Liquid Argon Gamma-ray Observatory
LArGO
A new concept of gamma-ray telescope
Eric Charles (SLAC)
for
G. A. Caliandro (CIFS/SLAC), E. Bloom (SLAC), R. Cameron (SLAC),
T. Shutt (SLAC), D. Akerib (SLAC)

LArGO

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Main goals●A new gamma-ray satellite has to be better than Fermi-LAT––●LArGO focus mainly on the improvement of the angularresolution––●Better sensitivity

LArGO can efficiently work as Compton telescope–LArGO

Gamma-ray polarization
Better sky mapping

Maintaining the conversion efficiency, and wide field of view asin the LAT–●

Not an easy task, because the LAT is very good!
A ground-braking new technique is necessary

Thus covering a very broad energy range ~100 KeV – 100 GeV

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Tracker-converter and Neutrino detectors●The tracker-converter has the double
goal to convert the incident photon,and to track the pair produced–

For the first aim is necessary large
converting material (high Z) tomaximize the effective area.

–For a good tracking the high Z
material should be minimized toreduce the multiple scattering.

LArGO●Neutrino detectors with the goal toobserve the vertex of the neutrino
interaction need a very massive target,
and a good tracking resolution–The solution adopted by ICARUS is aliquid Argon TPC where the massive
material is also active for the tracking.
A full active detectors without deadzones.

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LArGO●LArGO design elements––

a stack of 32 very thin (6.5 mm)
LAr-TPCs (TPC-layers),
Inter-layer distance 2.5 cm●

the e+e- tracks by 1 GeV photon
converted in a TPC-layer are
separated at the underlying layer
by twice the TPC pitch.

– 1 X0 diluted in 1 m
LArGO––Pitch of the drift charge readout plane
p = 100 µm
Spatial resolution 25 µm●–Current LAr-TPC have pitch and spatial
resolution of ~1 mm,

LAr close to triple point (84 K, 70 kPa)●

X0 = 20 cm. Minimum multiple scattering
4Performance in pair production regime

Top left: angular resolution of LArGO versus the energyof incident photons converting in the top of a TPC-layer(LArGO top), or at 1 mm from its bottom (LArGO bottom).
The red line is the average aperture angle of theconverted pair, while the shadowed red area shows its
95% probability distribution.
Right. Profile of the angular resolution versus the depthof the conversion point in a TPC-layer for photons withenergy 200 MeV, 500 MeV, and 1 GeV. L = 0 mm is thedeepest point of a TPC-layer.
LArGO

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Compton with LArGO●

Two consecutive Compton interactions inLArGO will happen in different TPC-layers–––●

attenuation length of 1 MeV photons in LAr at~70kPa is 180 mm
minimum distance d=2.5 cm, spatial
resolution σ=0.025 mm -> δθr negligiblethe angular resolution of LArGO in Compton
regime is dominated by its energy resolution

Energy resolution < 3%–strong anti-correlation between ionization andscintillation signals Crawford, et al., 1987, NIMP A, 256
Aprile, et al., 2007, PhysRev B, 76–●

Angular resolution of ~1° @ 1.8 MeV.

●Polarization above ~2 MeV––LArGOhigh photon detection efficiency of SiPMs

LArGO is able to track the scattered electronof the first Compton interaction
stopping power of the electrons in LAr at ~70
kPa is approximately 1.4 MeV/cm.
6Readout plane: wires vs printed circuits●

ICARUS uses 3 planes of wires (3 mmpitch) to record the drifting electrons–●This is a good technique to cover the verylarge areas of the ICARUS TPC

A valid alternative are multi-layer
printed strips–

This technique was developed in theICARUS R&D (P. Cennini, et al., 1994, NIMP A, 346),
and for the COIMBRA PET (Solovov, V. et al.,2002, NIMP A, 477)

Muli-layer printed strips arevery suitable●for covering small
surfaces (like for a
TPC-layer of LArGO),●and to realize strips withvery small pitch (~100 µm)

LArGO

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Multi-layer printed strips applied to LArGO●The strips are placed at
45° respect the Z-axis of aTPC-layer–––LArGO

The resolution of on-axis
tracks is optimized
The length of the strips is
as short as ~ 1 cm
This configuration is very
useful especially for thesignal to noise ratio, andthe power consumption8

Signal to Noise●Signal/Noise–––●A MIP in LAr produce ~5000 e-/mm. There is no charge amplification in LAr
The main source of noise is from the readout electronics.
ICARUS has ENC ~ 1000 e- per wire (3 mm pitch). S/N~15.

The ENC is proportional to the total capacitance of the detector–The inter-strip capacitance (Cis) dominates when the pitch is very small(C is =l a + b●●–––LArGOw+cl∝pp)l, w are the length and the width of the strip, respectively
a,b,c are constants depending by the substrate material

In ICARUS l/p ~500
In LArGO, l/p ~100, thanks to the 45° strip configuration
The ENC is reduced by a factor 5 just for the strip configuration in LArGO,
and their shortness9How to reduce the electronic noise●Low-noise electronics developed for new neutrino and dark
matter experiments have reached very low ENC values

ENC < 600 e-●

Other considerations––––LArGO

The electronic of ICARUS is not cold.
It is outside the Dewar. There are very long cables (few meters long) toconnect the readout planes with the electronic (50pF/m)
The total capacitance per wire in ICARUS is ~300 – 400 pF
The inter-strip capacitance estimated for LArGO with the formula in theprevious slide using the constant for a silicon substrate give Cis < 2 pF
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Power consumption and cryogenic●

Number of channels in LArGO is of the sameorder of magnitude than the LAT-tracker–●Power consumption of the LAT ~160 W

The power consumption is proportional tothe detector capacitance––––●

Pierre Jaron, CERN EF.

Hence to the length of the strips
Stript length ratio LAT/LArGO ~ 40
Same ratio for the power consumption
Power consumption of LArGO O(10 W)

The cryogenic temperature of~84 K required by the liquid argonin LArGO can be reached even bypassive radiators–Radiator heat lifts

At 80 K the radiators candissipate ~ 1W/m2LArGO

Collaudin B., Rando N., 2000, Cryo, 40, 797.

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Scintillation light and SiPM
●SiPMs●Scintillation
lightDrifting e-Micro strip plane

Fiducial
volumePrimary task of SiPM is to trigger anddefine the time stamp of an event
A fiducial volume of the track can beevaluated from the asymmetry of thescintillation light collected by SiPMs– Helpful for the reconstruction– Helpful to discriminate a secondevent arrived during the e- drift
time of the previous one●
Expected ~0.4 particles/300µs
in 50x50 cm2 TPC-layer

SiPMs
LArGO

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Single module

Dewar containing
a tracker module

Calorimeter
LArGO

Top and bottom
windows

The Dewar is needed
to keep the LAr
temperature (~84 K)
and pressure (~70 kPa)
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Readout

100 cmCathode50 cm

Satellite design hypothesis●220 cm

●

●
●

LArGO

110 cmDewar containing
a tracker module

55 cm

Height tracker
modules with a
total geometricalarea of 4 m2
FoV of each
module 45° x 30°
Full FoV >60° x 60°The 8 modules arecovered by an Anti
CoincidenceDetector

14Summary●The challenge of LArGO is to adapt the LAr-TPC technology to theneeds of γ-ray astrophysics–The performance of a γ-ray telescope is summarized by 3 parameters:●●

Main features of LArGO
––––

●

LArGO

Angular resolution, Effective Area (Aeff), Field of view (FoV)

Unprecedent angular resolution without reducing Aeff and FoV●LArGO has at 100 MeV approximately the same angular resolution thatcurrently Fermi-LAT has at 1 GeVWorking in both Compton and pair conversion regimes
Wide energy range ~100 keV – 100 GeV if coupled with a calorimeter 10 X 0deep
Polarization measurement in both Compton and pair conversion regime●Polarization energy range ~ 2 MeV – 1 GeV

LArGO is very promising for a future multipurpose γ-ray telescope15

Backup slides

LArGO

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Orbits and Rockets
OrbitsEquatorial LEO
Low CR bkg● High down link rate●

Langrangian point L2
Good for Cryogenic● No Earthalbedo/occultation●ISS●Possible humanoperations after thelaunchRocketsDiameter (m)Payloads Weight (t)Launch Site

Cost

Soyuz-2 (EU-Russia)

3.7

7.8 (LEO)
4.5 (GTO)

Plestsk
Kourou (FrenchGuiana)

50M EU?

Arianne 5 (EU)

5.4

16-21 (LEO)
7-10 (GTO)

Kourou (FrenchGuiana)

120M EU

Vega (EU)

3.0

1.5 – 2.0

Kourou (FrenchGuiana)~30M EU

Delta II (US)

2.44

2.7- 6.1 (LEO)
0.9 - 2.17 (GTO)
1 (open orbit)

Cape Canaveral

36,7M USD
(1987)

Delta IV (US)

5.0

8.6 – 25.8 (LEO)
3.9 – 10.9 (GTO)

Cape Canaveral/
Vanderberg

ATLAS V (US)

3.81

9.8 – 18.8 (LEO)
4.7 – 8.9 (GTO)

Cape Canaveral/
Vanderberg

Falcon 93.7

10.4 – 26.3 (LEO)
4.5 – 15.0 (GTO)

Cape Canaveral/
Kwajalein

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LAr TPC working principle and status of the art●Status of the art: ICARUS–

3 grid of wires with a pitch of 3mm
Spatial resolution in the drift
direction 0.7 mm
Spatial resolution in the readout
plane ~1mm
Edrift = 500 V/cm–

Electron drift velocity ~ 1.5 mm/µs–––

Electrons
path

Ionizing track

EdriftDrift●Status of the art: 3ton prototype–––

2 grid of wires with a pitch of 2 mm
Spatial resolution in the drift
direction 0.2 mm
Spatial resolution in the readout
plane ~0.6 mm

dE2

Induction 2dCollection
LArGO

E1Induction 1p18

Liquid Argon characteristic features

LArLXeSiCross Section(cm2/g @ 1 MeV)

5.7 x 10-2

5.2 x 10-2

6.3 x 10-2

Density (g/cm3)

1.39

3.06

2.33

Attenuation length(cm @ 1MeV)

12.6

6.3

6.8

Radiation length

14.3

2.77

9.3

LArGO

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γ-ray polarization●Pairs are produced preferentially in the plane identified by thelinear polarization of the incident photons and its direction–In case of small nuclear recoil (γ, e+, e- coplanar) the photon
conversion cross section is:σ0σ γ (ϕ)=[1+ P λ cos 2 (ϕ)]
2π–●Maximon L. C., Olsen H., 1962,
Phys. Rev. 126, 310

φ is the angle between the e+e- plane and the photon's electric field vector

The gamma-ray polarization can be detected looking for themodulation of the azimuthal angle φ of the pair planes.

LArGO

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Ingredients for the design of LArGO●●The purpose is to project a tracker for γ-ray telescope which
has better performance than Fermi-LAT, and which can work as
γ-ray polarimeter at least until photon energies of 1 GeV.
–Tracker depth ≥ 1X0–

Tracker full height ~1m

γ-ray polarizzation––

In order for the polarization to be detected, the plane of the converted pairneeds to be identified.
The tracker has to discriminate the two tracks with an angular resolution
lower than their aperture angle

4 meσθ ⩽,EγLArGO

for E γ ⩽1GeV

Aperture angle (W)distribution for E=100 MeV

21Compton regime●When a photon undergoes one or more Compton scatters within the detector,
and then is photoelectrically absorbed, its direction can be reconstructedfrom the kinematics of the scatterings.
The angular resolution is δ θ= δθ 2E +δ θ2r√δ θ E ∝ Δ E / E uncertainty due to energy measurement
δ θr ∝σ /d due to the error on the position of thescattering pointsθ●Tracking the recoil electron of the first Compton
interaction–

Solve the ambiguity in the sky direction of single
photon–the circle of possible positions on the sky is reduced
to a small arc proportional to the angular resolution(σa) of the electron track.

–the background of gamma-rays from diffuse emission
or close sources is reduced, and the sensitivity isimproved by a factor √ σ a /2 πd●LArGO

The measurement of the recoil electron allows thedetection of the gamma-ray polarization
2d σ Cs 1EE E02
2= re+ −2sin θ cos ϕdΩ 2
E0 E 0 E

( )[]

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Compton prototypes●

MEGA–––●

32 double sided Si-strip
detectors (not interleaved bytungsten foils) surrounded by apixelated CsI calorimeterenergy range is 0.4 – 50 MeV
angular resolution of 2°, energy
resolution of 8% @ 2 MeV

LXeGRIT–––liquid Xenon TPC
energy range is 0.3 - 10 MeV
angular resolution of 3°, energy
resolution of 8.8% @ 1 MeV

θBloser P., et al., 2003, NIMPA, 512, 220
LArGO

E. Aprile 1998, NIMP A, 412, 425
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Diffusion●The electrons drifting toward the anode are affected by diffusion

Transverse diffusion StS t =√ 2DLd /v d●●

D~4 cm2/s: diffusion coefficient
Vd = 1.5 mm/µs–For a drift length of 50 cm St~0.5 mm–St is small compared with thethickness of a TPC-layer (6.5 mm), butlarger than the pitch.

LArGO

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Space resolution in the drift direction

Cennini, P. et al., 1994, NIMP A, 345.

LArGO

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Particle rate
Electrons
Positrons
ProtonsHe

Earth albedo
neutrons
Earth albedo
gammas●lkj

LArGO

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