Chapter 4: A MIP Timing Detector (MTD) for CMS at HL-LHC
4.3 Studies to optimize BTL design parameters
4.3.1 Characterization of the sensor properties and optimizing the
Figure 4.6: Various BTL components. (Left) A BTL Readout unit, consisting of 24 BTL modules of LYSO:Ce bars + SiPMs. Each RU has a total of 768 SiPMs.
(Right) A BTL module consisting of 16 LYSO:Ce bars + 32 SiPMs. The module is connected to the FE boards using flex cables, as can be seen in the figure.
The modules will be read out with an ASIC, known as the TOFHIR (Time-of- flight, High Input Rate) chip (Ref. [102]). The TOFHIR chip is being designed to deliver precision timing information at the required rates for the BTL, and will be radiation tolerant, will satisfy the power requirements and will be able to operate at the low BTL temperatures (-35 ◦C). It will also have a dedicated noise cancelling circuit to mitigate the effect of DCR noise in the SiPMs. Appendix B discusses the commissioning tests performed on the two preliminary versions of the BTL prototype readout electronics, i.e., the TOFPET and the TOFHIR2A ASIC.
4.3.1.1 Impact point studies with laser
Before the bar shaped geometry was selected for the LYSO crystals, there was some interest in exploring a square tile shaped geometry for the crystals (Ref. [103]).
To understand the properties of such a geometry, we used 3×3 mm2 SiPMs from Hamamatsu and coupled them to 12×12×4 mm3 LYSO square tiles using optical grease (see left of Fig.4.7). We used a 373 nm UV laser to shine light directly into the the sensor configuration (with the light entering the LYSO crystal first, before the SiPM) , and recorded the output using a DRS4 digitizer. The laser pointer was moved slowly from one edge to the other edge of the tile configuration. Fig. 4.7 (Right) shows the amplitude dependence on the MIP impact point in this setup. It can be seen that in such a geometry, the centre of the tile (at the SiPM) records a 30% higher amplitude as compared to the edges, which can be understood from the smaller distance travelled by the scintillation light produced at the tile center, before reaching the SiPM.
Figure 4.7: Impact point studies with a laser. (Left) A schematic of a 3x3 mm2 SiPM coupled with a 12x12x4 mm3 LYSO tile using optical grease. (Right) Am- plitude recorded as a function of the MIP impact position on the SiPM +LYSO tile configuration.
This non-uniformity in amplitude also directly impacts the measured time resolution.
The time resolution measured at the centre of the tile is slightly better than the time resolution recorded at the edges, as can be seen in Fig. 4.8. To avoid this impact point dependence, the bar geometry was chosen for the LYSO:Ce crystals, with two SiPMs at the two edges.
Figure 4.8: The measured time resolution as a function of the MIP impact position on the SiPM +LYSO tile configuration. The time resolution measurement here includes a time walk correction.
4.3.1.2 Effect of wrapping the crystal on light collection efficiency
Before ESR was chosen as the reflector material for wrapping the crystals, we per- formed tests of the light collection efficiency by studying the difference between wrapped and unwrapped LYSO square tiles (not bars). We used 3×3 mm2 SiPMs from Hamamatsu and coupled them to 12×12×4 mm3LYSO square tiles using opti- cal grease for this study. We used a Keysight S-series scope for data acquisition, with a 6 GHz bandwidth and sampling rate of 20 Gsamples/second. Some specifications provided by the manufacturer for the LYSO tile are quoted below.
• Gain: 7×105
• Light yield: 33200 photons/MeV
• Photon detection efficiency (PDE): 25 %
Most photons hitting the LYSO will never reach the SiPM because of the low photon detection efficiency and also because the SiPM active area is much smaller than the LYSO tile area. The idea behind wrapping the crystals is to increase the probability of the photons reaching the SiPM, such that the scintillation light produced in the LYSO bounces back and forth several times before it exits the tile. To demonstrate the power of wrapping on the light collection efficiency (LCE), we placed a 22Na
Figure 4.9: 22Na spectrum as recorded from the charge deposited in the SiPM in two different cases: bare configuration and wrapped configuration.
source in front of the SiPM+LYSO configuration and recorded the source spectrum in the following two cases:
• no wrapping
• SiPM + LYSO wrapped in a reflective coating of Teflon tape .
The oscilloscope can be used to measure the22Na spectrum in a Volts vs time graph.
Using the known values of the circuit board amplifier (10) and the overall circuit resistance (50 Ohms), we can take an integral of the recorded pulse and divide it by the circuit impedance to obtain a charge (in pico-Coulombs) vs counts distribution.
The charge deposited by a MIP can be written as
Charge = Gain×LYSO light yield×PDE×LCE×𝑒 (4.2) where e is the charge of an electron. In an ideal case with LCE = 1, the charge deposited would be 475 pC for the 511 keV transition. From Fig. 4.9, the charge deposited corresponding to the 511 keV peak is≈149 pC with the teflon wrapping and≈50 pC with no wrapping. From this we can conclude that the LCE is about 3 times bigger in teflon wrapped LYSO+SiPM when compared to the unwrapped case.
On using Eqn. 4.2, we can caluclate 31% LCE for the teflon wrapped configuration and only 10.1% LCE for the bare configuration.
The final decision to use ESR instead of Teflon was made based on the poor radiation tolerance of Teflon.
4.3.1.3 Calibration of the number of photoelectrons deposited by a MIP
To measure the number of photoelectrons generated in the sensors per MIP, we need to first calibrate the energy per single photoelectron. To study this, we used a 3×3 mm2 bare SiPM (no LYSO tile), wrapped it in Teflon (with a tiny gap in the wrapping at the center) and mounted it on a circuit board. We used a Keysight S-series scope for data acquisition, with a 6 GHz bandwidth and sampling rate of 20 Gsamples/second. We recorded the integrated charge spectrum by shining a laser light directly into the SiPM (see Fig.4.10). For this, we placed an optical filter in front of the SiPM with ND = 2, where ND is defined as
𝑁 𝐷 =( 𝐼 𝐼0
)−𝑥, (4.3)
where x is the value of the ND filter (in this case ND = 2). I0 and I correspond to the intensities of the light before and after passing the filter. The laser light was also manually tuned to the lowest value where we could record a clean spectrum.
Figure 4.10: The SiPM is wrapped with Teflon and mounted on circuit board. A 373 nm UV laser is directly pointed at the center of the SiPM.
The charge spectrum is shown on the left in Fig.4.11. The first peak from the right is coming from the noise and trigger turn on. The second, third and fourth peak from the right correspond to one, two and three photo-electrons respectively. The peaks were then fitted with a Gaussian distribution and the integral is plotted against the number of photo-electrons, 𝑁
p.e. (with appropriate normalization), as seen on the right in Fig4.11. We obtain a Poisson distribution with mean 1 and a p-value of 0.67.
Figure 4.11: Single photoelectron measurements with a UV laser. (Left) The recorded charge spectrum. The different peaks correspond to different values of 𝑁p.e.. (Right) Poisson fit to the number of photo-electrons.
Figure 4.12: The number of electrons plotted against the number of photoelectrons.
We then plotted the the number of electrons (calculated from the integrated charge or area under the curve) from each peak against the number of photo-electrons (peak number), as seen in Fig. 4.12. The error on the number of photo-electrons is 0 and the error on the number of electrons is extracted from the error on the mean parameter from the guassian likelihood fit. We measure a gain of 7x105± 550.6, which is the number quoted by the manufacturer (see Sec.4.3.1.2).
We also recorded charge histograms by shining laser light into the SiPM through various optical filters. The optical filters used were : No Filter (ND = 0.0), ND = 0.1, ND = 0.2, ND = 0.3, ND = 0.4, and ND = 0.5. These charge histograms were fitted with a Gaussian to give the mean charge deposit. These were then plotted against the ND value (or normalized intensity) as shown in Fig4.13. The error on the mean
Figure 4.13: The charge deposited in a SiPM scales linearly with the intensity of the laser light.
charge is extracted from the mean parameter from the gaussian likelihood fit. Since the ND value (or normalized intensity) is proportional to the energy deposited, we can confirm that the energy deposited in the SiPM scales linearly with the charge (until the SiPM is saturated).
Once we know the relationship between the number of photo-electrons and the number of electrons (Fig. 4.12) and also the fact that the energy deposit scales linearly with integrated charge in a SiPM, we can determine the number of photo- electrons in a MIP energy deposit. We coupled the 3x3 mm2SiPM with a 12x12x4 mm3 LYSO tile using optical grease, wrapped the configuration with teflon and mounted it on a circuit board. The SiPM was biased at 57 V. We then recorded the spectrum of 22Na. In Fig. 4.14, the two beta decay transitions of the22Na source (511 keV and 1.27 MeV) are visible (2nd and 3rd peaks from the right, respectively).
The 1st peak is interpreted as a convolution of the lower compton edge, noise and trigger turn on. On fitting the peaks with a gaussian, the following results are obtained:
• For the 511 keV peak, the deposited charge is 85.5 pC which corresponds to 535M electrons. Thus the number of photoelectrons is estimated to be≈763 (from Fig. 4.12).
• For the 1.27 MeV peak, the deposited charge is observed to be 187.7 pC which is roughly 1173M electrons. This corresponds to 1674 photoelectrons (from Fig. 4.12).
Figure 4.14: The 22Na spectrum recorded by the 3x3 mm2 SiPM coupled with a 12x12x4 mm3LYSO tile using optical grease and wrapped in teflon.
The ratio of the deposited charge in the two peaks, 85.5/187.7 = 0.45 is within 10%
of the expected energy ratio of the two peaks, i.e., 0.51/1.27 = 0.40 (systematics not estimated). If we take the Compton edge of the 1.27 MeV photon to be around 160 pC from Fig. 4.14, the ratio of the charge in the compton edge with the 1.27 MeV peak is 160/190 = 0.842, which is less than 1% away from the expected value of 1068/1270 = 0.841. As we already know about the linear relationship between the energy deposited in the SiPM and the integrated charge (Fig.4.13), we can construct a simple relationship from the energy of the two peaks and the 1.068 MeV compton edge, given by
𝑁𝑝 .𝑒 . =1200.3×𝐸[𝑀 𝑒𝑉] +149.3. (4.4) This procedure was repeated several times, unwrapping and wrapping the tile again (leaving everything else untouched) in order to study the systematic effects produced by the stability of our wrapping procedure. The results obtained did not show any qualitative difference, with the variation in the charge deposited being less than 2%. Thus, we can conclude that for a 3 MeV MIP, the number of photoelectrons generated is≈3750. For a 4 MeV MIP, the number of photoelectrons generated is
≈5000.
Figure 4.15: The test beam setup at Fermilab. From left to right, the scintillator is used for trigger, the silicon tracker is defines the MIP impact position in the x-y plane, and the MCP-PMT is used to provide a reference time. The LYSO+SiPMs test setups, one for the 1-bar and the other for the 3-bar array, are positioned along the beamline. The beam direction is the z-axis.
Figure 4.16: Fermilab April 2019 test beam. (Left) The experimental setup at FTBF.
(Right) The box and mechanical structure capable of rotating the 3-bar assembly.