Transcript Document

Elena Vannuccini
(INFN Florence)
on behalf of
the PAMELA collaboration
PAMELA flight model
The PAMELA experiment
MAIN TOPICS:
 CR antiproton and positron spectra:
~104 antiprotons  80 MeV/c - 190 GeV/c
~105 positrons  50 MeV/c - 270 GeV/c
 search for light antinuclei
SECONDARY TOPICS:
 Modulation of GCRs in the Heliosphere
 Solar Energetic Particles (SEP)
 Earth Magnetosphere
More about PAMELA:
E.Mocchiutti – H01 – 14 July 11:00
M.Pearce – E19 – 18 July 09:55
• PAMELA on board of Russian satellite Resurs DK1
• Orbital parameters:
- inclination ~70o ( low energy)
- altitude ~ 360-600 km (elliptical)
- active life >3 years ( high statistics)
 Launched on 15th June 2006
 First switch-on on 21st June 2006
• Detectors in nominal conditions (no problems due to the launch)
• Tested different trigger and hardware configurations
• Commissioning phase successfully ended on September 15th 2006
 PAMELA in continuous data-taking mode since then!
Launch from Baykonur
Elena Vannuccini
PAMELA detectors
Main requirements  high-sensitivity antiparticle identification and precise momentum measure
+
Time-Of-Flight
plastic scintillators + PMT:
- Trigger
- Albedo rejection;
- Mass identification up to 1 GeV;
- Charge identification from dE/dX.
Electromagnetic calorimeter
W/Si sampling (16.3 X0, 0.6 λI)
- Discrimination e+ / p, anti-p / e(shower topology)
- Direct E measurement for e-
GF: 21.5 cm2 sr
Mass: 470 kg
Size: 130x70x70 cm3
Power Budget: 360W
Neutron detector
plastic scintillators + PMT:
- High-energy e/h discrimination
Spectrometer
microstrip silicon tracking system + permanent magnet
It provides:
- Magnetic rigidity  R = pc/Ze
- Charge sign
- Charge value from dE/dx
Elena Vannuccini
Antiprotons
Unexplored Region
Secondary component
• CR propagation
Primary source ((?))
• Dark matter
• Extragalactic primordial p-bar
Maximum energy determined by
spectrometer performances
(wrong determination of charge sign)
Spectrometer required performances:
4 mm resolution on the bending view (x)  MDR = 740 GV  spillover limit 190 GeV
( MDR = Maximum Detectable Rigidity  DR/R=1 @ R=MDR where R=pc/Ze )
Elena Vannuccini
The magnet
• 5 magnetic modules
• Permanent magnet (Nd-Fe-B alloy) assembled in
an aluminum mechanics
• Magnetic cavity sizes (132 x 162) mm2 x 445 mm
• Geometric Factor: 21.5 cm2sr
• Black IR absorbing painting
• Magnetic shields
Magnet elements
Magnetic tower
Aluminum frame
Base plate prototype
Elena Vannuccini
Magnetic module
The magnetic field
MAGNETIC FIELD MEASUREMENTS
• Gaussmeter (F.W. Bell) equipped with 3-axis probe mounted on a motorized positioning
device (0.1mm precision)
• Measurement of the three components in 67367 points 5mm apart from each other
• Field inside the cavity:
• 0.48 T @ center
• Average field along the axis: 0.43 T
• Good uniformity
• External magnetic field: magnetic momentum < 90 Am2
Elena Vannuccini
The tracking system
6 detector planes, each composed by 3 ladders
Mechanical assembly
• aluminum frames
• carbon fibers stiffeners glued laterally to the ladders
• no material above/below the plane
1 plane = 0.3% X0  reduced multiple scattering
• elastic + rigid gluing
Carbon fibers
LADDER
Test of plane lodging inside the magnet
First assembled plane
Elena Vannuccini
Silicon detector ladders
• 2 microstrip silicon sensors
• 1 “hybrid” with front-end electronics
Silicon sensors (Hamamatsu):
• 300 mm, double sided - x & y view
• AC coupled (no external chips)
• double metal (no kapton fanout)
• 1024 read-out channels per view
- strip/electrode coupling ~ 20 pF/cm;
- channel capacitance to ground:
- junction: < 10 pF
- ohmic: < 20 pF
Bias:
• VY -VX = + 80 V fed through guard
ring surrounding the strips
• Bias resistor:
- junction: punch-through, > 50 MΩ;
- ohmic: polysilicon, > 10 MΩ.
• Leakage current < 1 μA/sensor.
Elena Vannuccini
In-flight basic performaces
X view
Y view
N ~ 4 ADC counts
N ~ 9 ADC counts
cluster
S/N = 7/6 (x/y)
Y view
larger noise
 worse performances
S/N = 4
-2 -1 0 1 2
X view
lower noise
 better performances
Signal-to-noise
S/N 

Si
i
Ni
• Tracking system calibrated @
every orbit (95 min)
• Data acquisition in compressed
mode (~5%)
12 x 250 B ~ 3 kB/ev
(5 kB/ev all detectors)
 system is stable
 good signal-to-noise performaces
Elena Vannuccini
Charge identification capabilities
Beam-test data (@GSI 2006)
flight data
12C
projectiles on Al and
polyethylene targets
(track average)
4He
B,C
3He
d
Be
p
Li
Saturated clusters
X view
Y view
• Good charge discrimination of H and He
• Single-channel saturation at ~10MIP affects B/C discrimination
Elena Vannuccini
Spatial resolution
Sensor instrinsic resolution
Spatial resolution studied by means of beam-test of silicon detectors and simulation
Simulation
Simulation
COG
ETA4
ETA2
Center-Of-Gravity
• junction side (X):
• ohmic side (Y):
ETA2
ETA3
Non-linear algorythm with 2,3,4 strips
3 mm @0o, < 4 mm up to 10o ( determines momentum resolution)
8÷13 mm
Sensor alignment (relative to mechanical positions)
Track-based alignment: minimization of spatial residuals as a function of the roto-traslational
parameters of each sensor
@ground  proton beam and atmospheric muons (cross-check)  ~100±1 mm
@flight  observed displacements relative to ground alignment  ~10 mm
Necessary to align in flight !!
Elena Vannuccini
In-flight alignment
 Done with relativistic protons (high statistics)
Flight data
Simulation
Spatial residuals
(1st plane)
protons 7-100 GV
X side
Y side
(ymeas-yfit)
(xmeas-xfit)
After alignment:
• residuals are centered
• width consistent with nominal resolution
Elena Vannuccini
Momentum resolution
Iterative c2
minimization as a
function of track statevector components a
100 x
Beam test - protons
multiple scattering
MDR ~ 1TV
spatial resolution (x)
R (GV)
η = 1/R  magnetic deflection
Trajectory evaluated by
stepwise integration of
motion equations by means
of Runge-Kutta method
(not-homogeneous B field)
sR/R = sh/h
Maximum Detectable Rigidity (MDR)
def: @ R=MDR  sR/R=1
MDR = 1/sh
 Measured at beam test with protons of known momentum (CERN SPS, 2003)
 In-flight: (possible) global distortions after alignment procedure  deflection offset
 cross-check with electrons and positrons
energy measured by the calorimeter  DE/E < 10% above 5GeV
Elena Vannuccini
• z < 1 due to
Bremstahlung effect
in the material
above the
spectrometer
• the pdf of z
depends only on the
amount of traversed
material
Spectrometer systematics
z ~
1
PCal η Spe

1

PCal 1  ε  η Spe  Δη

Calorimeter calibration uncertanty
deflection offset
electrons
Positrons
5÷20 GeV
P0~10-5
P0~0.403
Kolmogorov probability between ze- and ze+ (P0 = 0 ÷ 1) with free parameter Dh
Dh ~ -10-3 GV -1
Elena Vannuccini
High-energy antiproton analysis
Event selected from 590 days of data
S1
Basic requirements:
• Clean pattern inside the apparatus
– single track inside TRK
– no multiple hits in S1+S2
– no activity in CARD+CAT
CAT
S2
TOF
• Minimal track requirements
TRK
c2
– energy-dependent cut on track (~95% efficiency)
– consistency among TRK, TOF and CAL spatial
information
S3
• Galactic particle
CAL
– measured rigidity above geomagnetic cutoff
– down-ward going particle (no albedo)
S4
ND
Elena Vannuccini
CAS
.
Antiproton identification
•
•
dE/dx vs R (S1,S2,TRK) and b vs R proton-concistency cuts
electron-rejection cuts based on calorimeter-pattern topology
-1  Z  +1
p (+ e+)
p
electron (17GV)
e- (+ p-bar)
“spillover” p
p-bar
Antiproton (19GV)
5 GV
Elena Vannuccini
1 GV
Proton spillover background
p-bar
“spillover” p
MDR = 1/sh
evaluated eventby-event by the
track fitting routine
10 GV
50 GV
MDR account for:
• number and distribution of fitted points along the trajectory
• spatial resolution of the single position measurements
• magnetic field intensity along the trajectory
Elena Vannuccini
p
Proton spillover background
Minimal track requirements
MDR > 850 GV
Strong track requirements:
•strict constraints on c2 (~75% efficiency)
•rejected tracks with low-resolution
clusters along the trajectory
- faulty strips (high noise)
- d-rays (high signal and multiplicity)
Elena Vannuccini
High-energy antiproton selection
p
p-bar
10 GV
50 GV
Elena Vannuccini
High-energy antiproton selection
p
p-bar
10 GV
50 GV
Elena Vannuccini
High-energy antiproton selection
p
p-bar
R < MDR/10
10 GV
50 GV
Elena Vannuccini
Antiproton/proton ratio
~300 p-bar
Elena Vannuccini
Antiproton/proton ratio
Secondary production
CR+ISM p-bar + …
Elena Vannuccini
Conclusions
PAMELA is in space, continuously taking data since July 2006
Detectors have been calibrated and in-flight performances has been studied
 PAMELA now ready for science!!
Magnetic spectrometer:
- basic performances (noise, cluster signal, spatial resolution...) are nominal
- tracking system alignment completed (incoherent+coherent)
 Spectrometer performances (momentum resolution) fulfill the requirements of the
experiment
 Preliminary results about high-energy antiproton abundance could be obtained!!
Work in progress to extend antiproton measurement further in energy
 thanks! 