NA62 Gigatracker Working Group 28 July 2009 Massimiliano Fiorini CERN

Software NA62 MonteCarlo fast simulation of the beam line based on Flyo called by Geant4 to generate primary particles beam particle passed to Geant4 at a given position along the beam line default: beam track passed just in front of the 1 st Gigatracker station only Gigatracker enabled: all other detectors excluded 3×10 6 events generated with two different “range cuts” for electrons in silicon: 1 st case: 1 mm (corresponds to 541 keV) 2 nd case: 1  m (corresponds to 990 eV) numbers and plots refer only to 1 st Gigatracker station

Beam Spectrometer Layout pπpπ pKpK pυpυ pυpυ θ πK 13.2 m9.6 m 60 mm GTK2 GTK3 GTK1 2 nd achromat 3 Gigatracker (GTK) stations: 60 mm × 27 mm dimension 300  m × 300  m pixel cells pixel structure and GTK geometry implemented in MonteCarlo by S. Bifani (see presentation on April 2009 Meeting): Si sensor (200  m thickness), Sn-Pb bump bond (15  m thickness, 10  m diameter cylinder), Si read-out chip (100  m thickness), carbon fiber support (100  m thickness)

1 st case

Number of hits on GTK1

Number of hits on GTK1 (zoom) Number of hits EventsEvent fraction [%] M k k k0.06 > 57.2 k0.24 Fraction of events with more than 1 hit: 0.72 %

New classes added (1) class GigaTrackerPixel gathers all the hits’ energy belonging to the same pixel sensitive volume methods implemented: AddEnergy(Double_t) GetNHit() GetEnergy() GetPixelID() GetPositionX() GetPositionY() SetClusterID(Int_t) GetClusterID()

New classes added (2) class GigaTrackerCluster collection of adjacent pixels (side-side, corner-corner) in a C++ vector methods implemented: AddPixel(GigaTrackerPixel) GetNPixels() GetPixelVector() GetDistance(GigaTrackerCluster) GetEnergy() GetPositionX() GetPositionY() GetWPositionX() GetWPositionY()

Number of pixels GTK1 Number of pixels Event fraction [%] Fraction of events with more than 1 pixel: 0.3%

Number of hits per pixel GTK1 Number of hits Pixel fraction [%] Fraction of pixels with more than 1 hit: 1.0 %

Number of clusters GTK1 Fraction of events with more than 1 cluster: 0.03 % Number of clusters Event fraction [%]

Number of pixels per cluster GTK1 Number of pixels Cluster fraction [%] Fraction of clusters with more than 1 pixel: 0.3 %

Energy release GTK1 per event mean energy: 72.4 keV (~20k e-h) most probable energy: 53.7 keV (~15k e-h) FWHM: ~25 keV (~7k e-h) minimum energy: ~29 keV (~8k e-h)

Hit position (X-Y) inside a pixel uniform pixel population (average on all pixel of one GTK station) r.m.s. = 86.8  m for both X and Y [300  m/SQRT(12) = 86.6  m]

Hit position (Z) inside a pixel in all cases the energy is released just at the entrance of the silicon sensor only for multiple hits, the energy is deposited also at different depths along the sensor, up to the full thickness (200 µm)

Hit position spread GTK1 hit position spread (with respect to first hit) in GTK1 r.m.s. (from un-zoomed plots) ~500  m for both X and Y

Position resolution single cluster arithmetic pixel position average (no weight) r.m.s. = 86.8  m for both X and Y

Charge sharing among pixels analyze single cluster formed by 2 pixels only (0.2%) pixel ordering given by Geant4 if both pixels have an energy below 28.8 keV (8 k e-h)  the event is inefficient found 9 inefficient events over 5595  (0.16 ± 0.05) % inefficiency

2 nd case

Number of hits on GTK1

Number of pixels GTK1 Fraction of events with more than 1 pixel: 2.4%

Number of hits per pixel GTK1 Fraction of pixels with more than 1 hit: 97.2 %

Number of clusters GTK1 Fraction of events with more than 1 cluster: 0.06 % Number of clusters Event fraction [%]

Number of pixels per cluster GTK1 Fraction of clusters with more than 1 pixel: 2.4 %

Hit position (Z) inside a pixel in all cases the energy is still released at the entry face of the silicon sensor now the energy is deposited much more often also at different depths along the sensor, up to the full thickness

Hit position spread GTK1 hit position spread (with respect to first hit) in GTK1 r.m.s. (from un-zoomed plots) ~100  m for both X and Y (before was ~500  m)

Position resolution single cluster arithmetic pixel position average (no weight) r.m.s. = ~86  m for both X and Y

Charge sharing among pixels analyze single cluster formed by 2 pixels only pixel ordering given by Geant4 if both pixels have an energy below 28.8 keV (8 k e-h)  the event is inefficient Found 28 inefficient events over  (0.05 ± 0.01) % inefficiency

Conclusions and To Do List use of new NA62 MonteCarlo with the aim of studying charge sharing effects among Gigatracker pixels two classes implemented for reconstruction purposes very preliminary results show a low inefficiency for particle detection in case that charge is distributed among 2 adjacent pixels To Do list increase statistics variations of other Geant4 parameters from the PhysicsList more detailed study of δ-rays production and “tracking” by Geant4 take into account other effects after-generation (diffusion, drift)