Rita Carbone, RICAP 11, Roma 3 26/05/2011 Stand-alone low energy measurements of light nuclei from PAMELA Time-of-Flight system. Rita Carbone INFN Napoli On behalf of the PAMELA collaboration

The PAMELA mission Search for antimatter Search for dark matter Study of cosmic-ray propagation Study solar physics and solar modulation Study of electron spectrum Study terrestrial magnetosphere First switch-on on June 21th 2006 Continuous data taking mode since 11th July 2006 Mission extended till December 2011 Launched on June 15th 2006

PAMELA apparatus Main requirements: high-sensitivity particle identification, precise momentum measure. Time-Of-Flight plastic scintillator strips + PMT: plastic scintillator strips + PMT:  trigger, albedo rejection;  trigger, albedo rejection;  mass identification up to E ~ 1 GeV;  mass identification up to E ~ 1 GeV;  charge identification from dE/dX.  charge identification from dE/dX. Magnetic spectrometer with microstrip Si tracker: with microstrip Si tracker:  charge sign and momentum  charge sign and momentum from the curvature; from the curvature;  charge identification from dE/dX.  charge identification from dE/dX. Electromagnetic calorimeter W/Si sampling; 16.3 X0: W/Si sampling; 16.3 X0:  discrimination e + / p, e - / p -  discrimination e + / p, e - / p - from shower topology; from shower topology;  direct E measurement for e-.  direct E measurement for e-.  charge identification from dE/dX  charge identification from dE/dX

The Time-of-Flight system provide a fast signal for triggering data acquisition in the whole instrument provide a fast signal for triggering data acquisition in the whole instrument measure the flight time of particles crossing its planes; rejection of albedo particles measure the flight time of particles crossing its planes; once this information is integrated with the measurement of the trajectory length through the instrument, their velocity can be derived. This feature enable also the rejection of albedo particles determine the absolute value of charge z of incident particles determine the absolute value of charge z of incident particles through the multiple measurement of the specic energy loss dE/dx in the scintillator counters 6 layers (3 planes, double view) Several paddles (scintillator + 2 PMTs) for each layer

Single paddle time resolution The position of the hit point on a paddle as reconstructed by the ToF is proportional to the difference of the time measurements operated at both ends for that event Plotting this TOF position vs. the position as reconstructed by the Tracker (assuming negligible the uncertanty in the projected position), the σ of the distribution of residuals gives us the single paddle time resolution. Time measurements Measured time resolution is ~200 ps for S1 and S3 paddles and ~150 ps for S2 paddles for Z=1 particles Measured time resolution is ~200 ps for S1 and S3 paddles and ~150 ps for S2 paddles for Z=1 particles For higher Z the time resolution improves: for Z=2 it is ~105 ps, for Z=6 it is ~70 ps For higher Z the time resolution improves: for Z=2 it is ~105 ps, for Z=6 it is ~70 ps This results are in agreement with some results from a beam test of February 2006 (in this case time resolution reached a value of 48 ps for Carbon ions) This results are in agreement with some results from a beam test of February 2006 (in this case time resolution reached a value of 48 ps for Carbon ions) As a consequence, the resolution in measurements of time of flight is ~300 ps for Z=1 particles and improves to ~100 ps for Carbon events As a consequence, the resolution in measurements of time of flight is ~300 ps for Z=1 particles and improves to ~100 ps for Carbon events

β measurement time of flight and β are related by: So each single measurement of time of flight allows a β measurement, whose resolution is given by the relation: The 6 ToF layers of PAMELA allow up to 12 independent measurements of time of flight and β, significantly improving this resolution! Plotting distributions of 1/β for relativistic samples of different nuclei we observe that also β resolution improves with increasing Z: σ ~ 14% for protons σ ~5% for Carbon σ ~ 5% for Carbon

Particle identification with ToF dE/dx vs. β distribution in a paddle dE/dx vs. β distribution in a paddle (after corrections for attenuation, gain variation and non linearity ) scaling for a Bethe-Block function Z measured in a paddle

A useful application: a completely ToF stand-alone B/C ratio measurement S11 S12 S21 S22 S31 S32 Used for selection Used for efficiency calculation All analysis is based only on flight data recorded till 31 January 2010

A useful application: a completely ToF stand-alone B/C ratio measurement Kinetic energy per nucleon E can be derived event by event from β measurement according to the relation E=amu(γ-1). Kinetic energy per nucleon E can be derived event by event from β measurement according to the relation E=amu(γ-1). As a consequence of resolution in β measurements, such energy measurement is realiable up to 2.5 GeV/n As a consequence of resolution in β measurements, such energy measurement is realiable up to 2.5 GeV/n Such energy determination is independent of the isotopic composition Such energy determination is independent of the isotopic composition All other sub-detectors are used, for this work, only for cross-correlation purpose in the “calibration” phase All other sub-detectors are used, for this work, only for cross-correlation purpose in the “calibration” phase

Boron and Carbon selection good quality sample of nuclei candidates (rejection of albedo particles and multiple event, removing South Atlantic Anomaly, etc.) good quality sample of nuclei candidates (rejection of albedo particles and multiple event, removing South Atlantic Anomaly, etc.) Using cross-correlation between ToF and Calorimeter we can calculate functions Z=f(E) and σ=f(E) for each nucleus on each ToF layer Using cross-correlation between ToF and Calorimeter we can calculate functions Z=f(E) and σ=f(E) for each nucleus on each ToF layer ToF layers S12, S21 and S22 are used for nuclei identification: |Z S12 -Z C S12 (E)|<1σ C S12 (E) AND |Z S2 -Z C S2 (E)|<1σ C S2 (E) ToF layers S12, S21 and S22 are used for nuclei identification: |Z S12 -Z C S12 (E)|<1σ C S12 (E) AND |Z S2 -Z C S2 (E)|<1σ C S2 (E) ~ B and ~ C selected events ~ B and ~ C selected events

Selection efficiency Efficiency sample: Same “quality” cut as for the analysis sample; Same “quality” cut as for the analysis sample; Monochromatic sample of B and C selected by S11 and S3 layers: |Z S11 -Z C S11 (E)|<1σ C S11 (E) AND |Z S3 -Z C S3 (E)|<1σ C S3 (E) Monochromatic sample of B and C selected by S11 and S3 layers: |Z S11 -Z C S11 (E)|<1σ C S11 (E) AND |Z S3 -Z C S3 (E)|<1σ C S3 (E) The efficiency of the selection is very law at low energy values, expecially for Carbon, because of the early saturation of the S2 plane. Efficiency of C selection Log10(E)

Stand-alone ToF B/C ratio in energy range GeV/n AMS: Aguilar et al., Astrophys.J.724: (2010) HEAO: Engelmann et al., Astron.Astrophys. 233, (1990)

Spare

Charge calibration of the ToF Many difficulties during calibration because of: 6 different groups of scintillators + 48 different PMTs → 48 independent calibrations Loss of linearity of the instrument due to Birks’saturation of scintillators and loss of gain of PMTs at high values of charge deposits Timing and damaging effects A complex 4 step charge calibration to go from ADC to dE/dx: 1. ADC counts-pC conversion + attenuation of the signal along the paddle 2. Compensation of non-linearity for relativistic sample 3. Evaluation of Bethe-Bloch functions and compensation of non linearity also for non-relativistic sample 4. Corrections for time dependence and aging effect