Detector Performance Analysis for the High Momentum Particle Identification Detector in ALICE

Victoria F. Owen

George Mason University, Fairfax, Virginia, USA

European Organization for Nuclear ResearchAdvisors: Antonello Di Mauro, Levente Molnar, Miranda Van Stenis

12 August 2011

CONTENTS

Abstract ....................................................................................................

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1 Introduction ......................................................................................

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1.1 The ALICE experiment ………………………………………..

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1.2 Particle Identification ……………………………... 3

1.3 HMPID …………………………………………

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1.3.1 Cherenkov Effect …………………………………………

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1.3.2 HMPID RICH …………………………………………

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2 Detector Background Analysis …………………………………………………...

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2.1 Particle Identification Strategies ………………………………

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2.2 Monte Carlo and Experimental Outcomes …………………… 5

2.3 Background Analysis ……………………………………..….

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3 Detector Background Analysis: Results …………………………………………

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3.1 Monte Carlo Results …………………………………………

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3.2 p-p Collision Results ………………………………………….

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4 Photon Pulse Height Study …………………………………………………….. 9

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4.1 Chamber by Chamber Analysis …………………………………

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5 Photon Pulse Height Study: Results ……………………………………………

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6 Preparation of Photocathode Test Samples ………………………………… 11

6.1 Preparing CsI Evaporation Boats …………………………………

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6.2 Five Sample Substrates ……………………………………………

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6.2.1 Materials Used ……………………………………………

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6.2.2 Mounting Samples / Preparing Chamber ……………. 13

6.2.3 Deposition of CsI ……………………………………………

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6.2.4 UV Scanning of Photocathode Samples ………….... 14

7 Preparation of Photocathode Test Samples: Results ……………………….

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8 Conclusion

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9 References

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ABSTRACT

The High Momentum Particle Identification Detector (HMPID) in the ALICE experiment of the LHC consists of 7 modules, each equipped with six large-area Cesium Iodide (CsI) photocathodes that are used as photon converters in the proximity focusing RICH layout. The objectives of this work were to study elements of detector performance such as: proton background events seen in both Monte Carlo and proton-proton collision data, photon pulse heights per chamber, and preparation of CsI coated samples for future analysis of photocathode substrates.

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1 INTRODUCTION

1.1 The ALICE experiment

A Large Ion Collider Experiment (ALICE) is an experiment at the Large Hadron Collider (LHC) for the study of heavy-ion collisions. The primary goal of this experiment is to study in detail the behavior of matter at high densities and temperatures. The detailed characterization of ion-ion collisions requires particles to be measured and identified over a large range of momentum.

1.2 Particle Identification

The particle identification system of ALICE is a series of sub-detectors: the Inner Tracking System (ITS), the Time Projection Chamber (TPC), the Time of Flight (TOF), the Transition Radiation Detector (TRD) and the High Momentum Particle Identification Detector (HMPID). A quasi background free particle identification in the HMPID can be studied using samples of particles from proton-proton collisions.

1.3 HMPID

The High Momentum Particle Identification Detector[1] (HMPID) of ALICE is devoted to the detection of the particles in the momentum range above the one covered by the other detectors, specifically charged hadrons of pt > 1 GeV/c (i.e pions, kaons, and protons). The HMPID employs a ring-imaging technique using Cherenkov light.

1.3.1 Cherenkov Effect

In a medium with a refractive index n, a charged particle moving with a velocity c can exceed the speed of light in that medium, c/n. When this occurs the particle emits characteristic electromagnetic radiation called Cherenkov radiation. The emission angle or Cherenkov angle c, of the Cherenkov radiation with respect to the particle track is derived from the way the charged particle polarizes the medium through which it travels, resulting in the equation below. [2]

Equation 1.1

From the curvature of the track in a known magnetic field the charge is determined. For particle identification, the energy loss of the particles (ITS, TPC), the time-of-flight (the time between the interaction and the observation in an fast detector such as the TOF) or the Cherenkov light created when the particle traverses a RICH detector is used. Equation 1.2 shows the relativistic relation between mass, velocity, and momentum.

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Equation 1.2

The momentum of the particles is obtained from the central tracking system (ITS; TPC) and can be correlated with the measured Cherenkov angle for particle identification.

Equation 1.3

The resolution in momentum is primarily determined by the TPC, and the resolution of the particle identification is determined by the HMPID detector parameters.

1.3.2 HMPID (RICH)

The momentum range in ALICE, 1 to 5 GeV/c, and the particles of interest, , , and p, leaves C6F14 as the only liquid for the radiator with adequate refractive index. The HMPID, due to particle density, space requirements, and photon absorption, employs proximity focusing technique. Proximity focusing does not use a mirror to reflect the photons onto the detection plane, but allows a small particle path relative to the distance from the radiator and the photon detection plane. Considering the particle densities involved, a solid CsI photocathode was the only reasonable choice.[1]

2 DETECTOR BACKGROUND ANALYSIS

2.1 Particle Identification Strategies

One method of particle identification is the variation of the Cherenkov angle in function of track momentum and mass. The relation between these two values relates to the different particles, pions, kaons, and protons as can be derived from Equation 1, 2, and 3.

2.2 Monte Carlo and Experimental Outcomes

The detector performance for both Monte Carlo simulations and proton-proton collision data was analyzed using the Cherenkov angle and track momentum relations. Figure 2.2.a shows the Monte Carlo results along with the theoretical values for pions, kaons, and protons.

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Figure 2.2.a Cherenkov angle (mrad) versus track momentum (GeV) for Monte Carlo Data with theoretical overlay for pions (red), Kaons (orange), protons (black).

The results for this data have a concentration of statistics between the pion and kaon bands at low momentum that does not correspond with any of the theoretical predictions. This background was also apparent in proton-proton collision data.

2.3 Background Analysis

Analysis of this background was concentrated to that of protons. Protons were initially chosen by selecting a PID (Particle Identification) probability of 100%, but later reduced to 80% to increase statistics, where the PID probability obtained from the HMPID reconstruction. Other quality cuts were made to the data to filter unfavorable events. These cuts included a Kink index (detection of decay vertex) equal to zero, number of TPC clusters (a measurement of the goodness of a track) greater than 80, and the MIP charge greater than 150 ADC.

With only protons selected, the background events for both Monte Carlo data and p-p collision data were clearly isolated from the theoretical proton line. Using a poly-line cut, these background events were selected for further analysis and compared with theoretical expectations.

3 DETECTOR BACKGROUND ANALYSIS: RESULTS

3.1 Monte Carlo Results

After protons were selected from the HMPID reconstruction of Monte Carlo data, proton background events were isolated using a poly-line cut line in ROOT. The track position in the chamber associated with these events were studied, along with the particle’s incidence

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angle, and number of photon hits per pad associated with a given track. In analyzing these variables, it was found that for HMPID PID proton selection from background, associated tracks were incident in only the upper-most and lower-most high voltage sectors of the HMPID chambers. No proton background events occurred in the central region of the chambers. The incident angle of the associated particles was small, ~0.19 rad or almost perpendicular to the plane of incidence. Also, the average number of photons was 8 compared to the expected Monte Carlo value of 5. Track multiplicity and vertex in three dimensions (x,y, and z) were analyzed with no abnormalities found. These results a shown in Figure 3.1.a (HMPID reconstruction) and Figure 3.1.b (Monte Carlo).

Figure 3.1.a HMPID reconstruction of Monte Carlo data for proton background selection.

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Figure 3.1.b Analysis of Monte Carlo background protons

Further analysis regarding the abnormal X and Y track positions was done and is discussed in section 3.2.

3.2 p-p Collision Results

Protons with a PID probability of greater than 80% (to increase statistics) were selected from LHC10e proton-proton (p-p) collision data period. As with analyzing reconstructed events from Monte Carlo data (section 3.1), background entries for events identified as protons was selected using a poly-line cut in ROOT. The X and Y track positions for tracks associated with these background events were plotted and demonstrated the same effect as seen with the Monte Carlo reconstructions. All background events were concentrated to the upper-most and lower-most high voltage sectors of the chambers. The incident angles of the tracks associated with these particles was found to be about 0.14 rad, again, nearly perpendicular to the plane of incidence. Figure 3.2.a.

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Figure 3.2.a Background from proton selection from LHC10e p-p data

It was suggested that incoming particles might be interfering with the HMPID space frame, so the particle’s charge and corresponding track positions were analyzed. This showed that background events from proton selection occurred from tracks incident in the upper and lower parts of each chamber and this observed background depends on the particle’s charge (Figure 3.2.b and 3.2.c).

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Figure 3.2.b Particle charge = -1 Figure 3.2.c Particle charge = +1

Because of a magnetic polarity of the ALICE solenoid and dipole was (-,-) for the LHC10e p-p run, particles with a negative charge curled clockwise, therefore interfering with the top supports, while positively charged particles curled counterclockwise interfering with the lower supports.

For the reconstructed events from Monte Carlo data, where the magnetic polarity was (+,+), the results were reversed such that negatively charged particles curled counterclockwise and positively charged particles curled clockwise. XY-track positions were confined to the upper-most part of the chamber for positive particles and to the lower-most part of the chamber for negative particles.

However, because proton acceptance in HMPID should be above 1 GeV, further analysis of the reconstruction program is being done to discover why particles with momentum less than 1 GeV are being identified with confidence as protons.

4 PHOTON PULSE HEIGHT STUDY

Each chamber of the HMPID is made up of six photocathodes separated by a 2 mm anode gap. This gives a display like the one in the figure[1] below.

Figure 4.a Schematic of pad cathode segmentation.

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Each of these cathodes can be broken, once more, into two separate high voltage sectors. The x and y coordinates of each high voltage sector is known precisely, allowing for events occurring in these specific regions to be analyzed independently.

4.1 Chamber-by-Chamber Analysis

The photon pulse height can be studied by examining photon cluster charge this is obtained from Single-Electron Pulse-Height Detection (SE-PHD). By studying each high voltage sector of each chamber individually, we are able to see specifically where events are occurring and being read out. Evaluating this distribution can allow for further simulation of detector behavior as well as understand the relationship between chamber gain and a primary particle avalanche and secondary electrons.

The shape of the pulse height distribution closely obeys the Furry distribution [1]

Equation 4.1where A is the current pulse height and A0 is the average pulse height.

In this study, each high voltage sector was isolated for each of the seven individual chambers. Using data from the LHC10e proton-proton collision run, several cuts were made to ensure the quality of the tracks being analyzed. These cuts included the number of TPC clusters greater than 80, the MIP charge greater than 150 ADC, and EsdTrackCut2010, a pre-parameterized analysis cut current for 2010 data.

5 PHOTON PULSE HEIGHT STUDY: RESULTS

Each high voltage sector for each of the seven chambers was analyzed separately, graphed in ROOT displaying statistical errors. For each chamber, a fit was made based on the Fury distribution from Equation 4.1, but rather than adjusting the equation by 1/A0, it was adjusted to accommodate each chamber’s specific distributions.

The lowest cathodes for both chamber 3 and 4 had very few events and therefore the fit was unsuccessful. The fit was very effective, however, for chambers with an approximately uniform distribution of events over the entirety of the chamber. For example, Chamber 1, as shown in the Figure below.

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Figure 5.a Pulse-height distributions (including errors) for each high-voltage sector of Chamber 1 for LHC10e p-p data on a log(y) scale displaying exponential fit.

Further analysis of these distributions will give more insight into detector behavior. These distributions could be studied further using a normalized Polya distribution for distributions when the Furry exponential fails, an indication that some avalanches originate from more than a single electron.

6 PREPARATION OF PHOTOCATHODE TEST SAMPLES

The HMPID uses a Copper-Nickel-Gold substrate on which to deposit CsI for use as photocathodes. Various other materials have been tested in the past, but the UV-scanning system has recently been improved, and therefore merits the testing of different

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substrates. The process for preparing these various substrates for future analysis is described in detail in this section.

6.1 Preparing CsI Evaporation Boats Molybdenum boats filled with CsI are used in the evaporation process. The boats are prepared ahead of time (the CsI is premelted) to assure uniformity during the final evaporation process. This is done in a vacuum chamber. The boats are filled with fine CsI powder. Each boat is filled and heated separately. The boat is mounted to a high voltage source inside of the chamber and the chamber is brought to very low pressure. Once the chamber is under vacuum, a voltage is sent through the source to heat the boat. The boat is heated slowly as not to lose any of the CsI powder. The boat is heated until the CsI has liquefied. At this point the voltage is cut off and the chamber is brought back to normal pressure. Once the boat has cooled completely, solidified CsI is removed such that the boat now contains CsI with an effective mass of .75grams. The samples are stored under vacuum until ready for use.

6.2 Five Sample Substrates

6.2.1 Materials Used

The QE of five different substrates is being studied: Aluminum (Al) , Copper-yellow (Cu-yellow), Copper-red (Cu-red), Gold coated Nickel-Copper films (Au-Ni-Cu), and Nickel coated Copper (Ni-Cu). These samples (approximately 4cm x 8cm) are mounted to a frame with electronics readouts to monitor current.

The frame is mounted upside down in the vacuum chamber, on a chariot, so that the four boats filled with premelted CsI (mounted to high voltage sources, also in the chamber) may deposit onto the face of the substrates. A side-view schematic of the CsI evaporation system (both deposition and scanning chambers) with mounted pads and four CsI sources is shown below in Figure[1] 6.2.1.a.

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Figure 6.2.1.a Side-view schematic of CsI evaporation system; CsI deposition chamber on the left and VUV scanner chamber on the right.

6.2.2 Mounting Samples / Preparing Chamber

The frame is mounted and its position and the coordinates of the samples (4 coordinates per sample pad) checked for correctness in both the evaporation chamber and scanning chamber. Once all points have been verified, the frame is brought back to and centered in the deposition chamber. The chamber is put under vacuum and reaches a pressure of around 6e-7mb in about 36 hours. The chamber is now prepared for deposition of the CsI onto the substrates.

6.2.3 Deposition of CsI

The deposition of the CsI takes place when the chamber containing the samples is under a pressure of about 6e-7 mb. Once this has been achieved, a voltage, starting at 10 V, is applied to the sources to which the CsI boats are connected. The voltage is incremented slowly and the pressure in the chamber, the current (in Amps), and the deposition thickness (from both monitors) are closely monitored during this time. Once the current to the CsI boats nears 100 A (~140 V) a deposition thickness in nm begins to register. The voltage is continuously increased to obtain a rate of about 1 nm per second. The readout of deposition thickness steadies, signifying the emptying of the CsI boats. Starting with 0.75 g of CsI, the deposition thickness, when all boats have been emptied, is about 350nm; in this case, occurring at about 200V.

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6.2.4 UV Scanning of Photocathode Samples

After the deposition of CsI, the frame to which the samples are mounted is moved electronically to the scanning side of the chamber, also under vacuum. The temperature in the chamber is about 60 C. It has been shown that samples treated at this temperature have better stability following exposure to air than those samples not heated[3]. Here the coordinates as measured and recorded before the frame was mounted in the chamber, are entered and registered into LabView. This program moves both the X and Y lever arms automatically to scan the deposition surface.

This system scans the samples using UV light (from a deuterium lamp source) and measures the resulting photocurrent from any given location on the photocathode. The measuring set up is shown in the figure[1] below.

Figure 6.2.4.a Cross-section schematic of QE evaluation based on measuring the photocurrent produced by the irradiation of CsI substrate samples by UV light from a

deuterium lamp source. For each sample two measurements are taken: one at full spectrum and one with a quartz window. Full SpectrumWithout a quartz window, there is a minimum loss of UV. Quartz WindowThere is a removable quartz filter, that when in use, cuts away much of the low UV wavelengths (lambda <170 nm).[3]

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When compared to full spectrum measurements, changes in the QE curve of the photocathodes can be detected. These measurements are taken when the samples are heated to around 60 C and again after the heater has been turned off and the samples have cooled to room temperature, about 25 C.

7 PREPARATION OF PHOTOCATHODE TEST SAMPLES: RESULTS

All samples have been prepared and scanned at both 60 C and 26 C, with and without a quartz window. Each sample pad was scanned multiple times at specific coordinates centered on the pad. The normalized current values, Inorm, is calculated based on the measured current on CsI photocathode ICsI , the current on the reference PM IPM, as well as the associated noise ICsI_noise and IPM_noise, measured in nano-Amps.[3]

Equation 7.1

The expected values for the normalized current range from about 3 to 3.5. The figures below show a comparison in normalized current values for each scan. For each temperature (60 C or 26 C) and optic set-up (with or without quartz window), the series of coordinates was measured six times.

Figure 7.a Normalized current ratio values for all measurements for set-up with and without the quartz window, both at T= 60 C.

Figure 7.b Normalized current ratio values for all measurements for set-up with and without quartz window, both at T= 26 C

Figure 7.c Normalized current ratio for coordinates for each set-up option. Only the first series of sample coordinates are displayed, four coordinates for each substrate, Cu-

red, Cu-Ni, Cu-Ni-Au, Al, Cu-yellow respectively.

These samples have been prepared and are ready for further analysis.

8 CONCLUSION

The HMPID is an essential part of the ALICE heavy-ion experiment for detecting high momentum particles outside of the range of the other ALICE detectors. Using Monte Carlo and proton-proton collision data to analyze background events and pulse height distributions gives insight into detector behavior and can lead to further simulations and

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improved reconstructions. Studying multiple substrates as possible candidates for future photocathodes, using new techniques, will allow future high momentum particle detectors to employ the most efficient and cost effective materials.9 REFERENCES

1. CERN / LHCC 9819, ALICE TDR 1, 14 August 1998.

2. D. F. Anderson et al., NIM-A 323 (1992) 626.

3. H. Hoedlmoser, Development of Large Area CsI Phototcathodes for the ALICE/HMPID RICH Detector, 18 March 2005

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