measurement of the hydrogen and helium absolute fluxes with the pamela experiment
DESCRIPTION
Measurement of the Hydrogen and Helium absolute fluxes with the PAMELA experiment. Elena Vannuccini On behalf of PAMELA collaboration. PAMELA Payload for Matter/antimatter Exploration and Light-nuclei Astrophysics. Direct detection of CRs in space - PowerPoint PPT PresentationTRANSCRIPT
ELENA VANNUCCINI
O N B E H A L F O F PAMELA COLLABORATION
Measurement of the Hydrogen and Helium absolute fluxes with the
PAMELA experiment
PAMELAPayload for Matter/antimatter Exploration and
Light-nuclei Astrophysics
•Direct detection of CRs in space•Main focus on antiparticles (antiprotons and positrons)
Launch from Baykonur
• 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 PAMELA in continuous data-taking mode since then!
PAMELA detectorsMain requirements:
- high-sensitivity antiparticle identification
- precise momentum measure
GF: 21.58 cm2 sr Mass: 470 kgSize: 130x70x70 cm3
Power Budget: 360W
Spectrometer microstrip silicon tracking system + permanent magnetIt provides:
- Magnetic rigidity R = pc/Ze = 1/|h|- Charge sign sign of h- Charge value from dE/dx
MDR* up to 1400GV*MDR = Maximum Detectable Rigidity R/R=100%
Time-Of-Flightplastic scintillators + PMT:- Trigger- Albedo rejection;- Mass identification up to 1 GeV;- Charge identification from dE/dX.
Electromagnetic calorimeterW/Si sampling (16.3 X0, 0.6 λI) - Discrimination e+ / p, anti-p / e- (shower topology)- Direct E measurement for e-
Neutron detectorplastic scintillators + PMT:- High-energy e/h discrimination
+ -
H/He selection
Single good-quality track in the spectrometer Particle rigidity (R = pc/Ze )
Downward-going (b>0) & positive-curvature (R>0) trajectory Positive-charge particle from above
Clean pattern through the apparatus Not an interaction product
Energy deposits in the tracking system consistent with H and He nuclei
He H
High-statistic (~108)sample of H and He (no isotope separation)Negligible bk of-interaction products-misidentified particles
H-flux vs L-shell
Galactic particles selected by requiring:
R >1.3 C
C = vert. Störmer cutoff
H flux
Polar regions
Equator
Selection efficiencies
General approach:•Efficiency evaluated from flight data Real performances
•Cross-checks and corrections from MC simulation Complete information Test of measurement procedure
•Evaluated every 2 months
Selection cuts
R<MDR
Protons
±4%
(MDR = 200÷1400GV)
Fiducial acceptance
Spectrum unfolding
Protons
10%
Real-energy spectrum (R)
• Physical effects ionization & m.scattering• Instrumental effects spatial resolution & alignment uncertainties
Measured-energy spectrum (Rm=R±)@ high energy: •Bayesian unfolding
•Spectrometer response matrix from MC
Spectrometer systematic uncertainty
Possibility of residual coherent misalignment (distortion) of the tracking system
Evaluated from in-flight electron/positron data by comparing the spectrometer momentum with the calorimeter energy
Upper limit set by positron statistics:
hsys ~10-4 GV-1
h~ 10-3 GV-1
A systematic deflection shift causes an offset between e- and e+ distribution
e+ e-
hsys
systrksyscalcal
trk
Δηηε1E1
EP
sys
Overall systematic uncertainties
At low R selection-efficiency uncertainties dominate
Above 500GV tracking-system (coherent) misalignment dominates
selection-efficiency
uncertainties
spectrometer systematic
error
Check of systematics
Fluxes evaluated by varying the selection conditions:
• Total vs time
• Total vs polar/equatorial
• Total vs reduced acceptance
• Total vs different tracking conditions ( different response matrix)
• …
Time interval (2 months)
Integral proton flux (>50GV)
3%
H & He absolute fluxes
• First high-statistics and high-precision measurement over three decades in energy
• Low energy minimum solar activity (f = 450÷550 GV)
• High-energy (>30GV) a complex structure of the spectra emerges…
Adriani et al. - Science - 332 (2011) 6025
PAMELA data Jul 2006 ÷ Mar 2008
H & He absolute fluxes@ high energy
Deviations from single power law (SPL): Spectra gradually soften in the range 30÷230GV
Spectral hardening @ R~235GV ~0.2÷0.3
SPL hp rejected at 98% CL
Origin of the structures?
- At the sources: multi-populations, non-linear DSA
- Propagation effects Sola
r m
odul
atio
n
Sola
r m
odul
atio
n
2.852.67
232 GV
Spectral index 2.772.48
243 GV
H He
H/He ratio vs R
Instrumental p.o.v.Systematic uncertainties partly cancel out
Theoretical p.o.v.Solar modulation negligible information about IS spectra down to GV region
Propagation effects small above ~100GV information about source spectra (eg. Putze et al.)
Power-law fit (c2~1.3)aHe-ap = 0.078 ±0.008
Solar activity during PAMELA life
PAMELA launched in 2006 @ beginning of last solar cycle.
Data collected over 5 years around minimum solar activity
Long-term flux variations
• Very large statistics collected
• Precise spectral measurement down to 400MV
Detailed study of solar modulation effect
Protons
Summary and conclusions
PAMELA has been in orbit and studying cosmic rays for ~4.5 years. >109 triggers registered
H and He absolute fluxes up to 1.2TV Most precise measurement so far. Complex spectral structures observed (spectral hardening at ~200GV!)
Step forward in understanding galactic CR origin and propagation!
Forthcoming results on long-term flux variations down to few hundred MV
Step forward in understanding propagation in the Solar System!
Thanks!!