文部科学省 [先端研究基盤共用・プラットフォーム形成事業 成果報告]
兵庫県立大学 高度産業科学技術研究所 ニュースバル放射光施設

ガンマ線偏光計HARPOの試験
所属

Ecole Polytechnique LLR

ビームライン

ＢＬ01

利用者氏名

Denis Bernard

利用分野

量子ビーム技術

利用年度

2015年度活用技術

ガンマ線利用

利用成果の概要
The HARPO time projection chamber (TPC) was installed in a polarised gamma-ray beamline at
NewSUBARU. This is a high angular resolution telescope for gamma-ray polarimetry. Data weretaken at photon energies from 1.7 MeV to 74 MeV, and with different polarisation configurations.
The experimental setup of the TPC and the photon beam are described attached figures. The
first results from the beam campaign are shown in attached papers.
HARPO時間投影チェンバー（Time Projection Chember)をNewSUBARU偏光ガンマ線ビームラ
インに設置した。これはガンマ線偏光測定のための高角度分解望遠鏡である。
光子エネルギー1.7MeVから74MeVで異なる偏光条件で、データの取得を行った。ガンマ線ビー
ムとTPCの実験配置を図に示す。実験結果の最初の形跡結果を、添付論文に示す。
<利用目的>
Test of HARPO, a high angular resolution telescope for gamma-ray polarimetry.
ガンマ線偏光計のための高角度分解望遠鏡HARPOの試験。
<実験方法>
Data were taken at photon energies from 1.7 MeV to 74 MeV, and with different polarisation
configurations.
The experimental setup of the TPC and the photon beam are described below.
光子エネルギー1.7MeVから74MeVで異なる偏光条件で、データの取得を行った。
ガンマ線ビームとTPCの実験配置は下図の通り。
NewSUBARU
Electron
StorageRing

http;//www.lasti.u-hyogo.ac.jp/NS/

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文部科学省 [先端研究基盤共用・プラットフォーム形成事業 成果報告]
兵庫県立大学 高度産業科学技術研究所 ニュースバル放射光施設<実験結果>
The first results from the beam campaign are shown in attached papers.
実験の最初の解析結果の論文を添付する。

<今後の見通し>This results will use development of HARPO for cosmic gamma-ray polarimeter.
本実験の結果は、宇宙ガンマ線偏光計HARPOの今後の評価・開発のために使われる。

問い合わせ先兵庫県立大学 高度産業科学技術研究所
ニュースバル放射光施設 共用促進室〒678-1205 兵庫県赤穂郡上郡町光都1-1-2
TEL : 0791-58-2543 FAX : 0791-58-2504
E-mail : kyoyo@lasti.u-hyogo.ac.jp

http;//www.lasti.u-hyogo.ac.jp/NS/

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arXiv:1503.03772v1 [astro-ph.IM] 12 Mar 2015

the 7th Symposium on Large TPCs for Low-Energy Rare Event Detection,
Paris, 15-17 December (2014).

HARPO: beam characterization of a TPC for
gamma-ray polarimetry and high angular-resolution
astronomy in the MeV-GeV range
Shaobo Wang
LLR, Ecole Polytechnique, CNRS/IN2P3, France
E-mail: wang@llr.in2p3.fr

Denis Bernard, Philippe Bruel, Mickael Frotin, Yannick Geerebaert,
Berrie Giebels, Philippe Gros, Deirdre Horan, Marc Louzir,
Patrick Poilleux, Igor Semeniouk
LLR, Ecole Polytechnique, CNRS/IN2P3, FranceDavid Attié, Denis Calvet, Paul Colas, Alain Delbart, Patrick Sizun
Irfu, CEA-Saclay, France

Diego Götz
AIM, CEA/DSM-CNRS-Université Paris Diderot, France
Irfu/Service d’Astrophysique, CEA-Saclay, France

Sho Amano, Takuya Kotaka, Satoshi Hashimoto,
Yasuhito Minamiyama, Akinori Takemoto, Masashi Yamaguchi,
Shuji Miyamoto
LASTI, University of Hyôgo, Japan

Schin Daté, Haruo Ohkuma
JASRI/SPring8, Japan
Abstract. A time projection chamber (TPC) can be used to measure the polarization of
gamma rays with excellent angular precision and sensitivity in the MeV-GeV energy range
through the conversion of photons to e+ e pairs.
The Hermetic ARgon POlarimeter (HARPO) prototype was built to demonstrate thisconcept. It was recently tested in the polarized photon beam at the NewSUBARU facility
in Japan. We present this data-taking run, which demonstrated the excellent performance ofthe HARPO TPC.

10 3

Nuclear conversion recoil momentum-5/4
unobserved (68% containment) (E )(degrees)σθ (mrad)1. Introduction
Gamma-ray astronomy is the primary means by which we can study the non-thermal processes
occuring in cosmic sources such as active galactic nuclei (AGN), pulsars and gamma-ray bursts(GRBs) [1]. The ability to measure the linear polarization in this energy range would provide a
powerful diagnostic for the understanding of the physical processes at work in these sources [2].
A telescope with the ability to perform polarimetry above 1 MeV has never been flown in space[3]. Gamma-ray telescopes have improved in sensitivity and resolution from COS-B [4] to the
Fermi-LAT [5], but the performance of these pair-creation telescopes decreases at low energy
where the background is problematic and the angular resolution is limited by multiple scattering.
The performance and polarimetry potential of a TPC have been studied in detail in Ref. [6]and [7]. Several e↵ects contribute to the angular resolution, which is shown as a function ofphoton energy in Fig. 1 for an argon gas TPC. The angular resolution for pair production islimited by multiple scattering of the electrons in the gas. Also, most conversions take place inthe field of a nucleus whose recoil momentum is impossible to measure because of the very shortrecoil path length. This puts a limit on the resolution at energies < 100 MeV. Even with theselimitations however, an improvement in angular momentum of up to an order of magnitude withrespect to that of the Fermi-LAT is achievable with a TPC.

10

10 2

Fermi (front evts)(E-0.78)1
101Multiple scattering
Ar 10 bar
-3/4
(E )
1

10

10

0.1

2

10

3

10

4

E (MeV)
Figure 1. Various contributions to the angular resolution of a TPC gamma telescope as afunction of the photon energy [6], compared to that of the Fermi-LAT [5]. The multiple scattering
contribution (green) is given for a 10 bar argon TPC, using an optimal tracking such as that
implemented in a Kalman filter, and with a track sampling of 1 mm and a spatial resolution of
0.1 mm. At high energies (> 100 MeV), it is limited by the multiple scattering of the electrons
in the gas. At lower energies, it is constrained by the unknown recoil momentum of the nucleus.
This limit can be overcome by looking at triplet conversion, but they are rarer and more difficult
to reconstruct.
The polarimetry of cosmic sources with a pair-creation telescope has long been considered
to be difficult or even impossible, as thick, high-Z detector elements are needed to convert theincoming photon, and the conversion electrons then undergo multiple scattering in the converter,
after which the information about the azimuthal angle of the conversion plane is blurred [8].
We have built and validated an event generator [7] which is full (5D, either nuclear or triplet
conversion) exact down to threshold and polarized. We have characterized the properties ofa telescope based on a thin homogeneous detector with optimal tracking and have established2the power laws that describe the dominating contributions to the angular resolution [6]. For
such a detector, even when the dilution of the e↵ective polarization asymmetry due to multiple
scattering is taken into account, polarimetry can still be performed with high precision (still
under the assumption of optimal tracking) [7].
The HARPO project aims to characterize the TPC technology as a high angular resolution
polarimeter and telescope in the MeV-GeV range, enabling us to obtain unprecedented
sensitivity for the detection of low-energy gamma rays. A TPC is a detector in which traversing
charged particles ionize the detector material. The electrons produced drift along an electricfield, E, and are then amplified and measured on the x-y readout plane. The drift time gives ameasure of the z coordinate. HARPO would be the first space polarimeter above 1 MeV.
In this paper we first describe the demonstrator that we have built to validate the performanceof the TPC technology, in particular the characterization of the GEM and micromegas
combinations used for gas amplification 1 . Then we present the recent experimental campaign,in which the detector was exposed to the quasi-monochromatic and almost fully polarized beamprovided by the BL01 line of the NewSUBARU facility, operated by the LASTI in University ofHyôgo in Japan.
2. The HARPO TPC demonstrator
The HARPO demonstrator [9] (Fig. 2) is a 30 cm cubic TPC which can be operated from low
pressure up to 5 bar. It is surrounded by 6 scintillator plates that provide an external trigger.
Amplification is performed with the combination of two GEMs [10] and one micromegas [11].
The readout comprises two series (x, y, perpendicular to each other) of 288 copper strips with
1 mm pitch. Signals are acquired with AFTER chips [12] using the Feminos system [13].

Figure 2. Left: the HARPO TPC demonstrator. Right: sketch of HARPO. A cubic TPC in
the center, with a readout plane of micromegas and GEM on the left. Six scintillator plates,
each equipped with a wavelength shifter and two photo-multipliers, are used for trigger. The
system fits in an aluminum cylinder which can operate up to 5 bar pressure.

1

The 2012 tests showed that with the 0.4 mm narrow collecting strips that we are using, the micromegas alone
does not provide sufficient amplification at 4 bar for routine operation in a safe configuration [9]. We have therefore
complemented the micromegas with two layers of Gas Electron Multiplier (GEM).

33. The HARPO amplification system
We first characterized the performance of the GEM+micromegas amplification system using aradioactive source. The amplification system was subsequently integrated into the TPC detector,
where it was characterized using cosmic rays.

events/second

3.1. Characterization of micromegas and GEM(s) combinations with a radioactive source
We characterized the combination of a micromegas and either one or two GEM in a gas mixture(Ar:95 %Isobutane:5 %) at atmospheric pressure. This was done in a dedicated test setup using
a 55 Fe source[16]. The successive amplification steps were kept at a distance of 2 mm from eachother by spacers (Fig. 3 left).
Vmesh Ebtrans VbGEM Edrift sect MCA
400 250 270 50
5
5V

104
103
102

Main peak, µM only

10

Main peak, µM+1GEM
Escape peak, µM+1GEM

1
0

200

400

600

800

1000

charge [ADC]
Figure 3.
Left: the layout of the test setup, with one micromegas and two GEMs. (Byapplying a null or inverted voltage to the top GEM, we only observe conversion in the twolower regions). Right: an example of a measured 55 Fe spectrum from amplification with one
micromegas and one GEM. The main peak (5.9 keV) is plotted after amplification with only the
micromegas (red) or with both micromegas and GEM (green). The escape peak is only visible
with full amplification (yellow).
In argon, the X-rays from a 55 Fe source deposit either 5.9 keV (main peak) or 2.7 keV (escapepeak) upon ionization. This conversion can take place either above or below a given GEM
sheet. The ionization electrons are therefore either amplified by that GEM or not. A typical
spectrum is shown in Fig. 3 (right). The main and the escape peaks are seen with amplificationfrom the micromegas and one GEM, and the main peak with micromegas amplification only.
The ratio of the position of the two main peaks provides a precise measurement of the absolute
GEM amplification gain. Further details on these measurements, including foil transparency
and extraction efficiency, can be found in Ref. [16].
3.2. Characterization with cosmic rays
The system of micromegas and 2 GEMs was commissioned in the detector. It was tested withcosmic rays, using the same gas mixture at several pressures. Most cosmic muons are relativistic
and therefore deposit the same average energy per unit length. The TPC was set ”vertical”, sothat most cosmic rays entered it through the amplification system. We selected events with acosmic ray that crossed the full z of 30 cm by triggering on the coincidence of the up and down
scintillator signals.
4total gain

Q

×106
70
60
50

104

P = 2.0 bar
P = 1.5 bar
P = 1.0 bar
test box, 1 bar
P = 0.5 bar

103

40
30
20

102

10
0
0

100

200

300

400

500

120 140 160

180 200 220 240 260

280 300

Vbottom
GEM [V]t[bin]

Figure 4. Left: average total charge per track as a function of time, for a run of cosmic rays
that traversed the full TPC length, entering through the anode and exiting through the anode.
We extract the drift velocity, the electron absorption per unit length and the amplification gainfrom such spectra. Right: the e↵ective gain measured from cosmic rays in the TPC for several
gas pressures. The results at 1 bar are consistent with those obtained with the radioactive source.

Table 1. Electron beam energy, laser wavelength, and -ray energy at the Compton edge
obtained from these.
Ee [GeV]Laser[µm]
Nd:YVO4 (2!) 0.532
Nd:YVO4 (1!) 1.064
Er(fiber)
1.540
CO2
10.55

0.618
13.4
6.76
4.68

0.982
E
33.3
16.9
11.8
1.74

1.233
[MeV]
52.1
26.6
18.5
2.73

1.480
74.3
38.1
26.5
3.93

Pulsing rate
[kHz]
20
20
200
CW

Polarization
P ⇡1
P ⇡1
P =0
P =0

The charge distribution over drift time is shown in Fig. 4 (left). The value of the drift velocity,
vdrift , was easily obtained using the basic relation of the TPC mechanism, z = vdrift ⇥ tdrift ,since the cosmic rays traverse the full z thickness of LT P C = 30 cm. The slope of the plateau
corresponds to electron absorption in the gas. The average charge per track for each run is used
to estimate the amplification gain. From this spectrum, the drift velocity, the attenuation lengthand the total amplification gain were extracted. Fig. 4 (right) shows the amplification gain forseveral TPC gas pressures as a function of the voltage on one of the GEMs. The measurements
are consistent with those obtained using the 55 Fe source.
4. Data-taking at NewSUBARUIn Nov. 2014 the detector was exposed to a beam of pseudo-monochromatic gamma-ray photonsdelivered by the BL01 beam line at NewSUBARU [15].
4.1. Laser-Compton Source (LCS)
The gamma-ray beam is produced by the inverse Compton scattering of laser photons byrelativistic electrons. The electron beam energy can be varied in the range 0.6 ⇠ 1.5 GeV.
The laser beams available are: Nd:YVO4 (2!) with wavelength = 0.532 µm, Nd:YVO4 (1!)= 1.064 µm, Er (fiber) = 1.540 µm and CO2 = 10.55 µm. This results in a gamma energy
range at the Compton edge between 1.7 MeV and 74 MeV as shown in Table 1. As the gamma
5has a maximum energy for forward Compton scattering, energy selection was performed byusing a collimator on axis. When collimation is applied, the polarisation of the laser beamis almost entirely transferred to the gamma beam: in that way, an almost fully polarized
gamma beam (P ⇡ 1) is obtained. A general issue in polarization studies is the control of a
possible systematic bias induced by the non-cylindrical-symmetric structure of the detector [17].
Therefore in addition to the fully polarized data, some data with a non polarized beam weretaken. To produce such a beam, a /4 plate is used. This changes the linear polarization
to circular polarization, which, as far as pair conversion is concerned, is equivalent to random
polarization (P = 0). Or vice versa.
4.2. The HARPO Trigger system
For the previous characterizations using cosmic rays, the TPC was triggered on a simple
coincidence of scintillator signals [9, 14]. In contrast, for data taking in beam, we wished tomaximize the fraction of selected gammas that converted in the gas. Therefore a dedicated,
more sophisticated trigger was designed. In addition it provides separately the control of theefficiency of each of its components.
4.2.1. Description of signals The HARPO trigger system is based on a PARISROC2 [19] chip
mounted on a PMM2 [18] board. This trigger is built from the discriminated signals of thescintillators, laser and micromegas mesh as shown in Fig. 6. The available signals are:• Laser: when a pulsed laser is used, the laser’s trigger signal L (Fig. 5) is used. It also
determines the time at which the event takes place, t0 , with a precision of a few 10 ns.

Figure 5. Schema of the Laser trigger used at NewSUBARU.

• Scintillator: the signal comes from 12 PMTs on 6 scintillators. The signal is recorded,
whether there is a trigger or not. It gives t0 with a hundred nanosecond precision. Thissignal determines t0 when the laser signal is not available (CO2 ).
• Micromegas mesh: the signal induced on the mesh is long and has an unpredictable shape:
it corresponds to the time distribution of the charge deposited by the event in the TPC gas
as it drifts towards the anode. The signal is amplified and derived through a 5 nF capacitor.
It is then discriminated with a constant-fraction discriminator (CFD), so as to determine
the rising edge tstart of the signal. The RC constant of the readout electronics of the mesh6

signal is ⇡ 1µs. It has been fixed as a compromise between the amount of electronic noise
given the large capacitance of the full mesh, of 8 nF, and the need of a precise determinationof the rising edge of the signal tstart .
Events that have a too small of a delay between the time of the event, t0 , and thediscriminated mesh signal rising edge, tstart , are rejected. This provides a veto on
background tracks created in the beam line upstream of the TPC and on gamma rays whichconverted in the material between the up scintillator and the active TPC gas (example: inthe PCB supporting the amplification system).

Figure 6. Global view of HARPO trigger system.

4.2.2. Gamma trigger line. The gamma trigger, T , is designed so as to select gamma rays
produced by LCS and that converted inside the TPC gas. The conditions are listed below:• the upstream scintillator signal (Sup ) is used as a veto,• at least one downstream scintillator signal (O, other than “up”) is required,• the laser trigger signal (L), when present, is used,• the events with a signal present in the mesh are selected, vetoing the prompt part as alreadyexplained, we require tstart t0 > 1µs. This “slow” selection of a mesh signal is denoted
Mslow .
The maincomponents:

trigger line is therefore defined as the combination of the four following
T ⌘ S up \ O \ Mslow \ L(1)

The distribution of tstart is shown in Fig. 8. Gamma conversions inside the TPC gas (blue)
are the signal events. The flatness of that part of the spectrum is due to the fact that theprobability of conversion per unit path length is constant for a thin detector. Tracks entering
the detector from upstream and gamma conversions in the detector material upstream of the
TPC gas, which escaped the vetos, form the (green) peak. If we had used a trigger without anyveto, the height of this peak would have been larger by several orders of magnitude, precluding
an efficient data taking. Events (red) which lie outside the normal time range (i.e., tstart < 100
or tstart > 400) are due to energy deposition inside the gas by events that are not related to thetrigger (pile-up).
7

4000

X

3500
3000

200

4000

channelchannel250

Eγ =74.3GeV

250

3500

Y

3000

LTPC/vdrift

200

2500

2500
150

2000

150

2000

100

1500

100

1500

1000
50

t0
0

0

500

tstart100 200 300 400 500

0

1000
50

t0
0
0

500

tstart100 200 300 400 500

t[30ns time bins]

0t[30ns time bins]#events

Figure 7. A gamma event that was selected by the gamma trigger, with definitions of timing
information.

104

tracksoriginating upstream

103

γ conversion in gas
1µs
102

noise

noise

10
0

100

200

300

400

500

tstart [30ns time bins]

Figure 8. Distribution of event rising edge tstart for one run (all trigger lines): tracks originating
upstream (green), gamma conversion inside the TPC gas (blue), charge deposition inside thegas by events not related to the trigger (red).

4.2.3. Other Trigger Lines. In addition to the main gamma trigger line, separate triggerlines have been formed with one of the trigger components omitted (such as O \ Mslow \ L,
S up \ Mslow \ L, S up \ O \ L, S up \ O \ Mslow , in the case of a pulsed laser). Analysis ofthese specific data will enable us to control separately the signal efficiency and the backgroundrejection factor for each component of the main trigger line. Other dedicated lines were used,8

such as the “traversing z track” line for calibration purposes. Most of these additional lines were
down-scaled so as to not saturate the bandwidth of the digitizing electronics, and were active
only for a fraction of the time, as denoted by the rainbow part of Fig. 6. Approximately 90%of the time was devoted to the main trigger line.
4.3. Gas
The pressure vessel was evacuated down to ⇠ 2 ⇥ 10 6 bar, rinsed with ⇠ 0.1 bar gas, evacuatedagain, and then filled with 2.1 bar of the gas mixture (Ar:95 % Isobutane:5 %). This same gas
was used in a sealed mode for 23 days.
At the end of the data taking, a pressure scan from 1 to 4 bar was performed, with thegas amplification tuned so that the signal amplitude was kept constant (Ee = 1.5 GeV, Nd 1!,
P = 1).
4.4. Monitoring
During data taking in the beam, some basic parameters were monitored, such as alignment,
amplification gain, trigger performance and event quality after some very basic tracking.

ystart[mm]xstart[mm]4.4.1. Alignment. The alignment of the HARPO detector in the collimated gamma beam wasmonitored by plotting the transverse (x or y) position of the gamma conversion candidate vertex
as a function of its longitudinal position z (Fig. 9). The vertex is defined here from the chargeweighted barycenter of the three first clusters of a selected gamma-conversion event (triggeredby the gamma trigger).

170

170

160

160
150

150

140

140

x=-0.0299t+150.52
130

130

120

120

y=0.0068t+145.75

100 150 200 250 300 350 400

100 150 200 250 300 350 400

tstart[30ns time bins]

tstart[30ns time bins]

Figure 9. Alignment plot for run 1277 (1 mm diameter collimator), with linear fit (gamma
trigger line).

9

Q[ADC]8000

absorption

7000
6000

Run 1237
Run 1569

Gain

5000

LTPC/vdrift

4000
3000
2000
1000
0
0

100

200

300

400

500

t[30ns time bins]
Figure 10. The average energy deposited at each time bin per traversing track along the z-axis
for two runs separated by a full week of high flux running.

4.4.2. Calibration. The calibration of the TPC, i.e., the determination of the drift velocity, ofthe amplification gain and of the electron absorption, is performed by the analysis of the time
distribution of the average charge per track as was described in section 3.2. A special triggerline (⇠ 2% of the events) was used to select the tracks traversing in the z direction (i.e., along
the drift time) without biasing the time distribution.
Fig. 10 and Fig. 11 show that there was no significant gas deterioration over the two weeks
in the beam. The small features that can be seen in the time charts (Fig. 11) are associatedwith changes in the gamma-beam energy and/or of the laser pulsing rate, meaning that thesesmall biases are related to the measurement, not to the quantity measured. The gas gain andthe drift velocity were found to be sufficiently stable and the electron absorption due to either
leaks or outgassing was found to be sufficiently small that we didn’t have to renew the gas.
5. Conclusions and perspectives
A gas TPC is the instrument suited to covering the performance gap in cosmic gammaray detection in the MeV-GeV range. We have built a demonstrator that includes a
micromegas+GEM amplification system. We have performed an experimental campaign
with this demonstrator using the high-flux, quasi-monochromatic, polarised photon beam at
NewSUBARU. Tests that were performed during data taking indicate an excellent detector
performance.
We have more than 1 TB of data (> 6 ⇥ 107 events, a large fraction of which is estimatedto be gamma conversions in the gas) to analyze so that we can study the gamma conversion toe+ e pairs and, in particular, measure the low energy polarization asymmetry, and characterize
the performance of the demonstrator both as a gamma telescope and as a gamma polarimeter.
Further development is ongoing to meet the constraints of a space environment [14], in particular,
tests on the behaviour of the detector in space with a dedicated trigger. The next step will thenbe to verify its operation with a balloon flight.
6. Acknowledgments
This work is funded by the P2IO LabEx (ANR-10-LABX-0038) in the framework“Investissements d’Avenir” (ANR-11-IDEX-0003-01) managed by the French National Research
Agency (ANR), and directly by the ANR (ANR-13-BS05-0002).
10

gain [a.u]

3

×10
3000
2500
2000
1500
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500
0

11-06 01h

11-07 01h

11-08 01h

11-09 01h

11-10 01h

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11-12 01h

11-13 01h

11-14 01h

11-15 01h

11-16 01h

Vdrift [cm/us]

time
3.4
3.3
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3.1
3
2.9
2.8

11-06 01h

11-07 01h

11-08 01h

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2.14

31

2.135

30

2.13
2.125

29
28

2.12

27

2.115

26

2.11

25

2.105

24

2.1
2.095

T [C]

P [bar]time

11-06 01h

11-07 01h

11-08 01h

11-09 01h

11-10 01h

11-11 01h

11-12 01h

11-13 01h

11-14 01h

11-15 01h

11-16 01h

23

time

Figure 11. Time variation of the gain, estimated by the average charge per track, corrected
for the track angle that is normalized to 30 cm track length, the drift velocity (x in red, y inblue) and the pressure (black) and temperature (green) of the gas.

This work was performed by using NewSUBARU-GACKO (Gamma Collaboration Hutch of
Konan University).
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[9] D. Bernard, HARPO - A Gaseous TPC for High Angular Resolution Gamma-Ray Astronomy and Polarimetry
from the MeV to the TeV, Nucl. Instrum. Meth. A 718, 395 (2013). [arXiv:1210.4399 [astro-ph.IM]].

11[10] F. Sauli, GEM: A new concept for electron amplification in gas detectors, Nucl. Instrum. Meth. A 386, 531(1997).
[11] Y. Giomataris, P. Rebourgeard, J. P. Robert and G. Charpak, MICROMEGAS: A high-granularity positionsensitive gaseous detector for high particle-flux environments, Nucl. Instrum. Meth. A 376, 29 (1996).
[12] P. Baron, D. Calvet, E. Delagnes et al., AFTER, an ASIC for the readout of the large T2K time projection
chambers, IEEE Trans. Nucl. Sci. 55, 1744 (2008).
[13] D. Calvet, “A Versatile Readout System for Small to Medium Scale Gaseous and Silicon Detectors”, IEEE
Transactions on Nuclear Science 61 (2014) 675.
[14] D. Bernard et al., Presented at SPIE2014. Proc SPIE 9144 M. [arXiv:1406.4830 [astro-ph.IM]].
[15] S. Amano et al., Several-MeV -ray generation at NewSUBARU by laser Compton backscattering, Nucl.
Instrum. Meth. A 602 2009 337.
[16] Ph. Gros, HARPO - TPC for High Energy Astrophysics and Polarimetry from the MeV to the TeV, TIPP 14
3rd Technology and Instrumentation in Particle Physics conference, 2-6 June 2014 Amsterdam, Proceedings
PoS(TIPP2014)133.
[17] M. Weisskopf, X-ray Polarimetry: from the early days to an outlook for the future, X-ray polarisation inastrophysics, August 25-28, Stockholm, Sweden.
[18] B. Genolini et al., PMm2: Large photomultipliers and innovative electronics for the next-generation neutrino
experiments, Nucl. Instrum. Meth. A 610 (2009) 249 [arXiv:0811.2681 [physics.ins-det]].
[19] S. Conforti Di Lorenzo et al., PARISROC, an autonomous front-end ASIC for triggerless acquisition in next
generation neutrino experiments, Nucl. Instrum. Meth. A 695 (2012) 373.

12

HARPO, a gaseous TPC as a gamma telescope andpolarimeter: first measurement in a polarised
photon beam between 1.7 and 74 MeV

A. Delbart∗Irfu, CEA-Saclay, Gif-sur-Yvette, France
E-mail: alain.delbart@cea.fr

S. Amanob , D. Attiéa , D. Bernardc , P. Bruelc , D. Calveta , P. Colasa , S. Datéd ,M. Frotinc , Y. Geerebaertc , B. Giebelsc , D. Götzae , P. Grosc , S. Hashimotob ,D. Horanc , T. Kotakab , M. Louzirc , Y. Minamiyamab , S. Miyamotob , H. Ohkumad ,P. Poilleuxc , I. Semenioukc , P. Sizuna , A. Takemotob , S. Wangc , M. Yamaguchib ,a Irfu, CEA-Saclay, Gif-sur-Yvette, Franceb LASTI, University of Hyôgo, Japanc LLR, Ecole Polytechnique, CNRS/IN2P3, Palaiseau, Franced JASRI/SPring8, Japane AIM, CEA/DSM-CNRS-Université Paris Diderot, France

Gamma-ray astronomy enables the exploration of the non-thermal emission and magnetic field
configuration of objects such as active galactic nuclei (AGN), gamma ray bursts (GRB) and pulsars. Presently, there is a sensitivity gap between 1 MeV and 100 MeV. Additionally, there is nopolarisation measurement above 1 MeV, although such a measurement could shed light on emission processes. A gaseous detector can achieve a much better angular resolution in the MeV-GeV
range than the current/past telescopes that use tungsten converters, thanks to the reduced multiple
scattering of the electrons and positrons. This translates to a greatly improved point source sensitivity and gives access to the linear polarisation of the photons through the azimuthal angle of thepair.
The HARPO time projection chamber (TPC) is a high angular resolution telescope for gamma-ray
polarimetry. It was installed in a polarised gamma-ray beam at NewSUBARU in Japan in 2014.
Data were taken at photon energies from 1.7 MeV to 74 MeV, and with different polarisation
configurations. The experimental setup of the TPC and the photon beam are described. The first
results from the beam campaign are shown.

The 34th International Cosmic Ray Conference,
30 July- 6 August, 2015
The Hague, The Netherlands∗ Speaker.

c Copyright owned by the author(s) under the terms of the Creative Commons Attribution-NonCommercial-ShareAlike Licence.

http://pos.sissa.it/

HARPO TPC gamma telescope and polarimeter: measurements in a polarised photon beam

A. Delbart

1. Science caseHigh-energy photons such as γ-rays emitted by cosmic sources are produced by non-thermal
phenomena that involve high-energy charged particles. They provide unique information on theprocesses at work in objects such as active galactic nuclei (AGN), pulsars and gamma-ray bursts(GRB). Presently γ-ray astronomy suffers from a gap in sensitivity between the energy ranges inwhich Compton telescopes (sub-MeV), and e+ e− -pair telescopes (above 1 GeV) excel. In principle,
pair telescopes could be sensitive down to threshold, but in practice the bad angular resolution at
low energy hinders their ability to assign a given photon to a particular source. This situation
is detrimental to the understanding of objects that have a multi-peak spectral energy distribution(SED), such as AGN, and to objects whose emission is peaking in the MeV region (the prompt
emission of many GRB). A better sensitivity is also needed to track high-energy cosmic-rays fromsuper-nova remnants (SNR) from their hadronic interactions, by the detection of the π 0 bump at≈ 100 MeV. An improved angular resolution is needed to observe crowded regions of the γ-raysky, such as in the centre of our galaxy.
At low photon energies, from radio-waves to soft-X-rays, polarimetry provides a powerful insight to phenomena at work inside cosmic sources, in particular it provides a means to measure
the magnetic field. Above a couple of MeV, this diagnostic is missing: not only does the Compton cross section decrease at high energy, but its polarisation asymmetry also decreases, like 1/E[1]. Polarimetry would allow, for example, to decipher leptonic from hadronic models of blazars[2]. In the case of a γ-ray production by synchrotron emission of high-energy protons, the linear
polarisation fraction stays high from X-ray to γ-ray energies. In the case of “leptonic” emission
though, X-ray emission is highly polarised, but the γ-ray part of the spectrum is completely washedout by the inverse Compton scattering of thermal photons, and the total emission is unpolarised.
Polarimetry is also of interest for searches of Lorentz invariance violation (LIV) using GRB [3]. In
the presence of LIV, the induced birefringence of the vacuum would result in an energy-dependent
rotation of the polarisation direction of a (supposedly) initially polarised emission, upon propagation. On average this would result to an unpolarised radiation: if a non-zero polarisation of distant
GRB is observed, an upper limit on LIV-producing effects by yet-unknown physics beyond thestandard model is derived [3]. As the rotation angle is proportional to the squared photon energy,
extending polarimetry to the γ-ray energy range would enable a huge gain in sensitivity.

2. Telescope and polarimeter with a thin homogeneous detector and an optimal
tracking
Past and presently active pair telescopes use an active target that consists of a “mille-feuille”,
a multi-layer set of alternatively high-Z converter slabs and sensor foils. In the case of the Si/W
tracker of the Fermi/LAT, the single photon angular resolution is dominated by the multiple scattering of the electrons in the slabs and scales like E −0.78 below 10 GeV, and amounts to ≈ 4◦ at
100 MeV [4]. Doing better with this technology would involve thinner and/or lower-Z converters,
a severe issue for space missions for which the number of electronic channels is limited. A way
out is the use of a low-density (here gas) detector such as a time projection chamber (TPC), a detector widely used in particle physics: a volume of (here homogeneous) material is immersed in a2

A. Delbart

HARPO TPC gamma telescope and polarimeter: measurements in a polarised photon beam

(here uniform) electric field. Upon occurrence of an “event”, that is of the conversion of a photon
to an e+ e− pair, the two tracks ionise the gas after which the ionisation electrons drift towards
an anode where they are amplified, collected and sampled in time. A segmented anode provides
a time “map” of the signal over the two directions x and y transverse to the electric field. Time
sampling provides a measurement of the drift duration, that is of the drift length z. In total a 3D
image of the event is provided at a reasonable cost in terms of number of electronics channels. The
single-photon angular resolution includes several contributions:• Recoil. In the case of a “nuclear” conversion, that is a conversion by interaction of thephoton with the field of a nucleus of the detector, the energy that is transferred to the nucleus
is small and the track length is too short to be reconstructed. The contribution of the recoil
momentum to the reconstructed momentum of the photon is therefore missing. Given thehigh-momentum tail of the recoil momentum distribution (Fig. 3 of [5]), we have estimatedthe contribution to the angular resolution by the angle that includes 68 % of the events [5] tobe ≈ 1.5 rad(E/1MeV)−5/4 .

momentumunobserved(68% cont)2(E-5/4)
10

back
front

Fermi (E-0.78)

10

-1

10

2

(degrees)

Nuclear conversion

10 3 recoil

Sensitivity E dN/dE (MeV / (cm s))σθ (mrad)Triplets. In the case of the conversion in the field of an electron of the detector, the trackof the recoiling electron can be long enough that it can be reconstructed, in which caseits contribution can be taken into account in the reconstruction: the event is then called a“triplet”, as three tracks are visible in the final state. Unfortunately these events are too few
to contribute significantly to the differential sensitivity (slide 20 of [6]). As the azimuthal
angle of the recoil is correlated with the direction of polarisation of the incident photon,
hope has raised to perform polarimetry with triplet ([7] and references therein). Alas, thecross section above a threshold recoil momentum gets low at low photon energy (Fig. 6 of[8]), and in total the potential of triplet for polarimetry is quite limited (section 5.3 of [8]).

(E )p resolution 10%

1

0.1

-3/4

1

(E )
multiple scatteringwith optimal fits

Gas 10 bar 1
Gas 1 barLiq (Sol)

10

10

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-4

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COSB

SPI
-5

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-6
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0

90 Gal Lat

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-7

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gas 10 bar, 10 kg
gas 1 bar, 10 kg
Liq (Sol), 100 kg

1

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Figure 1: (Colour online). Left: various contributions to the photon angular resolution; Right: point-likesource differential sensitivity [5].

• Next comes the multiple scattering of the tracks in the TPC material. These angular deflections induce correlated deviations between successive position measurements of a track, thatmust be taken into account in the reconstruction. It can be shown that this can be performed
in an optimal way by a Kalman-filter-based fitter [9]. In that case, in the continuous detector
3

HARPO TPC gamma telescope and polarimeter: measurements in a polarised photon beam

A. Delbart

limit [10], the expression of the RMS track angular resolution at its starting point is [5, 8]
1/6
σθtL = (p/p1 )−3/4 , where p is the track momentum, p1 ≡ p0 4σ 2 l/X03
is a characteristic momentum that parametrises the performance of the tracking under multiple-scatteringof a given detector, and p0 = 13.6 MeV/c is the constant that appears in the expression ofmultiple scattering (eq. (32.15) of [11]). In the expression for p1 , σ is the single-point spatial
resolution, l is the track longitudinal sampling, and X0 is the radiation length of the tracker
material. The single-track angular resolution is extended easily to the average single-photon
angular resolution by multiplication by a factor that is close to unity [5]. The p−3/4 variationcompares nicely with the experimental value of E −0.78 for the Fermi/LAT collaboration [4].
For l = 1 mm, σ = 0.1 mm and a 10 bar argon-based gas, σθtL = 0.26◦ at E = 100 MeV, while
the recoil contribution amounts to 0.27◦ (Fig. 1), The two curves cross close to 100 MeV, andthe total angular resolution amounts to 0.4◦ at 100 MeV, which is an order of magnitude improvement with respect to the Fermi/LAT. The expression for σθtL was cross-checked withan actual Kalman-filter (Fig. 18 of [8]).
• Detector resolution. At the highest energy, the angular resolution flattens to an asymptote
dominated by the detector resolution (Fig. 1 left).
• Momentum resolution. Track momentum can be measured either directly in the TPC, if
immersed into a magnetic field, or with additional external detectors such as a transition
radiation detector (TRD) or a calorimeter. For the lowest energies, the momentum can be
estimated by the multiple measurement of multiple scattering in the TPC alone ([5] andreferences therein). For a 10 % resolution, the contribution to the photon angular resolution
is negligible (Fig. 1 left).
The improvement in angular resolution translates to an improvement in point-like source differential sensitivity, as is shown in Fig. 1 right. Indeed, together with the high-performance Comptontelescope projects presently under development, closing the sensitivity gap is within reach.
Polarimetry has long be considered to be out of reach for e+ e− -pair telescopes as multiple
scattering blurs the azimutal angular information, that between the conversion plane and a direction transverse to the propagation of the photon. Following the approximation of [12], that is an
opening angle between the e+ e− pair equal to its most probable value, the resolution for the azimuthal angle is independent of photon-energy. In a mille-feuille telescope, after a path length of≈ 10−3 X0 in the conversion slab, the polarisation asymmetry already undergoes a dilution of thepolarisation asymmetry of a factor of two [8]. In a homogeneous detector though, the resolution
for the azimuthal angle varies like E 1/4 (eq. (18) of [8]): there is hope that in the lower part of theenergy spectrum of a pair telescope, where most of the statistics lies, the dilution stays high1 . We
have written an event generator which samples from the full, 5D, differential cross section, exact
down to threshold. It does not rely on any high-energy approximation, and includes polarisation[8]. We have used it to study the performance of an homogeneous tracker with optimal tracking as
described above, using the full 5D event probability density function (pdf). We found that for a gas
TPC the dilution stays indeed very close to unity over most of the energy spectrum (Fig. 20 of [8]).
1 We are using the high-energy physics definition of the dilution D of the asymmetry A , that is D ≡ A /A , that iseff

the higher the value of D, the better.

4

HARPO TPC gamma telescope and polarimeter: measurements in a polarised photon beam

A. Delbart

With a 1 m3 5 bar argon TPC, one full (exposure fraction of η = 1) year exposure to a Crab-likesource and with experimental cuts applied, a P precision of 1.4 % would be reached [8].

3. HARPO: the detector
The HARPO demonstrator is a 30 cm cubic TPC surrounded by 6 scintillator plates equippedwith a wavelength shifter and two photomultipliers (PMTs) for trigger and background rejection.
The gas container vessel is an aluminum cylinder designed to be operated from few mbar up to
5 bar in sealed mode (Fig. 2). The TPC is filled with a mixture of argon with 5% of iC4 H10and operated with a 220 V/cm drift electric field. The TPC end-plate is a "hybrid" MicroPattern
Gaseous Detector (MPGD) which is composed of two GEM foils[13] stacked with 2 mm spacing
above a 128 µm amplification gap bulk-micromegas[14]. The anode collection plane is segmented
in 2 times 288 strips in the two orthogonal x − y directions read by an electronic chain composedof two T2K/TPC Front-End cards, two FEMINOS back-end cards and a Trigger Clock Module[15]. An innovative trigger system, called FAST (Fully Automated Switched Trigger), makes useof the FPGA of a modified PMm2 trigger card to sequentially generates up to 16 trigger lines builtwith combinations of the signals from the 12 PMTs, micromegas mesh signal, and an external laser
trigger. This demonstrator was extensively studied in laboratory with a 55 Fe 5.9 keV X-ray source
and with cosmic rays to measure its performances and determine its operating conditions[16].

Figure 2: Left: a 3D CAD drawing of the HARPO demonstrator; Right: the gamma beam line and test
setup at NewSUBARU.

4. Detector test with a polarised gamma-ray beam
In November 2014, the HARPO demonstrator was exposed to a pseudo-monochromatic andalmost fully polarised γ-ray beam delivered by the BL01 beam line of the NewSUBARU storage ring [17] (figure 2) which is operated by the Laboratory of Advanced Science and Technology for Industry (LASTI) of the University of Hyôgo (Japan). The γ-ray beam is producedby the inverse Compton scattering of laser photons on the relativistic electrons stored in NewSUBARU (Laser Compton Source or LCS). We used four electron beam energies: 0.618, 0.982,
1.223 and 1.480 GeV. We could cover the γ-ray energy range from 1.74 MeV up to 74.3 MeV
through four available laser photon wavelengths: λ = 0.532 µm and λ = 1.064 µm with a 20 kHz
5

A. Delbart

HARPO TPC gamma telescope and polarimeter: measurements in a polarised photon beam

Vdrift [cm/ µs]

gain [a.u]pulsed Nd:YVO4 (2ω and 1ω) laser, λ = 1.540 µm with a 200 kHz pulsed Er (fibre) laser andλ = 10.55 µm with a CW CO2 laser (Fig. 5). In order to select the Compton energy edge, we used
two 100 mm long collimators made of lead bricks and that we precisely aligned on the beam axisby γ-ray energy measurements with a NaI detector. When collimation is applied, the laser beam
linear polarisation (measured to be above 99.9%) is almost fully transferred to the γ-ray beam.
For systematic bias studies, the laser beam was also changed to circular polarisation by use of aquarter-wave plate, which, for an experiment that is not sensitive to the polarisation of the leptons,
is equivalent to random polarisation (P = 0).
The HARPO demonstrator was filled with gas at 2 bar after successive vacuum pumpings and rinsings. The main effects of gas degradation are a modification of the amplification gain and a loss of
signal due to capture of electrons along the drift. Figure 3 shows a measure of the relative gain andelectron drift velocity over the two weeks of data taking, with a slight variation of gain and drift
velocity (around 5%). A monitoring of the temperature and pressure shows that we had a very low
leakage rate.
9000
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time 31

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T [C]

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time

Figure 3: Evolution of the gain, electron drift velocity, temperature and pressure during data taking.

A special trigger line was developed for selecting γ-ray from LCS conversion in the gas. It
uses the signal of the 6 scintillators, the start signal from the pulsed laser, and the signal from themicromegas mesh. It was designed to select events with conversion in the TPC gas, while rejecting
conversions in the upstream material. This was done by applying a veto on "early" signals in themesh, which come from electrons with zero or short drift. This trigger system is suitable for a
configuration in beam and not for a space telescope. Additional trigger lines were included tocheck the efficiency of the different trigger elements. Figure 4 shows a selection of conversion
events. Our raw output is a pair of 2D maps, representing the x − z and y − z projections, where z
is obtained from the arrival time of the electrons. In most cases, we see nuclear conversion, where
only the e+ e− pair is visible. Occasionally, we see triplet conversions, where the recoil electron
forms a third track. An estimation of the rate of gamma conversions in the gas comes from looking
at the position of the "start’" (i.e. z/t minimum, the point closest to the beam side) of an event.
This point is an approximation of the vertex position (Fig. 4). If that point is within the transverse
geometrical location of the γ-ray beam and longitudinal location of the TPC gas, we estimate that6

A. Delbart

HARPO TPC gamma telescope and polarimeter: measurements in a polarised photon beam

Figure 4: Raw data from 3 pair conversion events. Left : a 74.3 MeV photon; Middle : a 4.68 MeV photon.
Right : a 74.3 MeV photon with recoil electron (triplet conversion). The top and bottom represent the x − z
and y − z projections. The signal is spread out by diffusion and electronics shaping. This will be taken into
account in the reconstruction algorithms.

Ee (GeV)

1.6
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Nγ /Nrecorded [%]X Channel

the event contains an interaction of the beam with the gas. Figure 5 shows how this selection isdone, and gives the estimated fraction of recorded events which show interactions of γ-rays in thegas volume. The rate of acquisition of gamma events reached up to 50 Hz for a total of nearly
60 million events, so that at least 30 million events are due to an interaction of the photons with thegas. At the considered energies, most of these events should be pair conversions.
250
200
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10.55µm
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Eγ beamFigure 5: Left: γ energies as a function of electron energies and laser wavelengths; Middle: distribution of
the position of the first signal in the TPC for each event in one run. The central region (in red) represents
the beam position. Events in that region are considered to contain an interaction of the γ-rays with the gas;
Right: resulting fraction of all recorded events which contain γ-rays for most of the photon energies used.

5. Conclusion and Prospectives
The HARPO TPC demonstrator was exposed to polarised gamma photons with energies between 1.74 MeV and 74.3 MeV. The data taking was efficient with more than 50% of high quality
events coming from gamma conversion to e+ e− pairs inside the TPC gas volume. A complete reconstruction and characterisation of these events is on-going to measure the angular resolution of
the detector, as well as its sensibility to the photon polarisation. In the perspective of a stratosphericballoon flight test, several R&D programs are on-going on the design of a rad-hard electronics, the7

HARPO TPC gamma telescope and polarimeter: measurements in a polarised photon beam

A. Delbart

optimization of the MPGD by use of a thin gap micromegas optimised for high pressure, and thedevelopment of an advanced trigger and a gas recirculation system.

6. Acknowledgements
This work is funded by the French National Research Agency (ANR-13-BS05-0002) and was
performed by using NewSUBARU-GACKO (Gamma Collaboration Hutch of Konan University).

References[1] D. Bernard, Compton polarimetry revisited, arXiv:1507.02824 [astro-ph.IM].
[2] H. Zhang and M. Böttcher, X-Ray and Gamma-Ray Polarization in Leptonic and Hadronic Jet Modelsof Blazars, Astrophys. J. 774, 18 (2013) [arXiv:1307.4187 [astro-ph.HE]].
[3] D. Götz et al., GRB 140206A: the most distant polarized Gamma-Ray Burst, Mon. Not. Roy. Astron.
Soc. 444 (2014) 3, 2776 [arXiv:1408.4121 [astro-ph.HE]].
[4] M. Ackermann et al., The Fermi Large Area Telescope On Orbit: Event Classification, Instrument
Response Functions, and Calibration, ApJS 203 (2012) 4, [arXiv:1206.1896 [astro-ph.IM]].
[5] D. Bernard, TPC in gamma-ray astronomy above pair-creation threshold, Nucl. Instrum. Meth. A 701(2013) 225 [Nucl. Instrum. Meth. A 713 (2013) 76] [arXiv:1211.1534 [astro-ph.IM]].
[6] D. Bernard, A thin detector as a high angular resolution gamma ray telescope (and polarimeter) in
the MeV-TeV energy range; Towards a post-Fermi mission ? Gamma2012, 5th International
Symposium on High-Energy Gamma-Ray Astronomy, Heidelberg, July 9-13 2012.
[7] V. F. Boldyshev et al., Measurement of linear polarization of photons by using the asymmetry of recoil
electrons in the photoproduction of triplets, Phys. Atom. Nucl. 58 (1995) 39 [Yad. Fiz. 58 (1995) 43].
[8] D. Bernard, Polarimetry of cosmic gamma-ray sources above e+ e− pair creation threshold, Nucl.
Instrum. Meth. A 729 (2013) 765 [arXiv:1307.3892 [astro-ph.IM]].
[9] R. Frühwirth, Application of Kalman filtering to track and vertex fitting, Nucl. Instrum. Meth. A 262,
444 (1987).
[10] W. R. Innes, Some formulas for estimating tracking errors, Nucl. Instrum. Meth. A 329, 238 (1993).
[11] K. A. Olive et al. [Particle Data Group Collaboration], Review of Particle Physics, Chin. Phys. C 38
(2014) 090001.
[12] Yu. D. Kotov, Methods of measurement of gamma-ray polarization, Space Science Reviews 49 (1988)
185.
[13] F. Sauli, GEM: A new concept for electron amplification in gas detectors, Nucl. Instrum. Meth. A
386, (1997) 531.
[14] I. Giomataris et al., Micromegas in a bulk, Nucl. Instrum. Meth. A 560, (2006) 405.
[15] D. Calvet, A Versatile Readout System for Small to Medium Scale Gaseous and Silicon Detectors,
IEEE Trans. Nucl. Sci. 61 (2014) 675.
[16] S. Wang et al., HARPO: beam characterization of a TPC for gamma-ray polarimetry and highangular-resolution astronomy in the MeV-GeV range, [arXiv:1503.03772 [astro-ph.IM]].
[17] S. Amano et al., Several MeV γ-ray generation at NewSUBARU by laser Compton backscattering,
Nucl. Instrum. Meth. A 602 (2009) 337.

8

The Vienna Conference on Instrumentation on February 15-19, 2016.
https://indico.cern.ch/event/391665/

Measurement of 1.7 to 74 MeV polarised

rays with the HARPO TPC

Y. Geerebaerta,⇤ , Ph. Grosa,⇤ , S. Amanoe , D. Attiéb , D. Bernarda , P. Bruela , D. Calvetb , P. Colasb , S. Datéf , A. Delbartb , M. Frotina ,B. Giebelsa , D. Götzc,d , S. Hashimotoe , D. Horana , T. Kotakae , M. Louzira , Y. Minamiyamae , S. Miyamotoe , H. Ohkumaf ,P. Poilleuxa , I. Semeniouka , P. Sizunb , A. Takemotoe , M. Yamaguchie , S. Wangaa LLR, École Polytechnique, CNRS/IN2P3, Palaiseau, Franceb CEA, Irfu, CEA-Saclay, Francec AIM, CEA/DSM-CNRS-Université Paris Diderot, France
d IRFU/Service dAstrophysique, CEA-Saclay, France
e LASTI, University of Hyôgo, Japanf JASRI/SPring8, Japan

Abstract
Current gamma-ray telescopes based on photon conversion to electron-positron pair, such as Fermi, use tungsten converters. Theysu↵er of limited angular resolution at low energies, and their sensitivity drops below 1 GeV. The low multiple scattering in a gaseous
detector gives access to higher angular resolution in the MeV-GeV range, and to the linear polarisation of the photons through theazimuthal angle of the electron-positron pair.
HARPO is an R&D program to characterize the operation of a TPC (Time Projection Chamber) as a high angular-resolution andsensitivity telescope and polarimeter for -rays from cosmic sources. It represents a first step towards a future space instrument.
A 30 cm cubic TPC demonstrator was built, and filled with 2 bar argon based gas. It was put in a polarised gamma-ray beam at
the NewSUBARU accelerator in Japan in November 2014. Data were taken at di↵erent photon energies from 1.7 MeV to 74 MeV,
and with di↵erent polarisation configurations.
The electronics setup is described, with an emphasis on the trigger system. The event reconstruction algorithm is quickly
described, and preliminary measurements of polarisation of 11 MeV-photons are shown.
Key words: TPC, gamma-ray, polarimetry, gaseous detector
1. High angular resolution -ray astronomy and polarimetry in the MeV - GeV energy range

5

10

15

20

-ray astronomy provides insight to the understanding of the
non-thermal emission of among the most violent objects in the 25
Universe, such as pulsars, active galactic nuclei (AGN) and ray bursts (GRB), and thereby to the understanding of the detailed nature of these objects.
Alas, between the sub-MeV and the above-GeV energy
ranges for which Compton telescopes ( e ! e ) and pairtelescopes ( Z ! Ze+ e ) are, respectively, highly performant,lies the MeV-GeV range over which the sensitivity of past mea- 30
surements was very limited, in particular as the degradation of
the angular resolution of pair telescopes at low energy ruins
the detection sensibility. This hinders the observation and theunderstanding of GRBs, the spectrum of most of which peaksin the MeV region, it biases the description of the Blazar se- 35
quence. More generally, it limits the detection of crowded regions of the -ray sky such as the galactic plane to its brightest
sources : to a large extent, the MeV-GeV sensitivity gap [1] isan angular resolution issue.
⇤ Corresponding author

Email addresses: yannick.geerebaert@polytechnique.fr(Y. Geerebaert), philippe.gros@llr.in2p3.fr (Ph. Gros)

40

The angular resolution of pair telescopes can be improved,from the Fermi/LAT’s ⇡ 5 at 100 MeV [2] to 1 2 , by theuse of pure silicon trackers, i.e. without any tungsten converter
plates [3, 4, 5]. An even better resolution of ⇡ 0.4 can beobtained with a gas detector such as a time-projection chamber (TPC), such that together with the development of highperformance Compton telescope, filling the sensitivity gap for
point-like sources at a level of ⇡ 10 6 MeV/(cm2 s) is withinreach [6] and Fig. 1.
Furthermore the measurement of the linear polarization of
the emission, which is a powerful tool to understand characteristics of cosmic sources at lower energies in the radio-wave
to X-ray energy, is not available for -ray above 1 MeV [7].
The use of a low density converter-tracker, such as a gas detector, enables the measurement of the polarization fraction before
multiple scattering ruins the azimutal information carried by thepair [8].
-ray polarimetry would provide insight to understanding thevalue and turbulence of magnetic fields in the -ray emitting
jets structure of most -ray emitting sources, and for example
enable to decipher the leptonic and hadronic nature of the emitting particles in blazars [9].

Preprint submitted to Nuclear Instruments & Methods in Physics Research, Section A

March 17, 2016

-4

Comptel
OSSE

10

COSB

65

EGRET

gal=0

IBIS

10

gal=30-6

gal=9010-7Fermi/LAT P7V6

10 bar
1 bar

-1

10

1

10

10

2

10

3

10

Figure 3: Left: Layout of readout plane of MICROMEGAS. Only the copper of the PCB-based plane is shown. One x strip is coloured in red and one
y pseudo-strip is in blue. y pseudo-strip are segmented in pads, connected together through via by an internal layer (green). Right: Top view of real PCB
with pillars, keeping the mesh at a distance of 128 µm of the readout plane.

4

E (MeV)

2. The HARPO detector

50

70

We have built a TPC which is using argon-based gas mixturesin the range 1 5 bar [10]. The drifted electrons are amplified bya hybrid amplifier whose performances have been characterized
75
in detail [11].
We have exposed the detector to a tunable -ray source, using
the head-on inverse Compton scattering of a laser beam on the
0.6 1.5 GeV electron beam of the NewSUBARU storage ring(Hyōgo, Japan) [12]. The detector was put so that the photon
beam would be aligned with the drift direction z of the TPC, 80
coming from the readout side, and exiting through the cathode.
StripsAs shown on figure 3, the signal is collected by two orthogonal series of strips (x & y), which, in our case, reduces thenumber of channels by a factor 144 compared to the equivalent pixel sensor. This reduction is only possible if the channel
occupancy is low enough to avoid unsolvable ambiguities andcomes at the cost of the need for o↵-line association of each x
track to a track in the y view. Then, only 576 channels (x & y
strips, 1 mm pitch) are read out and digitized at 33 MHz (up to
100 MHz) by eight AFTER chips mounted on two FEC boards.
Channel data are then zero suppressed and sent to a PC via Gigabit Ethernet by two FEMINOS boards synchronized by one
TCM board. These versatile boards were originally developedat IRFU for the T2K and MINOS experiments[13]. To mitigate
the dead time induced by readout and digitization (1.6 ms) wedeveloped a sophisticated trigger, with a multi-line system so as
to provide real-time efficiency monitoring of each component.
Channel250

X, 288 ch, 1mm
Y, 288 ch, 1mm
Daq

200

FEC+FEMINOSTrigger

h
es
M

T
PM

Laser

Clock &
Synchronisation
TCM

Fully
Automatic
Switched
Triggerγ

Data storageConﬁguration

100

Monitoring

50
00

Trigger
eﬀiciency
monitoring

PARISROC2 on PMm2

60

100

200

500
300
400
Time [30ns bin]

Laser

PC

Scintillator
Signal on mesh
Main trigger line

Figure 2: Global view of HARPO electronics.

55

Active gas volume

Scintillator

Argon TPC 10 kg

Figure 1: Di↵erential sensitivity as a function of energy (argon-gas-based
HARPO TPC, green) compared to the 90 galactic latitude performance of the
Fermi-LAT [2] and of the Compton telescope COMPTEL [1]. Adapted from
[6].

45

tron Multiplier) + one MICROMEGAS (128 µm-gap bulk Micro MEsh GAseous Structure).

SPI

-5

2

2

Sensitivity E dN/dE (MeV / (cm s))

10

The present detector is aimed at ground-validation tests, but
we have designed it taking into account the constraint of space
operation.
HARPO produces very fine 3D images of -conversions toe+ e pairs by tracking these events. This is done at a low costin terms of power consumption and data flow in the presence ofa large number of background noise tracks.
85
The TPC is a 30cm cubic field cage, enclosed with a copper
cathode and a readout plane anode including a hybrid multistage amplification system composed of two GEM (Gas Elec2

Figure 4: Typical timing of trigger in TPC.

To provide presence and timing informations on events,
six scintillators, readout by PMT and PARISROC2 chip[14]mounted on a PMM2 board[15], surrounds the TPC.
The timing of the charge induced on the mesh is used fortrigger. The signal, which is long and with unpredictable shape,corresponds to the time distribution of tracks in the TPC. To
get a signal from the rising edge, we use a constant fraction

90

95

discriminator (CFD), which shows the beginning of the signal,and therefore the position of the beginning of the track. We canmeasure the delay between the start of the event and this signal
to build a veto on tracks created upstream from the TPC.
125
The main line of the trigger select pair creation events which
follows from the interaction of photon with nucleus of gas
atom in the TPC. It is composed of:• Veto on upstream scintillator (which reveal an interaction130
before the active gas region),100• signal on mesh of MICROMEGAS with a veto basedon presence of very early signal (we reject most of the
gamma-rays that convert in the material of the readout
135
plane),• signal in at least one of the five others scintillators,• laser signal, whenever available (for pulsed laser), in coincidence with signal in scintillators.

115

120

uz u+x! = arctan
uz u+yu+z u xu+z uy!

(1)

A distribution of ! is shown in Fig. 6 for several orientationsof the detector with an unpolarised beam. The distribution isdominated by systematic fluctuations due to inefficiencies of
the reconstruction algorithm in some track configurations andrelated to the cubic shape of the detector.
dN/dω [rad-1]

3. Analysis of data from a polarised photon beam
0.04

We took data with HARPO TPC in a polarised photon beamat the NewSUBARU [12]. The photon is aligned with the drift
direction z and arrives from the readout side. The detector was
rotated aroung the z-axis to study the systematic angular effects related to the cubic geometry of the TPC. A total of about
60 million events were taken, with 13 di↵erent photon energies
from 1.7 to 74 MeV, and 4 TPC orientations. Both polarised
and unpolarised beams were used.
Figure 5 shows a pair conversion event as observed in the
HARPO TPC. The two electron/positron tracks are visible ineach of the two projection X-Z and Y-Z.
X channel

110This trigger suppressed the huge background rate from theaccelerator (up to 5 kHz) by a rejection factor greater than140
two orders of magnitude[18] during the data taking campaign,
where 60 millions of events, at 13 energy points, fully polarized
and not polarized, have been recorded on disk.

0.035
0.03

0.025
0.02
0.01
0.005
0

2500

145

2000
1500
1000

100

500
50

100

200

300

250

400

500

time [30ns bin]

4000
3500

150

3000

200

2500
150

2000
1500

100

1000
50
0

500
0

100

200

−2

−1

0

1

2

3

ω [rad]

3500

150

0

−3

Figure 6: Azimuthal angle ! for one configuration of the TPC with 11 MeV
photon beam. Only the statistical uncertainties are shown. Systematic e↵ects
dominate the distribution.

3000

0

ω0 = -45oω 0 = 0o
ω0 = 45oω0 = 90o

0.015

4000

Eγ = 11 MeV

250
200

Y channel

105

The electron tracks were reconstructed on each projection using a clusest neighbour search based on a Kalman filter [16].
Unfortunately, since the two tracks are difficult to disentangle
near the vertex, it was impossible to use the Kalman filter to estimate the track direction near the vertex. The identified tracks
were therefore fitted with a straight line. Then, the tracks reconstructed on each projection (X-Z and Y-Z) were paired together
using their charge profile as a function of the drift time (equivalent to the Z coordinate). This way we can define tracks in
3D.
For each pair of reconstructed 3D track, point and distanceof closest approach (POCA and DOCA) were calculated. We
selected only the track pairs where the POCA was close enoughto the vertex position, as estimated from the beam geometry.
The azimuthal angle ! for a pair e+ e with direction u~+ and u~
is defined in Fig.1 of [17]. Using the fact that our photon beamis aligned with the z direction, ! is:

300

400

500

155

time [30ns bin]

Figure 5: Example of a raw event in the HARPO TPC. The two tracks fromthe pair conversion of a 11 MeV photon are clearly visible.

3

To compensate for these systematic e↵ects, we took datawith di↵erent orientations of the TPC with regard to the photon beam. By combining the data taken at di↵erent angles !0of the TPC we obtain the distributions of ! !0 shown on theupper two plots of Fig. 7. Most of the systematic fluctuations
are averaged out, although a few remain. I these distributions
we can already see a di↵erence between the unpolarised (top)and polarised (middle) beam data.
Finally we divide the distribution for a polarised beam, by theunpolarised one. This way all the systematic e↵ects cancel out,
and only the polarisation asymmetry remains. Figure 7, bottom
plot, shows this asymmetry. The measured polarisation angle
14.7 ± 6.5 is consistent with the expected value of zero. Thee↵ective asymmetry, estimated at 6.25 ± 1.42%, is significantly
smaller than the theoretical value of 17 % (see Fig.21 in [8]).

dN/dω [rad-1]

dN/dω [rad-1]

0.016
0.014
0.012
0.01
0.008
0.006
0.004
0.002
0 −3
0.016

NP=100%/N P=0%

0.014
0.012
0.01
0.008
0.006
0.004
0.002
0 −3
1.4

1.2
1
0.8

Y
VER

References

RY
INA
LIM

PRE

190

1 + A cos(2(ω - ω0))
A = 5.54 ± 0.986%, ω0 = -31.5 ± 5.17o
χ2 / NDF = 434 / 33 = 13.2

Eγ =−211MeV−1
P = 100%

0

1

ARY

2

3

ω - ω 0 [rad]

IN
LIM

195

RE
YP

VE1R+ A cos(2(ω - ω ))0
A = 8.15 ± 0.943%,
ω = -5.4 ± 3.29oχ2 / NDF = 465 / 33 = 014.1

Eγ =−211MeV−1
ratio (P=1)/(P=0)

Y
VER

0.6

0

1

ARY

2

3

ω - ω0 [rad]

0.2

−3

L
PRE

−2

−1

200

N
IMI

1 + A cos(2(ω - ω0))
A = 6.25 ± 1.42%, ω0 = 14.7 ± 6.51o
χ2 / NDF = 20.7 / 33 = 0.628

0.40Eγ = 11MeV
P = 0%

0

1

205

2

3

ω - ω0 [rad]Figure 7: Distribution of the reconstructed azimuthal angle ! !0 from pair210
conversions of 11 MeV photons in the TPC. Top: unpolarised photon beam.
Middle: fully polarised photon beam. Bottom: ratio of the two distributions
above. The systematic e↵ects are cancelled, and an e↵ective polarisation asymmetry of 6% is visible.
215

160

4. Discussion and outlook
220

165

170

175

180

185

The HARPO TPC was successfully used to measure pair conversions of photons from a few MeVto 74 MeV. The trigger using photomultipliers and the direct signal from the micromegas
mesh performed a suppression of the background by nearly two225
orders of magnitude, with about 50 % selection efficiency as
shown in [18].
A first reconstruction algorithm has been applied to the data
to extract the azimuthal angle of the electron-positron pair. By230
comparing data from polarised and unpolarised beam, we extract a first observation of the polarisation modulation for photons with energy 11 MeV and higher. The reconstruction efficiency is still very low, and many systematic e↵ects remain to235
be understood. These results show that photon polarimetry in
the pair production regime can be achieved with a TPC. A more
appropriate reconstruction is being developped to increase theefficiency and the resolution.
240
These preliminary results demonstrate that the design of aspace TPC is viable. A sophisticated hardware and software
allows a reduction of the number of channels by several ordersof magnitude. The next step will be the design of a balloon-245
borne TPC. It will be used to validate the trigger wich is a key
point for a successful space telescope. The trigger will haveto extract approximately 10 Hz photon conversion signal fromabout 5000 Hz single-track background.
5. Acknowledgements
This work is funded by by the French National Research
Agency (ANR-13-BS05-0002).
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