Nuclear Instruments and Methods in Physics Research A 732 (2013) 419–422

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Nuclear Instruments and Methods inPhysics Research A

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Efficiency and uniformity measurements of a light concentrator incombination with a SiPM array

Mariana Rihl a,b,n, Stefan Enrico Brunner a,c, Lukas Gruber a,c, Johann Marton a, Ken Suzuki a

a Stefan Meyer Institute for Subatomic Physics, Austrian Academy of Sciences, Boltzmanngasse 3, 1090 Vienna, Austriab University of Vienna, Faculty of Physics, Boltzmanngasse 5, 1090 Vienna, Austriac Vienna University of Technology, Faculty of Physics, Karlsplatz 13, 1040 Vienna, Austria

a r t i c l e i n f o

Available online 7 June 2013

Keywords:Silicon photomultipliersSiPMLight concentratorCherenkov detectorPosition sensitive photon detector

02/$ - see front matter & 2013 Elsevier B.V. Ax.doi.org/10.1016/j.nima.2013.05.187

esponding author at: Stefan Meyer Institute foy of Sciences, Boltzmanngasse 3, 1090 Vienna3 6507885300.ail address: mariana.rihl@assoc.oeaw.ac.at (M.

a b s t r a c t

A position sensitive Cherenkov detector was built, consisting of 64 SiPMs with an active area of3�3 mm2 and a pixel size of 100� 100 μm2. The sensitive area is increased by a light concentratorwhich consists of 64 pyramid-shaped funnels. These funnels have an entrance area of 7�7 mm2 and anexit area of 3�3 mm2, guaranteeing a sufficient position resolution e.g. for the barrel DIRC detector ofthe PANDA experiment at FAIR. The efficiency and uniformity of the light concentrator in combinationwith the SiPM array was tested by scanning the array in two dimensions, using a pulsed light beam.Results of these tests and comparison with simulations are given here.

& 2013 Elsevier B.V. All rights reserved.

1. Introduction

Silicon Photomultipliers (SiPMs) are multi-pixel APDs operatedin Geiger mode. This solid-state photon detection technologyprovides good single photon detection capability and high photondetection efficiency. Further features are their compact size,insensitivity to magnetic fields and cost efficiency, which makethem suitable for many research fields that require photon detec-tion, such as particle physics, nuclear physics or medical imaging.

A position sensitive Cherenkov detector was built, consisting ofan array of 8�8 SiPMs (Hamamatsu S10931-100P) with an activearea of 3�3 mm2 each and a pixel size of 100� 100 μm2. Thesignals are amplified with four 16 channel amplifiers that werebuilt in-house and are based on Photonique amplifiers. In addition,a suitable light concentrator consisting of 64 pyramid-shapedfunnels was developed. With an entrance surface of 7�7 mm2

and an exit surface of 3�3 mm2, this light concentrator, which ismade out of brass and coated with aluminium, increases thedetection area of the module, while providing sufficient positionresolution, e.g. for the barrel DIRC detector [1] at the PANDAexperiment at the FAIR facility in Darmstadt [2]. Increasing thedetection area of the detector by this method gives severaladvantages. One essential advantage is that the signal-to-noiseratio improves by increasing the sensitive area using light focusingand keeping the dark count rate constant [3]. Another benefit is

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r Subatomic Physics, Austrian, Austria.

Rihl).

that the number of read-out channels can be kept low, thus themodule can be built very compactly.

In previous work, simulations for the collection efficiency wereperformed [4] as well as a scan with a laser beam to measure thecollection efficiency of the module. However, the beam spot diameterwas as large as 1 mm and the step size was 250 μm [5]. These twoparameters have been improved significantly in the new tests,providing a more detailed picture of the characteristics of the SiPMsand the light concentrator. Also, a scanwith a finite incident angle wasperformed. The new data allows to further optimise the light guide.

2. Test setup

To test the position sensitive photon detector, the completesetup was put inside a dark box. The test setup consists of thedetector module, a light source and two stepping motors whichmove the beam spot across the area of the scanned SiPMs.

The Hamamatsu 10931 3�3 mm2 SiPMs with a pixel size of100� 100 μm2 were chosen because they have the highest photondetection efficiency and an adequate dynamic range. The 10931sensor series has the photon detection maximum at λ¼ 440 nm.For the scan, a light source with a wavelength near that maximumlooked reasonable and an LED with a wavelength range of465 nmoλo475 nm was used.

The light source was set to emit pulses instead of a continuouswave in order not to saturate the sensor. The pulse rate of the LEDwas about 900 kHz with a pulse width of about 6.5 ns.

To reduce the beam spot diameter from 1.370.1 mm at the LEDexit to 10874 μm at the SiPM surface, an optical setup, includingthree biconvex lenses and a 10 μm pinhole were included into the test

Fig. 3. Test setup inside dark box. On the left side of the box the optical and motorsetup is mounted. On the right side of the box sits the detector prototype.

M. Rihl et al. / Nuclear Instruments and Methods in Physics Research A 732 (2013) 419–422420

setup. This optical apparatus, which is presented in Fig. 1 was movedby the two steppingmotors, which changed the beam spot position onthe detector and the array by steps of 100 μm. This guaranteed thateach pixel of the SiPM was triggered by the light beam.

During the tests, the coordinate convention was defined asfollows: the x- and z-axis build a plane parallel to the detectorsurface and the y-axis is parallel to the beam direction. Fig. 2shows a schematic of the optical setup and its mounting on thestepping motors. Furthermore, it gives an overview of the chosencoordinate convention. Due to the fact that the motors are highprecision tools and that the weight had to be completely poised inorder to keep the precision of the motors at its high level, somemeasures had to be taken. The beam spot could be moved in an x-and z-direction. In order to reduce the wiggling of the motor tips,cage plates were mounted to serve as stabilisers. The opticalapparatus is fixed via fixation cage plates on the x-axis motortip, the beam direction is parallel to the y-axis of this setup.

Fig. 3 shows the opto-motoric setup together with the detectormodule inside the dark box.

3. Scanned channels and scanning mode

Due to timing restraints not all 64 sensors could be scanned.Thus, three adjacent SiPMs were chosen randomly for the test.These sensors are referred to as F2, F3 and F4. Their position on thedetector module surface can be seen in Fig. 4.

The sensors were scanned in three different ways. In the firsttwo setups, all three sensors were scanned at once, with andwithout light concentrator. In order to test the behaviour of thecollection efficiency in dependence of the incident beam angle,each sensor was scanned separately with light concentrator and anincident beam angle of about 151.

4. Data acquisition

For the data acquisition, a LeCroy 735Zi WavePro digitaloscilloscope was used. Three channels were used to acquire thesignal, while the fourth one was used as trigger input.

Fig. 1. Schematic of optomecanical items and laser beam. A: LED beam exit. B: biconvexlens with f ¼30mm. C: 10 μm pinhole, serves as point-like light source. D: collimatingbiconvex lens with f ¼100 mm. E: focusing biconvex lens with f ¼200 mm.

Fig. 2. Schematic of motor and optical setup, the coordinates x, y and z ofmovements are defined.

Fig. 4. Detector module with light concentrator. The scanned sensors are high-lighted by the rectangular frame.

The scope of the experiment was to extract the pulse heightfrom the signal of the respective SiPM. The amplitude of the signalwas measured by acquiring the minimum of each waveformduring the acquisition window of 200 ns. To achieve good statis-tics, 1000 samples were taken per position of the photon sourcefor each of the three channels respectively. The oscilloscopecalculated the mean and standard deviation of 1000 samples ofthe amplitude. The acquired data for each channel was back-ground corrected and then added up. The data is referred to as⟨a⟩LC and ⟨a⟩noLC for the mean amplitude with and without lightconcentrator respectively.

These two data values (per channel) were saved into a text file,together with information about the coordinates of the beamposition.

Taking into account the number of data points that need to beacquired during the scans, it is obvious that an automation routineis beneficial. Such a routine was created with LabVIEW andregulates the beam spot movement by the motors as well as thedata acquisition by the oscilloscope and the saving of the data.

Fig. 5 shows a snapshot of the data acquisition with theoscilloscope.

M. Rihl et al. / Nuclear Instruments and Methods in Physics Research A 732 (2013) 419–422 421

5. Results

5.1. Qualitative analysis

The data acquired during the scans was transformed into twodimensional histograms, using routines based on C++ and ROOT.Fig. 6 shows the two dimensional histograms from a top view. It ispossible to clearly distinguish between the original sensitive areaand the enhanced sensitive area when the light concentrator isapplied. Also, the reduced collection efficiency due to an incidentbeam angle is evident in Fig. 6(c).

As can be seen in Fig. 6, it can be distinguished between activeareas and the areas where no photons get detected. One reason forthe inactive area is the finite rim which separates the funnels fromeach other. At these areas, photons get reflected. Another reason isthat the sensors were not soldered in perfect alignment, resultingin an offset between the exit area of the light concentrator and theactive area of the SiPMs. Fig. 7 shows a comparison between thetwo dimensional histograms and microscope photos of the respec-tive channels, illustrating the offset of the sensors in relation to thelight concentrator.

Fig. 5. Data acquisition with trigger and SiPM signals. Due to the beam diameter ofabout 108 μm (FWHM), only one SiPM sends a signal at a time, represented here bysensor F3.

Fig. 6. Two-dimensional histogram of the scan data for the three sensors (a) with-out LC, (b) with LC and (c) with LC and an incident beam angle of about 151. Thecolour scheme gives the mean intensity of signal height of the SiPMs in mV.

5.2. Collection efficiency

The collection efficiency of the light concentrator can becalculated by comparing the data from the scans with lightconcentrator to the scans without the light concentrator. Thecollection efficiency ϵcol of one funnel of the light concentrator isdefined by

ϵcol ¼nd

α � nd0; ð1Þ

with nd being the number of photons detected with light con-centrator, nd0 the number of detected photons without lightconcentrator and α¼ ð73Þ2 � 0:93 an area factor [5]. The 0.93 in thearea factor α is the geometric fill factor and puts into account thefact that the edges are rounded.

The area factor α represents the enlargement of the detectionarea of a SiPM and is in this specific case Aentrance=Aexit , whereAentrance and Aexit represent the entrance and exit area respectively.The collection efficiency ϵcol was calculated, using the followingequation for a certain funnel:

ϵcol ¼∑⟨a⟩LC

∑⟨a⟩noLC � α ð2Þ

Table 1 shows the results for the collection efficiency for eachsensor with incident beam angles of 01 and 151 respectively. Themean collection efficiency is also given.

Fig. 7. Histogram of mean intensity and photo of the sensors with the lightconcentrator on top. The arrows indicate areas where no photons get detected as aresult of imperfections of the alignment of the sensor array and the lightconcentrator.

Table 1Collection efficiencies for the evaluated three channels at two different photonincident angles. Standard deviations of the collection efficiencies are also shown,indicating the fluctuations of the collection efficiency funnel by funnel.

Channel Angle (1) Collection efficiency, ϵcol ð%Þ

F2 0 88.6F3 0 83.4F4 0 86.0

Mean 0 86.0 s¼ 2:6ÞF2 15 56.8F3 15 55.4F4 15 58.4

Mean 15 56.7 ðs¼ 1:5Þ

100%

87%

55%

27%

0%

funnel length

col, max

col, LC

Fig. 8. Simulation of the collection efficiency in dependence of the incident beamangle and different funnel lengths [4].

M. Rihl et al. / Nuclear Instruments and Methods in Physics Research A 732 (2013) 419–422422

5.3. Comparison to simulations

Comparing the measured mean values with simulations of thecollection efficiency of the light concentrator shows that theresults are in good agreement with the simulations. The simulatedcollection efficiency for a light concentrator with a funnel length of4.5 mm and an incident beam angle perpendicular to the detectorsurface is about 86%. The mean of the measured collectionefficiency for the light concentrator with an incident beam angleof 01 is also about 86%. Applying an incident beam angle of 151results in a mean collection efficiency of about 57%, compared tothe simulation value of 61%. Fig. 8 shows the results of thesimulation for the light concentrator, which was done previouslyby the authors [4]. The figure displays the collection efficiency fordifferent funnel lengths. The simulated collection efficiencies aregiven in dependence of the incident beam angle.

6. Conclusion and outlook

A prototype of a position sensitive SiPM array with a lightconcentrator was tested in order to evaluate the collectionefficiency by scanning with a narrowly-focused LED light. Thescans were performed with a light source of a beam spot diameterof 10874 μm and a stepping size of 100 μm. These parametershave been improved significantly to earlier tests, giving a moredetailed picture of the collection efficiency and uniformity. Inaddition, the performance of the light concentrator collectionefficiency was tested for two different incident light beam angles,01 and 151. The simulation agrees well with the data and can beused to further optimise the geometry of the light concentrator.

Ideas to optimise the detector include better alignment of thesensors to the concentrator or a slightly narrower exit area inorder to remove the gaps in-between and to develop a differentkind of light concentrator with plexiglas cones instead of ametal grid.

References

[1] B. Seitz, et al., Nuclear Instruments and Methods in Physics Research Section A628 (2011) 304.

[2] W.F. Henning, Journal of Physics G: Nuclear and Particle Physics 34 (2007) 551.[3] S. Korpar, et al., Nuclear Instruments and Methods in Physics Research Section

A 610 (2009) 427.[4] G. Ahmed, et al., Nuclear Instruments and Methods in Physics Research Section

A 639 (2011) 107.[5] L. Gruber, et al., Journal of Instrumentation 6 (2011) C11024.