Impact of Fabrication-related Concentrator Imperfections

The gains in light yield and CRT predicted in the previous section were based on simulations that used some idealizations, for instance a perfect alignment of the LC on the SiPM or reflector walls without surface scattering. In this section, the results of a simulation study are presented, that analyzes the impact of fabrication-related imperfections on the performance of an LC. All these studies considered a concentrator with H_C = 4 µm.

Horizontal Alignment of Concentrator on SiPM If the LC is fabricated as an inde-pendent device apart from the SiPM, the mounting on the sensor surface can only be achieved with limited accuracy depending on the assembly process. Typically, pick and place robots are used in the semiconductor industry for automated positioning combining high precision and fast processing. High end systems can provide an accuracy down to 2 µm (standard deviation). To evaluate the sensitivity of the LC approach on the horizontal alignment, the concentrator was placed off-center by a certain distance (see Fig. 5.15), first along one axis (∆x) and then diagonally along both lateral axes (∆x= ∆y). The optical MC simulations indicate, that the gain in light yield through the LC is still larger than 10% for a one-dimensional displacement of up to 2 µm (see Fig. 5.16a). For the more realistic situation of LC-misalignment in both lateral dimensions (see Fig. 5.16b), a 2µm offset already decreases the benefit in light yield to 7%. The negative effects on the timing resolution are even more pronounced and a misalignment of ∆x= ∆y = 2 µm leads to a CRT equal to the reference.

Vertical Gap between Concentrator and SiPM If the LC device is bonded to the SiPM surface using an additional material as glue, it will have a certain vertical distance

∆z to the sensor. In the case of using a transparent glue, the inactive areas of the SiPM become accessible to photons despite of the mounted concentrator. This situation was studied through vertical displacing the LC, assuming it was perfectly aligned in the horizontal plane

x y z

¢x ¢z

Active area Concentrator

Figure 5.15: Illustration of the fabrication related imperfections of LCs on SiPMs studied.

A horizontal misalignment of the LC relative to the SiPM microcells is given through ∆x and ∆y (not shown). Mounting the LC on the SiPM surface might lead to a vertical gap

∆z.

CHAPTER 5 RESULTS

Figure 5.16: Results of the simulation studies evaluating the sensitivity of the LC approach to imperfections such as horizontal misalignment in one (a) or two dimensions (b), a vertical gap between LC and SiPM (c), and reflector walls with rough surfaces (d). The values at the abscissa "Ref" correspond to the results obtained without LC. Error bars are omitted, since the statistical inaccuracy of the simulations was below 0.1%.

and the glue had the same RI as the SiPM epoxy window. The simulation results show, that vertical distances larger than 1 µm cause the improvement in light yield to drop below 10%

(see Fig. 5.16c). The CRT was decreasing even faster and a timing resolution inferior to the reference was predicted for an offset of 3 µm.

Surface Scattering of Reflector Walls Depending on the fabrication process used for the LC, the reflecting walls can have a certain roughness that scatters light. The impact of this was studied using the optical simulations with implementations of LCs that had roughness valuesσ_αfrom 5^◦ to 15^◦. Remarkably, the results indicate that for the combination of the investigated detector configuration with a 4µm-LC, the scattering of photons has only little impact on the light yield and CRT. However, care must be taken for transferring this conclusion to other LC geometries with larger heights and in combination with different scintillator geometries.

100

5.2 CONCENTRATOR FOR IMPROVED LIGHT COLLECTION

5.2.5 LSO-based Light Yield and Timing Measurements

The measurements of light yield and timing using the two LSO/PMT setups were conducted with the encapsulated Cr grid chip and the LC1 sample, since these had almost identical values of f_Geo. The histograms of detected scintillation photons of the two samples are compared in Fig. 5.17 for the (4×4×7) mm³ and the (4×4×20) mm³ LSO crystal. The differences in the abscissas of the photopeak between the Cr grid andLC1 are clearly visible and amount to N_Det^LC1/N_Det^Cr = (9±2)% for both scintillator geometries (average ± standard deviation of all acquired data sets). Since the experiments used the same LSO crystal and test PMT, these increases in light output can be attributed to changes in the photon transfer caused by the LC in comparison to the partly absorbing/reflective metal grid mimicking an SiPM. Optical MC simulations of the experimental setup predicted gains in light yield of 8%

for both LSO geometries. This agrees with the experimental results within the accuracy of the measurement.

The increase in light collection can also be seen in the pulse shapes of the test PMT depicted in Fig. 5.18a,b. The configurations equipped with theLC₁ sample exhibit larger amplitudes than the Cr grid setups. The increased light yield and PMT pulse amplitudes also lead to an improvement in the temporal resolution as can be seen in the timing curves δt_Test(U_LED) in Fig. 5.18c,d. The best timing resolution δt_Test for theLC₁ configuration was (279±3) ps and (231±3) ps for the 7 mm and 20 mm high LSO, respectively. Using the Cr grid sample, the minimal δt_Test values were (289 ±4) and (239±3). All values represent the average

± standard deviation of the acquired data sets. These results indicate an improvement in timing resolution of 3% (7 mm LSO) and 4% (20 mm LSO), which correlates well to the observed gain in light yield. Although these differences are only modest, they represent significant changes with respect to measurement accuracy and reproducibility.

1

(4bb4bb7)bmm³bLSO (4bb4bb20)bmm³bLSO LC1bsample

Figure 5.17: Histogram of the amount of detected photonsN_Detcomparing the results of the LC₁ and the Cr grid sample for the (4×4×7) mm³ LSO (a) and the (4×4×20) mm³ LSO (b).

The abscissas corresponding to the centers of the 511 keV photopeaks are highlighted with dashed lines and illustrate the gain in light output through the concentrator. The plateau in the range from 5·10³ to approximately 15·10³ corresponds to the Compton continuum described in Sec. 2.2.3.

CHAPTER 5 RESULTS

Figure 5.18: Pulse shape of the PMT output comparing the LC1 and the Cr grid sample for the (4×4×7) mm³ LSO (a) and the (4×4×20) mm³ LSO (b). The larger amplitudes in the LC₁ curves are a consequence of the increased light output through the concentrator.

This also leads to improved timing resolutionδtTest as shown in the timing curves (c,d). The error bars represent ±one standard deviation of the repeated measurements.

102