened in February and April 2012 and the PMTs were
nium commissioning runs the panel veto was installed over 10 m apart. Hence a direct influence can be ruled
so that the complete veto was operational at the start out.AthirdPMTwaslostrightafterinstallationdueto
of the physics runs of Gerda in November 2011. Until apuncturedcable(PMT305)andafourthPMTceased
working during the installation and could be immedi-
ately exchanged (PMT 203). In July 2013, the Gerda
e 1.0 GERDA 15/04 800s water tank was drained, the veto inspected and two of
o duty cycl0.8 670000live day t4h0e1s)e.APlMloTtshewrePreMsTucscsetsisllfuwlloyrkexacshianntegnedde(dPaMnTdssh3o0w5e&d
vet0.6 500 little to no signs of deterioration.
400
0.4 300
3.1 Simulation studies
0.2 un 25 un 28 un 31 un 34 un 37 un 40 un 43 un 46 120000
r r r r r r r r The performance of the muon veto was simulated with
0.0 0
Jul/11 Jan/12 Jul/12 Dec/12 Jul/13 theGeant4-based[31]frameworkforGerdaandMa-
Fig. 10 Duty cycle of the muon veto. The veto uptime jorana (MaGe) [32]. First, the simulations were used
(black) and accumulated live days (red) are marked as well
to find initial placements and efficiencies [7] and re-
as the Gerda physics runs for Phase I (filled light red and
peatedoncetheexactgeometriesoftheapparatuswere
green)
finalized[9].ThemuonspectraprovidedbytheMacro
7
experimentwereusedasinputforthesimulationofcos- 102 103 104
mogenic muons [5]. The simulations were mainly used a.u.] 105 cosmGicER DmA 15/04 105
to determine the efficiency of the apparatus in case the s [ pillbox
muon caused any energy deposition in the germanium vent104 simulation 104
e
crystals. 103 103
For the efficiency of the Cherenkov veto the sim- 102 102
ulation was undertaken in two parts. Firstly, muons
were simulated with the Cherenkov effect switched off. a.u.] 103 102 103 deetd. eligph. t m[P.E.1] 04103
Theprimaryverticesofthosemuonsthatcausedenergy nts [102 pillbox 102
e simulation
deposition in the germanium detectors were extracted ev 10 10
from these events. Secondly, these selected muons were
1 1
used in a second simulation with Cherenkov effect en-
10-1 10-1
abled. This two-step procedure was applied in order to 102 103 det. light [p.e.] 1 04
accelerate the simulation as only a minute fraction of
Fig. 12 Photo-electron spectra for all cosmogenic muons
the muons interact with the germanium detectors and (top)andthosewithenergydepositioninthegermaniumde-
becausethesimulationofopticalphotonsisaresource- tectors (bottom). In each panel the total recorded p.e. spec-
trum (red) is compared to the spectrum which is recorded
demanding procedure. A detection efficiency for the
just in the pillbox (green). Spectra derived from simulations
veto was derived by determining the fraction of energy
(blue)arenormalizedtothe sameexposure
depositing muons which caused a trigger signal in the
muon veto. The trigger condition were the same as for
themuonvetoDAQdescribedinSec.2.4.Fortheentire
3.2 Multiplicity and photon spectra
veto a detection efficiency of
The light yield of a single muon is determined by ob-
εMC =(99.935±0.015)% (2)
µd serving the total number of recorded p.e. in all PMTs.
For comparison, the pillbox is treated as an individual
for muons with energy deposition in the germanium volume. In Fig. 12 histograms of the recorded and sim-
detectors was found in the simulated data. ulatedeventsareshownfortheperiodofPhaseI.Inthe
ByremovingcertainPMTsfromtheefficiencycalcu- spectra for the cosmogenic muons, apart from the very
lations, a veto degradation (e.g. possibly broken PMTs lowlightyieldtherearemaximaatabout167p.e.inthe
or malfunctioning FADCs) was simulated. Even with pillbox data and about 605 p.e. for the total spectrum.
the first two FADCs removed (14 PMTs in total, two These broad peaks correspond to the mean traversed
in each of the seven rings shown in Fig. 4), the effi- distance of 1.8 m for the pillbox and of 9 m for the wa-
ciency is still (99.525+0.025)%. However when only four ter tank for muons with a mean incident angle of 60◦.
−0.035
PMTs in the pillbox are removed the value drops to Light from muons in the water tank is subject to at-
(97.855±0.065)%.ThepillboxPMTscanhencebecon- tenuation, the attenuation length of photons in water
sidered the most critical ones in case of a break-down. being ∼10 m. Assuming a mean distance of 5 m of the
The light produced inside the pillbox can illumi- PMTsfromthemuontrackinthewatertankand2min
nate the main water tank through two small manholes. the pillbox, muons of any track deposit approximately
However, the light coming from these two holes is not the same amount of light. Thus, each muon generates
sufficient in order to generate a trigger. The insensitiv- (115±39)p.e./m.Thisisreproducedbythesimulation.
ity against a loss of a few PMTs gives high reliability A peak structure is visible in the data for muons
of the efficiency of the veto against small variations of which deposit energy in the germanium detectors as
trigger conditions or changes in the amplitude of the well. The peak for the pillbox is at slightly lower p.e.
PMTs. valuesbecausetheaverageincidentzenithangleislower
In an earlier work the efficiency was estimated as andthusthetracklengthshorter.Thep.e.spectrumfor
εMC = (99.56±0.42)% and thus some what lower de- all PMTs shows a double peak feature. This is due to
µd
spite lower trigger conditions [7]. The previous simula- the fact that the muon has to cross the cryostat in or-
tion neglected the optical connection between the “pill- dertodepositenergyinthegermanium.Themuoncan
box” volume and the main water tank completely. Sev- deposit light in the water tank before and after inter-
eral other simulation studies have been performed like acting with the germanium detectors. The higher peak
the veto response to regular cosmogenic muons which correspondstomuonswhichpassthetanktwiceandthe
are shown and compared to experimental data in the smaller corresponds to muons that pass the water tank
next sections. just once (e.g. shallow angles close to the neck of the
8
u.] 106 m events, 18 P.E. cut GERDA 15/04 3.3 Coincident muon-germanium events
s [a.105 panel trigger The muon veto and the germanium systems have been
ent sim. events, FADC cut operational in common during Phase I of Gerda over
v
e104 a time of 491 d. During this period an exposure of
E = (21.6 + 6.2 ) kg·yr of germanium data was
103 enr nat
recorded. During Phase I the muon veto was only shut
102 down during pauses in the germanium data taking and
hence there is no additional loss of exposure due to the
10
veto.
0 10 20 30 40 50 60
PMT multiplicity M The two data streams were correlated by using the
Fig. 13 PMTmultiplicityspectrumoftheCherenkovveto. timestamps of the events. Prior to Phase I both DAQ
Themultiplicityofallevents(green)iscomparedwithevents systems were operated with their own internal clock
where the panels have fired as well (red) and with simulated whichpermittedundesiredjumpsinthetimeoffsetbe-
data(blue).Histogramsaftercutsaremarkedbydashedlines
tween the two systems. For Phase I both DAQ systems
were equipped with the same GPS timing system so
that events can be correlated via the timestamp with
high precision. The length of the germanium trace of
cryostat). Again, the simulations agree with this even 160 µs is taken as a coincidence window. Most interac-
though the double peak structure is less pronounced. tionsbetweenmuonsandthegermaniumarrayhappen
within ±10 µs, however delayed interactions cannot be
Another characteristic of a muon event is the num- excluded. The germanium DAQ is described in detail
ber of fired PMTs. This multiplicity M is shown in in Ref. [1].
Fig. 13 for several classes of muon events. The spec- Bothsystemswerephysicallyandelectronicallyvery
trum of all measured events (green line) shows a peak stableovertime.AfterthedeploymentofthenewBEGe
at M>60 and another one at M<10. The peak at high detectorsinthesecondhalfofPhaseItheset-upofop-
M is the regular response of the veto to muons. This erational modules was unchanged for the rest of the
is verified by either the simulated data (blue line) and data taking with four BEGes, six enrGe and one natGe
a subset of all events in which the panel veto has to coaxialdetector.Inthisperiodthemeanratewasr =
µ
havefiredaswell(redline).Theshapeofthespectrais (4.01±0.04)×10−2/s for the veto (no cuts applied),
slightly different which is due to different incident an- r = (2.87±0.06)×10−2/s for all germanium detec-
Ge
gles (panel trigger) or the lack of random coincidences torsandr =(9.5±0.6)×10−5/stherateofphysical
coin
at low multiplicities in the simulated data. In addition, coincidences. Due to the low rate of veto and germa-
four PMTs were lost in the Cherenkov veto which en- nium random coincidences are negligible compared to
hances the peak at M>60. This is due to events that the true coincident rate.
would trigger all PMTs are now recorded as having an
M=(66−x),wherexistheamountoflostPMTs.Thus,
the counts at or just below 66 are shifted to lower mul- 3.4 Muon rejection efficiency
tiplicities.
A muon rejection efficiency (MRE) can be obtained by
Only the measured spectrum from the Cherenkov definingacutforclearlyidentifiedmuonhitsintheger-
trigger shows the low multiplicity enhancement, which manium detectors and testing for coincident veto sig-
ischaracterizedbynotonlyalownumberoffiredPMTs nals. The rejection efficiency ε is given as the ratio
µr
butalsoanunusuallylowamountofrecordedp.e.With of these events which are vetoed in comparison to the
acutof18p.e.thisenhancementcanbealmostentirely entire set. The following cuts were applied to the ger-
suppressed. This is the standard cut condition for the manium events of the Gerda Phase I data to identify
Cherenkovvetodata.Thesourceofthisenhancementis muons:eitherasinglehitshowedanenergydepostionof
discussed in Sec. 3.7. A cut which emulates the trigger morethan8.5Mevorthesummedenergyofamulti-hit
conditionimplementedintheDAQdescribedinSec.2.4 exceeded 4 MeV. This cut excluded energy depositions
(“FADC cut”) was applied to the simulated data. The from the U and Th decay chains. In addition, the ger-
resulting spectrum (dashed blue line) indicates the be- manium test-pulse and quality cuts were applied, but
havioroftheCherenkovvetowithoutunphysicalevents no muon veto cuts. Out of the 848 candidate muon
atlowM likerandomcoincidencesorthelowmultiplic- eventsidentifiedaccordingtoenergyreleaseandmulti-
ity enhancement. plicityinthe Germaniumdetectors,841areactuallyin
9
coincidence with a valid muon veto signal. This leads 3.6 Panel detection efficiency
to a MRE of:
Inordertodeterminetheefficiencyofthepanels,adata
ε =(99.2+0.3)% (3)
µr −0.4 sample of clearly identified muons was selected. A cut
This is a conservative number since the studied events on the Cherenkov events of M ≥20 was chosen which
are not standard events either in germanium multiplic- discards unphysical events at low M. The pre-selection
ity or in energy range where saturation might set in. of muon events by the Cherenkov veto is necessary be-
The derived MRE is slightly lower in comparison causethestandardtriggeroftheplasticvetocannotbe
to the efficiency derived from the simulation given in used. For the panels a simple cut on the pulse heights
Eq. 2. Assuming that the given MRE can be projected is insufficient due to the γ coincidences at low pulse
to standard events, i.e. with multiplicity m = 1 and heights (see Fig. 7). In addition to the pulse height cut
withenergiesatoraroundtheROI,therejectionpower definedbythetriggerthreshold,acutontheproductof
of the veto is reliably high and close to unity. twopulseheightsintheformofEq.1wasapplied.The
detection efficiency of one panel can now be given as
the ratio of events, in which the top and bottom panel
3.5 Muonic background index
of a stack have fired in comparison to the events where
all three panels in a stack have fired.
Inordertoestimatetheimprovementofthebackground
index given in cts/(keV·kg·yr) due to the muon veto, Thedatasetcontainedeventssincethebeginningof
a ±100 keV window was chosen around Q . Analog the muon data taking with the panels in August 2011.
ββ
to the germanium background a blinding window of Inthisset,30044eventswerefoundwhichtriggeredthe
±20 keV around Q was omitted from the analysis topandbottompanel.Oftheseevents29951triggered
ββ
hence the ROI of this study is 160 keV wide. Out of the third panel as well. This leads to an average muon
a total exposure of E=27.8 kg·yr of germanium data, detection efficiency per panel of:
14 vetoed events were found in this ROI with a ger-
εP =(99.70+0.03)% (7)
manium multiplicity of one. Were these 14 events not µd −0.05
vetoed,theywouldhaveledtoacontributiontotheBI
Since this value is an average over different panels, it
of:
can be seen as an approximation for a general panel
BI =(3.16±0.85)×10−3 cts/(keV·kg·yr) . (4) efficiency. The efficiency of a triple stack of panels is
µ
hence (εP )3 =(99.10+0.09)%. Due to insensitive areas
A simulated value for the muonic background in the µd −0.15
at the panel borders (scintillator edges, encasing) the
germaniumarraysurvivinganti-coincidencecutsis[33]
effective area of the panels is reduced by ∼5 mm per
border or less than <0.25% of the area.
BI (MC)=(1.6±0.1)×10−3 cts/(keV·kg·yr) . (5)
µ
As this simulation was undertaken before construction
of the experiment was finalized the geometry differs to 3.7 Low multiplicity enhancement
what was realized. Due to the different geometry, the
low statistics and the subsequent large errors, both re- The enhancement at low multiplicities is characterized
sults can be considered sufficiently in agreement. by a very low number of recorded photo-electrons (in
WithBI andthepreviouslyderivedMRE,anesti- most cases one or two p.e. per PMT) without any ob-
µ
mationoftheunvetoedbackgroundcontributioncanbe servableclusteringeffectsandamountstoabout8.7%of
given. It is assumed that the MRE is constant over the the overall Cherenkov activity (i.e. 3.1×10−3/s). This
entire energy range of the germanium detectors. The behavior suggests a very faint source of light inside the
given vetoed BI is equivalent to the amount of success- watertankthatisnotdirectlycausedbymuons.Events
fully vetoed muons, i.e. 99.2%. An amount of unvetoed which trigger the panel veto and can hence be consid-
muons is found: ered true muon events do not show this anomaly. It
was already suggested that this anomaly is caused by
BI =(2.87±0.77)×10−5cts/(keV·kg·yr) . (6)
µ,unvet. scintillation of the reflective foil under irradiation by α
The design goal of Phase II of Gerda aims to reach a sourcesinthestainlesssteelofthewatertank[1,8].The
totalBIof10−3 cts/(keV·kg·yr).Thus,withthecurrent foilhasahighlyreflectivefrontsideandiscoveredwith
settings of the muon veto unchanged, unvetoed muons an adhesive on the back side. According to the manu-
wouldcontribute1/40oftheBI“allowance”.ForPhaseI facturer the foil itself has a thickness of 66 µm and the
of Gerdathisisequivalentto0.16eventsina200keV adhesive a thickness of 38 µm [24]. If the adhesive can
analysis window. be assumed to be an organic compound a mean free
10
path of ∼30 µm for a 5 MeV α particle is expected. the solid angle (8.5% of 4π) and efficiency (0.25) of the
Hence it is unlikely that α particles coming from the PMT used for this test, the efficiency (0.3) and sur-
backsideareabletodepositenergyinthefoilandpro- face coverage (0.005) of the PMTs in the Gerda water
ducescintillationphotonswhichcanexitthefoilonthe tank the effect of this scintillation can be calculated. It
frontside.Thiswastestedbyilluminatingthefoilwith isfoundthatthebulkoftheseeventswoulddeposit2-6
an 241Am α source and by recording the scintillation p.e. in the PMTs per event. These events will on aver-
light on the front side with a 9235Q PMT. When the age not fulfill the trigger conditions given in Sec. 2.4 It
foil was illuminated from the back side, i.e. through is still likely that about 1% of these events could still
the adhesive, almost no additional light was recorded. triggertheveto.Thiswouldputtheexpectedandmea-
Whentheadhesivewasremovedfromthebacksidethe sured rate as well as the expected photon yield of one
samemeasurementyieldedasmalleffect.Ifilluminated totwop.e.perPMTintothesameorderofmagnitude.
onthefrontsidesufficientphotonswererecordedwhich Thus, the scintillation caused by β particles has to be
could explain the enhancement if applied to the condi- considered the most likely source of this enhancement.
tionsinthewatertank.However,thissuggeststhatthe
α source is either solved in the ultra-pure water or ad-
heringtothefrontsideofthefoil.Theactivityofwater 4 Summary
fromthisplantwasmeasuredtohaveanoverallαactiv-
ity in the range of 10−6–10−7 Bq/kg [23] and measure- In this work, the muon veto deployed during Phase I
ments of Gerda water samples agree with these val- oftheGerdaexperimentwasintroduced,itshardware
was presented and its performance was shown. In ad-
ues[34].Thisactivityistoolowtoexplaintheseexcess
dition, the cosmogenic components of the background
events. If the front side of the foil had a radon contam-
in Gerda was systematically identified, analyzed and
ination, a higher rate would have been expected after
compared to Monte Carlo simulations.
theopeningofthewatertank.Ahigherratewasindeed
Thehardwarethresholdswerechoseninaway,that
measured but this was in accordance to a higher dark
by design a very pure muon sample is recorded with
rateafterprolongedexposuretolightoftheCherenkov
only a few percent contamination by random coinci-
PMTs.
dencesorothersourcesofbackground.Withthispower-
Another explanation for the origin of this anomaly
fulmuonvetoovertwoyearsofdatahavebeenrecorded
are β sources since electrons have much higher specific
including the 491 days coincident with the germanium
ranges in comparison to α particles. The stainless steel
of the Gerda water tank exhibits a low level of ra- detectors.
The Monte Carlo simulations of earlier works were
dioactivityandoneofthestrongestcontributiontothe
radionuclides in the steel is the β emitter 60Co with extendedtoaccommodateforamorerealisticset-upof
an activity of ∼20 mBq/kg [35] which leads to a 60Co themuonveto.PMTmultiplicityandp.e.spectrawere
found in good agreement with the data and the light
activity on the surface of the water tank of ∼5 Bq. To
testtheeffectthefoilwasilluminatedbya60Co-source deposition of cosmogenic and energy-depositing muons
could be related to their different track-lengths in the
(Q =0.35 MeV) and the results are shown in Fig. 14.
β
water tank. With the updated geometry a detection
The foil was illuminated from the front and from the
back side (adhesive not removed). The back illumina-
tion shows only a slightly smaller scintillation effect in
s GERDA 15/04
comparison to the front illumination and in both spec- ent104 back illumination
v
tra a low-energy β spectrum emerges. In order to de- e
front illumination
termine an efficiency for this process, the scintillation 103
single photon
rate of a 5 mm thick sheet of plastic scintillator was
recorded that is assumed to have an efficiency of unity. 102
By comparing the rate of the foil with the rate of the
scintillator, the efficiency of the foil towards 0.35 MeV 10
β particles can be calculated:
1
εfoil =(12.0+1.1)%. (8) 0 10 20 30 40 50 60 70 80
β −1.0 detected light [p.e.]
With this efficiency, the activity expected from 60Co Fig. 14 The effect of irradiation of the VM2000 reflective
foil by a β source (60Co) recorded by a PMT. The single
from the steel is reduced to ∼0.6/s which is still too
photonresponse(red)iscomparedtoanirradiationfromthe
high in comparison to the measured rate. Using the
front(blue)orbackside (green)whichcarriestheadhesive
meanlightrecordedbytheilluminationmeasurements,