Measurement of the FD camera light collection efficiency and uniformity

Transcription

1 GAP Roma, 1 March 2000 Measurement of the FD camera light collection efficiency and uniformity P. Facal San Luis Sezione INFN di Roma II, Roma, Italy and Universidad de Santiago de Compostela, Santiago de Compostela, Spain P. Privitera Università di Roma II, Tor Vergata, and Sezione INFN di Roma II, Roma, Italy Abstract. Detailed measurements of the light collection efficiency and uniformity of the FD camera with a set up which closely simulate the optics of the FD telescope are presented. We observed that light collection at the pixel edges is very much improved when mercedes are used. The light collection efficiency averaged over the FD focal surface is 93%. 1 Introduction The Fluorescence Detector (FD) camera is composed of a matrix of 20x22 hexagonal pixels, positioned on the focal surface of a spherical mirror. Details of the geometry and the mechanical characteristics of the camera are given in [1]. Hexagonal phototubes are used to instrument the focal surface as fluorescence light detectors. Even if their hexagonal shape represents the best approximation to the pixels geometry, a significant amount of insensitive area is nevertheless present. In fact, some space between PMTs is needed for a safe mechanical packaging on the focal surface; moreover, the effective cathode area is smaller than the area delimited by the PMT glass envelope. A crucial property of the FD camera is its efficiency of collection and uniformity. In fact, large corrections on the energy estimated from the PMT signals would be needed in case of significant losses of light in the pixel borders. 1

2 The systematic uncertainties on the measurement of the cosmic rays energy and direction due to this effect could spoil the performance of the FD. For this reason, the hexagonal phototubes of the AUGER FD camera are complemented by a simplified version of Winston cones [2], which maximize light collection and guarantee a sharp transition between adjacent pixels, The mechanical characteristics of these light collectors (mercedes) and their mounting were described in [1]. In this note, we present a measurement of the camera light collection efficiency and uniformity, based on tests of a small prototype of the camera instrumented with 7 phototubes, (the sunflower). The experimental set up used for the measurement is described in Section 2. A light source system, reproducing the fluorescence light and the mirror optics, was moved over the surface of the sunflower. Results of light scans over the surface of the sunflower are presented in Section 3. Conclusions on the camera uniformity are given in Section 4. 2 The experimental set up A sketch of the experimental set up used for the measurement of the camera uniformity is shown in Fig. 1. A small version of the full size camera body holds seven Philips hexagonal phototubes XP3062, arranged in a sunflower configuration (Fig. 2). High voltage, low voltage and signals from the PMTs are handled by the distribution board [3]. Signals are sampled by an 8-bit flash ADC (CAEN V534) at 20 MHz, and readout by a VME controller Motorola MVME2700. A Xenon flash lamp (Hamamatsu L2360) provides light pulses of approximately 1 µs width with high stability. The time distribution of the light pulse is shown in Fig. 3. The distribution of the light pulse maximum is presented in Fig. 4, showing a stability of 1.5%. The FD detector optics [4] produce a light spot of 1.5 cm diameter on the camera, with light rays angles of incidence in the interval between approximately 10 to 30 degrees. The upper limit is determined by the aperture of the diaphragm while the lower limit results from the shadow of the camera. In order to simulate the optics, the Xenon lamp is attached to a light diffusing cylinder, sketched in Fig. 5. Teflon disks placed inside the cylinder diffuse the light from the Xenon flash lamp. A black disk of 10 cm diameter provides the correct angles of incidence range for light rays passing through the exit hole at the end of the cylinder. The 1.5 cm diameter exit hole simulates the light spot. The nm wavelength range corresponding to the fluorescence light is selected by an UG-1 filter placed at the exit hole. The light diffusing cylinder is fixed on a frame which allows X-Y movements over the sunflower surface. The reflecting surface of the light collectors used for the tests presented in this note was obtained glueing aluminized mylar on the mercedes. 2

3 3 Measurement of the camera uniformity The uniformity of the camera response was measured with the following procedure. The light diffusing cylinder was moved over the sunflower surface in steps of a few mm. The light from the exit hole would hit one or more PMTs depending on its position over the surface. For each step, the signal of each photomultiplier was measured, and normalized to the value obtained when the light spot was positioned on the center of the PMT. Then, the sum ɛ of the normalized signals was calculated. For a full light collection efficiency, ɛ is expected to be close to unity. A first set of measurements was taken without the light collectors, placing the exit hole of the light diffusing cylinder very close to the PMTs photocathodes. Afterwards, the mercedes were mounted on the small camera body and the light scans were repeated placing the exit hole of the light diffusing cylinder very close to the edges of the light collectors. The results of a scan along a line passing over the mercedes arms are shown in Fig. 6 (see Fig. 2 for the coordinate system). The loss of light in the borders between phototubes is clearly observed. Another scan which passes over the mercedes vertices, where the light loss is maximal, is presented in Fig. 7. On the basis of the results obtained, we can draw the following conclusions: significant losses of light, up to 70%, are present in the pixel borders when the light collectors are not used. Also, the response within the same pixel shows a typical structure, which can be attributed to the shape of the collecting electric field at the edges of the hexagonal photocathodes. the light collectors are efficiently recuperating the light loss, which is reduced to less than 15% even in the worst case with the spot over a mercedes vertex. The uniformity within the same pixel is also improved, since light rays which were hitting the photocathode borders are now redirected by reflection over the mercedes in the central region of the photocathode. A complete map of the sunflower equipped with light collectors was performed. In Fig. 8 we present the measured ɛ as a function of the light spot position over the sunflower. In the region covered by PMTs, a value always larger than 85% is measured, with bumps corresponding to the PMTs positions on the surface. Several scans are shown in Fig To guide the eye, lines connecting the measured efficiency as a function of the step movement are presented. The contribution from individual PMTs is also shown. In Fig. 13 we present the distribution of the efficiency ɛ for the subset of measurements which were performed on the area covered by the central pixel of the sunflower. Notice that the measurements were performed with constant density over the surface of the sunflower. Thus, the average efficiency of 93% obtained from this distribution corresponds to the light collection efficiency averaged over the FD focal surface. 4 Conclusions We have performed detailed measurements of the response to light of the FD 3

4 camera with a set up which closely simulate the optics of the FD telescope. We observed that light collection at the edges of the pixel is very much improved when mercedes are used. Also, the uniformity of response to light improved since mercedes cover the photocathode edges, where photoelectron collection is less uniform. The light collection efficiency changes from 100%, when the spot is over the pixel center, to a minimum value of 85% in the worst case with the spot over a mercedes vertex. The light collection efficiency averaged over the FD focal surface is found to be 93%. In terms of shower energy reconstruction, an average correction of 7% to the signal measured by each pixel would already reduce the systematic uncertainty on the energy measurement to the 5% level. It is conceivable that more elaborated corrections depending on the shower image position over the focal surface will further reduce the systematic uncertainty. We acknowledge the suggestion given by Paul Sommers of using a light diffusing cylinder to simulate the FD optics. The technical contributions of Francesco Bracci, Gianni Vitali and Enrico Tusi were essential for the realization of this work. References [1] C. Aramo et al., The camera of the AUGER Fluorescence Detector, Auger Tech. Note GAP [2] R. Winston and W.T.Welford, High collection nonimaging optics, Academic Press, 1989 [3] R. Cardarelli et al., The baseline design of the backplane distribution system of the FD camera, Auger Tech. Note GAP [4] G. Matthiae and P. Privitera, The Schmidt telescope with corrector plate, Auger Tech. Note GAP

5 Figure 1: Sketch of the experimental apparatus used for the measurement of the camera uniformity. 5

6 Figure 2: The sunflower geometry. The grey circle on the upper right corner represents the light spot. 6

7 Figure 3: Time distribution of the Xenon flash lamp light pulse. The histogram shows the average over 1000 pulses. The dotted line indicates the pedestal level. Figure 4: Distribution of the light pulse maximum. 7

8 Figure 5: The light diffusing cylinder used to simulate the FD optics. 8

9 ε Figure 6: Measurement of the light collection efficiency along a line passing over the mercedes arms. The full dots represents the measurements performed with mercedes, while the open dots the measurements without mercedes. 9

10 ε Figure 7: Measurement of the light collection efficiency along a line passing over the mercedes verteces. The full dots represents the measurements performed with mercedes, while the open dots the measurements without mercedes. 10

11 Figure 8: Light collection efficiency as a function of the x and y position of the light spot over thesunflower. Seven bumps corresponding to the PMTs are visible. 11

12 ε ε Figure 9: Measurements of the light collection efficiency along lines parallel to the y axis (full lines). The dashed lines represent the contribution from individual pixels. 12

13 ε ε Figure 10: Measurements of the light collection efficiency along lines parallel to the y axis (full lines). The dashed lines represent the contribution from individual pixels. 13

14 ε 2 3 ε Figure 11: Measurements of the light collection efficiency along lines parallel to the x axis (full lines). The dashed lines represent the contribution from individual pixels. 14

15 ε ε Figure 12: Measurements of the light collection efficiency along lines parallel to the x axis (full lines). The dashed lines represent the contribution from individual pixels. 15

16 Figure 13: Distribution of the light collection efficiency ɛ in the central pixel. 16