Positron Detection: LYSO screens

Potential Upgraded LYSO (0.5mm x 0.5mm pixel; 10mm long)

light-collection efficiency and SNR for single ~GeV positron light_collection.pdf
ATTENTION: the formula for the light-collection efficiency is wrong:
I assumed 1.1 x 10^-3 for the light-collection efficiency:
- move the camera closer (500 mm instead of 760mm)
- use a lens with f=0.95 (Navitar)
- assume that the light is reflected from the back side due to ESR (2pi instead of 4pi emission)
- pi (0.95*50mm/2)^2 / (2pi* (500mm)^2) = 1.1 x 10^-3

2024 Summer Downtime Changes

We removed the 1st LYSO to make space for the positron tracker installation
We moved the camera closer to the mirrors to improve light collection

2024 August: light collection

Camera: Hamamatsu Orca Flash 4.0 camera (https://www.hamamatsu.com/eu/en/product/cameras/cmos-cameras/C13440-20CU.html)
Lens specifications: 50mm focal length
Magnification factor: lower end of ruler (pixel 1864) 140mm marker: (pixel 345): 1 pixel (6.5 micrometer) ↔ (140 mm / (1864-345)): M = 140 mm / ((1864-345) * 6.5 micrometer) = 14.18
Distance screen to lens: s; distance lens to sensor: s': thin-lens formula: 1/s + 1/s' = 1/f
Magnification: M = s/s' = (s/f - 1) → s = 50mm (14.18 + 1) = 760 mm
LYSO dimensions: 2mm x 2mm → imaged to (2mm / (14.18*6.5 micrometer))^2 ~ 471 pixel
We gain a factor of 16 if we reduce the crystal size to 0.5mm x 0.5mm
Light collection efficiency (lower estimate; doesn't take directionality of the emission into account; f-number: 1.2): pi (50mm / (2*1.2))^2 / (2 pi * (760 mm)^2) ~ 3.8 x 10^-4

2024 May: calibration measurements

See the full presentation for details: 9-24 presentation.pdf

Estimation of LYSO2 sensitivity in counts/positron:

LYSO1 seems to have a defect around some pixels, so only LYSO2 was used for analysis.

Experimental setup - relevant quantities for energy axis (from measurements made in tunnel + CAD model):

dy = 67.36mm; obtained by using distance from PDC downstream pipe center and zero dispersion axis from PDC CAD model (-125.93 mm) + pipe radius (+7.01 inch) + measurement in tunnel of the distance from the top of the pipe to the bottom of the LYSO2 screen (+0.6 inch)
d_nom = 24.81 mm; obtained from d_nom = 57.53 mm at DTOTR1 (Spectrometer Energy Calibration) linearly scaled to LYSO2 z-position as:

using z_B5D36 = 2005.95 m, z_PDC_END = 2009.37 m, z_LYSO2 = z_PDC_END + 0.59 (tunnel measurement), and z_DTOTR1 = 2015.26 m. Note that using d_nom measured at LFOV (Spectrometer Energy Calibration) gives d_nom = 24.49 mm at LYSO2, which is within the assumed uncertainty on d_nom below (+/-7%).

Positron energy spectrum calculation steps:

Images of LYSO screens were collected by camera and averaged over shots (3000 shots for 0.1mm Al foil, 100 shots for 1mm Pb glass)
For background subtraction details, see presentation above
Images are projected along non-dispersion (x) axis to obtain a raw positron spectrum in counts per pixel versus y position in pixel
y pixels are converted to mm using resolution ~5.63 px/mm (calculated by comparing screen size on image to physical measurements made in tunnel)
y axis in mm is converted to energy axis in GeV using the formula:

(All energies are in GeV)

Raw positron spectrum is converted from counts per pixel to counts per mm using pixel resolution, and then from counts per mm (dN/dy) to counts per GeV by chain rule:

GEANT4 simulation for Al 0.1 mm is used to calibrate from counts per GeV to positrons per GeV. The 1 mm glass Pb simulation is linearly scaled up by 3x to better match experimental data and is used to compare the general shape (and not the absolute value) of the positron spectrum between experiment and simulation.

Figures showing both the GEANT4 simulations and the experimental measurements using the methods described above:

In these figures, the dotted lines represent the part of the positron spectrum where significant number of positrons are obstructed by the ceiling of the dipole exit window.

For 0.1mm Al foil data, we use the energy range from 2.5 GeV to 3.5 GeV where positron loss from the transport from IP to LYSO is expected to be negligible, and we compare total camera counts (14889 counts) with the number of positrons from the GEANT4 simulation (1048 positrons). This provides the experimental calibration value of ~14 counts/positron.
Assuming dy is accurate to +/-1cm and d_nom is accurate to +/-7% (corresponding e.g. to a shift of B5D36 z location by +/- 0.5 m), calibration value is accurate to +/- 2 counts per positron.

Final experimental sensitivity: 14 +/- 2 counts per positron.