An e.m. calorimeter on the Moon surface

R.Battiston, M.T.Brunetti, F. Cervelli, C.Fidani

MoonCal

Mooncal could :

Resolve the difference of electrons spectra proposed by the many diffusion models for sources in the TeV region from Vela, Cygnus loop and Monogem

Detect an excess of positrons and electrons by the excellent energy resolutionand the observations with high statistics

Provide complementary observations on gamma’s with respect to Glast by a better energy resolution above 100 GeV. Targets : Galactic and extra-Galactic diffuse components, supernova remnants, pulsars, AGN’s, GRB’s

Observe line gamma rays from SUSY particles annihilation. As the energy resolution is better at higher energies,Mooncal will precisely measure the signature of line gamma rays

Physics Perpectives

O graziosa luna, io mi rammentoche, or volge l’anno, sovra questo colle io venia pien d’angoscia a rimirarti G. Leopardi

The regolith as sampling material for an EM

calorimeter

Simulation of Regolith Composition

GEANT4 derives the Regolith Radiation Length (0) from chemical composition and relative densities

Regolith Radiation Length : 14.4 cm

Gamma Rays at 100 GeV: Energy Deposit vs Depth

Depth of Maximun Energy Deposit

Gamma Rays at 100 GeV: Total Energy Deposit vs Depth

Gamma Rays at 100 GeV: Tranverse Development

r = 1 cm

L = 150 cmorL = 300 cm

d = 7.5 cm

d = 7.5 cm

Scintillator Geometry

The layout (1)

The layout (2)

The layout (3)

Gamma Ray at 100 GeV (1)

Gamma Ray at 100 GeV (2)

Gamma Ray at 100 GeV (3)

Gamma Ray at 100 GeV (4)

MoonCal simulation: Boundary conditions and restrictions

• CALORIMETER GEOMETRY:– Cylinder of 3 m radius and 1.5 m height filled with regolith and

scintillators of 1 cm radius and 1.5 m height separated by 7.5 cm (on a xy grid).

• ENERGY RESTRICTIONS:– Lower energy cut for gammas 550 keV– Lower energy cut for electrons and positrons 1.4 MeV

• GEOMETRY RESTRICTIONS:– Incident angle: 45° ≤ ≤ 80° longitudinal containment– Incident area: inside a disk of 1 m radius lateral containment

Scintillator distance = 7.5 cmScintillator radius= 1 cm

Distance vs Scintillator Diameter (1)

The resolution E/E has been fitted according to:

E

ba E +=

RESULTS

Energy Resolution : d = 7.5

Scintillator distance = 4 cm

Scintillator radius= 0.5 cm

Distance vs Scintillator Diameter (2)

Energy Resolution: r = 0,5cm d= 4cm

Energy Resolution

Energy Resolution vs Incident Angle (1)

Energy Resolution vs Incident Angle (2)

Analysis Cuts:

-First plot: resolution between 0.1 and 1 GeV with the following lower cuts:

Gammas, electrons, positrons 1 keV

-Second plot: resolution between 1 and 50 GeV with the following lower cuts:

Gammas 2 keV, electrons 356 keV, positrons 347 keV

Low Energies Studies

SiPM concept GM-APD gives no information on light intensity

SiPM first proposed by Golovinand Sadygov in the mid ’90

A single GM-APD is segmented in tiny microdiodes connected in parallel, each with the quenching resistance.

Each element is independent and gives the same signal when fired by a photon

output signal is proportional to the number of triggered cells that for PDE=1 is the number of photons

Q = Q1 + Q2 = 2*Q1

substrate

metal

Features of a SiPMThe characteristics of a SiPM are:• capability to detect extremely low photon fluxes (from 1 to few hundred) giving a proportional information;• extremely fast response (determined by avalanche discharge): in the order of few hundreds of ps.

Other features are:• Low bias voltage (20-60V)• Low power consumption• Insensitive to magnetic fields• Compact and rugged

First prototypes

The wafer includes many structurediffering in geometrical details

The basic SiPM geometry iscomposed by 25x25 cells

Cell size: 40x40m2

1mm

1mm

SiPM @ ITC-irst

40 cm of regolith

T=-20 ± 3 C

Regolith Physical Properties

Further Developments

Longitudinal segmentation of Scintillator Rods

Steps: 5 or 10 cm

Numbers and Weight as a CONCLUSION

Area covered by a single rod Surfice covered by Moon Cal d = 4 cm Area ~ 7 cm2 ~28 m2

d = 7.5 cm Area ~ 24 cm2

d = 15 cm Area ~ 97 cm2

Scintillator rod : Wrapping in Carbon fiber

Weight of a single rod (length: 150 cm):

r=0.5 cm .15 Kgr= 1cm .5 Kg

Energy deposit on scintillators

Properties of a SiPMThe properties described for the GM-APD are valid for the SiPMwith two additional complications:

1) Further term in the photodetection efficiency:

PDE = Npulses / Nphotons = QE x P01 x Ae

Ae = (Active area) / (total SiPM area)

Dead area is given by the structuresat the edges of the microcell (metal layers, trenches, resistor…)

Properties of a SiPM2) Optical cross-talkDuring an avalanche discharge photons are emitted.

3x10-5 photons with energy higher than 1.14eV emitted per carrier crossing the junction.[from A. Lacaita et al., IEEE TED, vol. 40, n. 3, 1993:]

Those photons can trigger the avalanche in an adjacent cell:

optical cross-talk.

Solutions:- operate at low over-voltage => low gain => few photons emitted- optical isolation structure:

cell1 cell2

cell1 cell2

SiPM manufacturersRussian groups:• Obninsk/CPTA, Moscow (Golovin)• Mephi/PULSAR, Moscow (Dolgoshein)• JINR, Dubna (Sadygov)

They have been working on this since the beginning.

New labs/companies involved in SiPM production:• ITC-irst• Hamamatsu• SensL• MPI

SiPM @ ITC-irstDevelopment of SiPM is done in the framework of anagreement between INFN and ITC called MEMS:

• role of ITC-irst: to produce a matrix of SiPMs with detection efficiency optimized in the short-wavelength region.• role of INFN (Pisa, Perugia, Bologna, Bari, Trento): to couple the SiPM with a scintillator, to develop a read-out electronics for calorimetry, PET, TOF applications.

Project started at the beginning of 2005.

Details on the status of the project: http://sipm.itc.it

First productionCompleted in september 2005.Completed in september 2005.

Characteristics of the first fabrication run:

1) 11 photolithografic masks1) 11 photolithografic masks 180 process steps (3 months in clean room)180 process steps (3 months in clean room)

2) Substrate: p-type epitaxial, 42) Substrate: p-type epitaxial, 4m thickm thick 3) Quenching resistance made of doped polysilicon3) Quenching resistance made of doped polysilicon

4) No structure for optical isolation4) No structure for optical isolation

5) Geometry not optimized for maximum PDE5) Geometry not optimized for maximum PDE

Main objective was to study the breakdown properties!

Electrical characterization

1.E-10

1.E-09

1.E-08

1.E-07

1.E-06

1.E-05

1.E-04

1.E-03

-40 -30 -20 -10 0Vbias [V]

Cu

rre

nt [

A]

IV characteristics of 10 devices

• Breakdown voltage 31V• Uniform VBD all over the wafer surface

position ofthe tested devices

T=22oC

Signal characteristics

-0.3

-0.25

-0.2

-0.15

-0.1

-0.05

0

0.05

-1.0E-07 0.0E+00 1.0E-07 2.0E-07 3.0E-07

Time (s)

Vo

ltag

e (

V)

VBIAS=35.5V

Dark signal

single cell signal

doublesignal

(optical cross-talk)

Single signal @ 32.5, 34 and 35.5V

Rise time ~1nsRecovery time ~70ns

SiPM read-out by means of a wide-band voltage amplifier on a scope

-0.2

-0.15

-0.1

-0.05

0

0.05

-5 5 15 25 35 45 55 65 75 85 95Time (ns)

Vol

tage

(V

)

Vbias=34V

Vbias=35.5V

Vbias=32.5V

Single electron spectrumSingle electron spectrum in dark conditionIntegration time = 100ns.

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

-2.0E-09 -1.5E-09 -1.0E-09 -5.0E-10 0.0E+00

QDC

No

rma

lize

d C

ou

nt

35V 34V 33V

0.0E+00

2.0E+05

4.0E+05

6.0E+05

8.0E+05

1.0E+06

1.2E+06

1.4E+06

1.6E+06

1.8E+06

2.0E+06

31 32 33 34 35 36Bias Voltage (V)

Ga

in

GainGain vs Bias voltage

Q=Cmicrocell*(Vbias-Vbreakdown)

=> C = 80-90fF

T=22oC

Linear dependence,as expected.

Preliminary optical characterization

Trigger from pulse gen.

Dark signals

Example of a Signal Response to light excitation (=470nm)

VBIAS=33.5V

T=22oC

bunch of photons

No measurementof PDE has beendone yet.

Preliminary optical characterization

1pe 2pe 3pePulse height spectrum from low-intensity light flashes (red LED)

Each peak corresponds toa different number of fired cells

Very good single photoelectron resolution!

T=22oC

V=1.5V

T=22oCV=2V

1pe

2pe

Conclusion• September 2005: first production of Silicon Photomultipliers at ITC-irst.

Extremely good results: Gain ~ 106

Dark count ~ MHz Recovery time ~ 70ns PDE measurement in progress, encouraging first results

• Second production run just completed. Implemented trenches for optical cross-talk isolation. Characterization in progress.

• Next goal: to reduce dark count acting on the technology