Progress of upgrading alpha-ray imaging detector
in low radioactivity background
H. Ito1*, H. Ishiura2, K. D. Nakamura3, and K. Miuchi2
1Tokyo University of Science, Chiba, Japan
2Kobe University, Hyogo, Japan
3Tohoku University, Miyagi, Japan*itoh.hiroshi@rs.tus.ac.jp

Poster #168

Abstract: In underground particle physics, current detectors arerequired massive volume of target using ultra-pure material.
Uranium or thorium impurities on the surface of the detector could
be reduced fiducial volume and produced background source via
emanation radon. So, α-ray imaging detector based on μ-TPC in alow BG has been developed to screening the material surface.

1. Alpha-ray Imaging Chamber (AICHAM)

gnitteselpm
Sa

NIM A 953(2020)163050.
Cu mesh2.4. Electronics and trigger data acquisition system
Drift plate (Cu)
10cm
to, T. Hashimoto, K. Miuchi et al.
Nuclear Inst. and Methods in Physics Research, A 953 (2020) 163050
131
The electronics for the µ-PIC readout consisted132
of amplifier-shaper discriminators [16] for 768 anode133
and 768 cathode10cmsignals and a position-encoding
134
module [17] to reconstruct the hit pattern. A data
Drift axis
Sample window
135
acquisition system consisted of a memory board(9.5×9.5 cm2)= ray
2.136 µ-PIC:
maicro-pixelchamberE
to record tracks and a flash analog-to-digital conIJ
137
verter (ADC)
for (x)the andenergy
measurement. The• Readoutof anodecathodeCF4 gas 0.2 bar
Volume:simultaneously.
138 (y)flash
ADC with 100 MHz sampling recorded the(35×35)×31cm3• 400
pitch ofstrips
139
sumμmsignal
the cathode strips with a full time
140
range μ-PICof 12 µs.
The anodesum signal issued the• Low-αdevelopedby replacing
31cm
to epoxyin triggering,order to in contrast to141 PItrigger.
Withmaterial,
this way ofµ-PIC (30×30cm2)
reducesurfaceemission.
142
the trigger
byαsignal(for example, primary scintil143
lation) in the TPC before the drift, the absolute
Fig.
2. Schematiccrosssection
section ofof detector
detector setup.
Sample window
size
is144
positionalong the drift direction cannot be meaFig.
2:
Schematiccrosssetup.
Όͱͧΐ̡̳̥͐
BasedTimechamberusing
9.5 cm onù 9.5 cm.
Electricprojectionfield is formed bya drift plate biased
at *2.5 kV and copper
145
sured. However, because the alpha particles werewires
with
1cm
pitchconnecting
with
chain
registers.
μ-PIC with low α-ray emission, 3D α-ray
146
expected to be emitted from the sample, the drifttracks are reconstructed.
147
along coordinate of the emission point was assumed
Then the image of α-ray emission for the@ Kamioka mine, Japan148
to be the position of the drift plate.
sample would be obtained.
130

Po-210anode(strip behind)

Calib-α

1. Photograph of the experimental setup. The detector system is composed ofmicro-TPC, a gas circulation system, and electronics. The stainless-steel vessel isovered so that the outer view of the TPC field cage can be viewed.

3. Demonstration of Alpha-ray imaging measurement

149

cathodeNIM A
977(2020)164285.

3. Performance check3.1. Alpha-particle source
2210
ocess.- Therefore,
a position-sensitivealpha detector is required in151
A10×10cm
copperplatewith
Pb accuCondition
1)MPPCs
2) Reflector sheet by PICOLON group
der to determine the site and perhaps the process associated with152
mulated on the surface was used as an alphaAfterinstalling cooled charcoal system.
materials
contamination.
153
particle source for the energy calibration andThis paper
is organized
as follows. The details of the alpha-particle
7MPPCs back side measuring
MaskedPCBector, setup, low-↵ micro pixel chamber ( -PIC), gas circulation sys154
energy-resolution measurement [11]. The source
Not andreplacing
field cagetime.
m, electronics,
trigger and new
data acquisition
systemsin
are thisdescribed
155
emits
alphaparticleswithanenergyof5.3
MeVSection 2. The performance check that uses the alpha-particle source,
210
156
asa
decayofPo. The alpha-particle emission
ample test, and background estimation are described in Section 3.
Fig. 3. Drift plate with a sample window (hole size is 9.5 cm ù 9.5 cm) and copper
Current
Performancee remaining background of the detector and future prospects are157
rate(hereinaftercalledtheαrate)ofthesource
supportmesh.
͊Ǌ̳ʜ̳ɧM̴Ƚ@ķcussed in Section 4. Finally, main conclusions are presented inEnergy Resolution: σE=7% @5.3 MeV
158
plate was calibrated to be 1.49 ± 0.01 α s−1 for 4.8–
ction 5.
Fig. 3: Drift plate with a sample window (hole size is159
5.8
MeVbyusing
theUltra-Lo
1800
[11].
Position Resolution: σx=7 mm 9.5 × 9.5 cm2 ) and copper
7MPPCsfrontside
measuringsupport mesh.
150

track length and the detector stability. The track length was expectedǊ͐Ă͋Ļ̘̥
Alpha-particle
imaging detector
on gaseous micro-TPC̒ʯΌͱͧΐMBĘ̫̀̒̌͌
Sensitivity:~10-3based
α/cm2/hr
to be longer, which improved the tracking performance when the gas

Count rate (α/hr/cm2)

Anode (cm)Alpha/hrPTEP(2021)
043F01160
3.2.
Energy calibrationpressures were low, while the discharge rate of the -PIC increased.
The@1
monthmeasurementE>3.5MeV−3 alpha/hr/cm2 (90% C. L.).
A new alpha-particle detector was developed based on a gaseous<1.77×10
range of 5 MeV alpha particle is Ì8 cm in 0.2 bar CF4 gas, which
would
161
An
energy calibration was conducted with the116 whichbackgroundsource,andso
theimpurities
wererecro-TPC upgraded from the NEWAGE-0.3a detector [14]wasMPPCbacksideSample region the detector size.
provide a reasonable detection
efficiency consideringSamplè͉̰͌̎Ă͋Ļ̘̥
edto-searchfor darkpoint
matter from2013. [14]. Details of the device with the new ma- 162 alpha-particle source. The energy was calculated
Unique- September, 2008 to January,
117
moved
The electric field in the drift volume, E = 0.4 kV_cm_bar, was formed
BG region
BG
e detector consisted of the micro-TPC using a low-↵ -PIC as readout,
163
from the flash ADC waveform. Figure 4 showsby supplying
a negative
voltageof 2.5 kVandplacing
field-shaping
118terial,alow-αµ-PIC,willbedescribedinRef[15].
1.
Severalsamples
canbemeasured
inas circulation system, and electronics, as shown in Fig. 1. The TPC
patterns with chain resistors every centimeter [15]. The drift
164 velocity
a typical energy spectrum of the alpha-particles enclosed
in a stainless-steel
vessel for the gas seal during thesametime.
was 7.4 ± 0.1 cm_ s. The -PIC anode was connected to +550 V. The
165
source.
The
energyresolution
wasestimatedtobeasurement.
3Calib.
source2. Since the BG data has been taken typical gas gain of -PIC was 10 at Ì 500 V.
166
6.7% (1σ) for 5.3 MeV, which is worse than the119
2.3.
Gas
circulation
systemsimultaneously,sys. uncertainty for gas. Setup andconfiguration
167
Ultra-Lo 1800 resolution of 4.7% (1σ) for 5.3 MeV.
2.2. Low-↵ -PIC
MPPC front side
condition can be canceled out.
168
This deterioration was thought to be due to theFig. 2 shows a schematic view of the detector, where the120 gas volumeAgascirculation
systemthatuses
activatedchar3. Calib. source is put at edge in orderto The backgroundstudy
for the direction-sensitive dark matter search
35 cm ù 35 cm) ù 31 cm. The detector was placed undergroundattheEnergy
(MeV)
169
gain
variation of the µ-PIC detection area.
121
coal
pellets
wasdevelopedforradon-background
238
232
Cathode(cm)suggests that -PIC has radioactive impurities of
U andTh whichmonitorgas statusdependinga time.
mioka facility
in the Institutefor Cosmic
Ray Research,onJapan.
Ansuppression
protect
gain
deteriemitandalphatoparticles[5]. aA against
survey witha HPGe
detector revealedygen-free copper plate with a surface electro-polished 122
to a roughness
Thealpharatewas
frombothside are similar.
3.3.
that-PIC’s
glass
cloth
themainbackgroundsource, 170
and sothe Event reconstruction
123 as oration
A pump (EMP,
0.4 m and a size of (35 cm ù 35 cm) ù 0.1 cm was usedthe drift due to the outgassing.
As a wereresult,
α rate=polyimide
0.58±0.04
α/hr/MPPC
oneside
impurities
removed.
Thewithglass
clothinthe-PICte. The drift plate had an opening with a size of 124
9.5 cmMX-808ST-S)
ù 9.5 cmand witha needle-type
circulatemeter
171
Figure5 shows a typical event display with thewasreplaced
anewmaterialofpolyimide
andepoxy.
Details
ofa sample window. A copper mesh made of 1-mm- wire in 1-cm
125
(KOFLOC,
wereto flow
the-PIC,
gaswillat bea described172
tracksand flash ADC waveform data for alphathePK-1250)
device with thenewusedmaterial,
a low-↵
inch (aperture ratio of 0.81) was set on the drift plate to hold the3210
4.
ProgreesofUpgrading
Ref.
[16,17].
126
rateof∼500
cm/min.
Thegaspressure
wasmon173particleemission
fromPo. The hit points
weremple at the window area, as shown in Fig. 3. The electrons ionized
Before mask(2)operation
Suppressed
PCB
α-ray174 determined based on coincidence with anode andthe alphadrift towards
the -PIC
with a vertical
upward- to ensure the stable
963.9 hr127
itoredof thecircula(1)particles
Installing
CooledCharcoal
SystemE>2MeV
2.3. Gas circulation systemnting electric field E. CF4 gas (TOMOE SHOKAI Co.LTD, 5N grade:128
tionsystemandasmaintained
withinanincrease
175
cathodedetections.
Figure5(c)showstheanode•
PCB:
Print
CircuitBoard
includesofglass
fiber,
withU/Th
radiopurity of 99.999% or more), which was also used in the NEWAGE∼ 2% for severalweeks. system
176 atcathodeplane forthe track. The open circles are
A gas circulationthat uses
activatedcharcoalpellets
impurities,
alpharays emittededge of fiducialarea.
a, was used because of the low diffusion properties.129The ofpressure
One month measurement
s set at 0.2 bar as a result of the optimization between the expected(Molsievon, X2M4/6M811)
theissuppressionof
radonthe alpha rays were shield.
• Thewasreddeveloped
shade forareamaskedthen3!""
Unit: ×10 α/cm /hrsource•Wecomparedimage
beforeandafterthemask,andtheedge
2 &".$'
OFF: 0.76!".$%
eventsarereduced
but
it
will
be
more
suppressed
innextplan.
144&".")
ON: 0.17!"."(After mask

Energy (MeV)

Alpha-ray rate from Rn was suppressed to 1/4.

←masksource(3) New field cage replacement

15cm• New field cage was designed as a half length of previous height,
hnew=15 cm.
• Remained radon would be reduced to half, and diffusion due tothe drift would be small, i.e., S/N and the position resolution
would be improved.
• Replacement has been completed in this summer, and theperformance test after the replacing is ongoing.

793.6 hrE>2MeV

5. Conclusion• We have been developing alpha ray imaging detector based on μ-TPC
in low radioactivity background• The detector has been upgrading and the performance estimation isongoing after the add new system.
• In parallel, the sample measurement have been done, e.g. MPPC andreflector sheet.