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Measurement of the optical properties of the EB Module 0 tiles


ATLAS Internal Note CAL-NO-XXX March, 1998

Measurement of the optical properties of the EB Module 0 tiles
Martine Bosman, S lvia Bravo, Matteo Cavalli-Sforza, Eugeni Grauges
Institut de F sica d'Altes Energies (IFAE) Universitat Autonoma de Barcelona Edi ci Cn E-08193 Bellaterra (Barcelona), Spain.

Abstract
Optical properties of the tiles equipping the Tilecal Extended Barrel Module 0 (EB0) assembled at IFAE (Barcelona) have been measured. The light yield as a function of the tile size, the uniformity of the tile response and the stability of the production process are presented.

1 Introduction
To equip the EB0 modules assembled in 1997, a new batch of tiles was produced by IHEP (Protvino) and wrapped with masked tyvek envelopes at Michigan State U. About a third of the sample, approximately 1600 tiles, was used for the EB0 assembled in Barcelona. The optical properties of the tiles were measured with a movable 90 Sr source before their insertion in the module, in April and May 1997. The goals of the measurements were several: to evaluate the spread of the response of each tile type to check the uniformity of the production process; to study the tile response as a function of the tile size and check the e ectiveness of the tile masking, by measuring the light output along the central line of each tile type (see gure 1); to investigate the tile response in regions that are relevant for the source calibration data, and the data taken with muons impinging at 90o at the tile plane at the center of the tiles.

Figure 1: Schematic view of a tile

2 Experimental Setup
The measurements were performed with a setup schematized in gure 2. Mechanically, the setup consisted of a plotter, with a precision of positioning of 25 m. The pen is substituted by a collimator containing a 90Sr radioactive source with an activity of 2 mCi. The collimator is cylindrical with 6 mm thick steel walls that shield transversally the radioactive source. It is covered by a 5 mm thick steel cap, with a 1 mm diameter hole 1

Figure 2: Experimental Setup Tile type length1 length2 Width Area Width/Area (mm) (mm) (mm) (cm2) (cm?1) 1 221 231 97 219.2 .04425 2 231 241 97 228.9 .04237 3 241 251 97 238.6 .04065 4 251 262 128 328.3 .03899 5 262 274 128 343.0 .03731 6 274 287 128 359.0 .03565 7 287 301 147 432.2 .03401 8 301 316 147 453.5 .03241 9 316 331 147 475.6 .03091 10 331 351 188 641.1 .02933 11 351 367 188 674.9 .02786 Table 1: Dimensions of the 11 tile formats used in EB0, the notations are de ned in gure 1 in the center. A support structure holds the tile above the plotter, at a distance of about 1-2 mm from the collimator. The light produced by the source in the tiles , is collected at the edges by two optical bers that channel the light to the PMT's (Phillips XP2012). These WLS Y11(200)M bres of Kuraray were inserted into the new pro les used in the assembly of the EB0 modules. Good contact between the bers and the tiles was ensured by the use of small steel clips that pressed the pro les against the tile. All the above elements were contained in a light-tight black box. The current at the anode of the PMT is integrated by an electronic circuit that provides a DC voltage proportional to the mean current of the PMT. A VME ADC, digitizes the 2

voltage after every plotter movement. The speed of the plotter is approximately 1 step (5mm) every 5 seconds. The Data Acquisition is controlled by Labview running in a PC that controls the plotter and the VME crate. The digitized data are written to a le for further analysis.

3 Measurements and available data
The dimensions of all tile types are summarized in table 1. A subset of 154 tiles of the full sample used to equip the EB0 assembled in Barcelona, was scanned: every tile of two tile rows (#5 and #6) of the cell B12 has been scanned (altogether 60 tiles), before installing them in the EB0. the full batch was sampled, at the rate of 1 out of 17, or a sample of 94 tiles; these tiles were not installed in the EB0. The granularity of the scans, one step every 5 mm, was a compromise between time and precision. Each tile scan lasted about 20 minutes; half of the time was needed for stabilization of the pedestals. To monitor the stability of the system , one tile was chosen as reference and was measured three times per day. The system was stable during the measurement of the tiles of the cell B12. The RMS of the response of the reference tile as measured with PMT1 and PMT2 was 1.6% and 1.3%, respectively. This re ects not only the stability of the gain of the PMT but also the repeatibility of the tile- ber coupling. For the measurement of the full EB0 sample, some instability appeared. It is illustated in gure 3. Table 2 summarizes the results.

Stability period

RMS of the response PMT 1 (%) PMT 2 (%) Cell B12 1.6 1.3 Full EB0
0.9 7 2.4 4

Non-stability period

periods.

Table 2: System stability , in terms of the RMS of the response of the reference tile for the two scanning For the cell B12 the average signal of PMT1 and PMT2, has been used as reference. For the full EB0 sample, PMT1 was used as reference for the stable period, and PMT2 for the unstable period correcting for the monotonic decrease in gain.

4 Uniformity of the tile samples
To study the uniformity of the tile response in a production batch, the light output was measured for a sample of each tile type, by moving the source along the central line (see 3

EB0 - Pmt stability
22000 21500 21000 20500 20000 19500 23/4/97 24/4/97 25/4/97 28/4/97 29/4/97 30/4/97 5/5/97 6/5/97 6/5/97

20000 19500 19000 18500 18000 23/4/97 24/4/97 25/4/97 28/4/97 29/4/97 30/4/97 5/5/97

Figure 3: System stability. Response of the \reference tile" as a function of time, during the scanning

of the full EB0 tile sample. Each point corresponds to the average of the three daily measurements. The vertical axis gives the tile response in ADC counts. The vertical line separates the measurements in two sets. In the rst set where both PMT's were stable. The second set, the response of PMT2 decreased monotonically, whilst the other was more irregular.

gure 1, central length) in steps of 5 mm. An example of the signal output from the two bers, together with their sum, is shown in gure 4. The characteristic response of a tile was obtained by summing all measurements along the central line from the two bers after pedestal substraction (this sum is only used for comparison of tiles of the same type). The spread of the tile response for each tile type is summarized in table 3: the gaussian as well as the number of tiles with a light yield di ering from the mean more than 3 are given (in these cases the measured light yield is always smaller than the average), and the RMS of the full sample for each tile type are quoted. Figure 5 shows, as an example, the correlation between measurements from the two PMTs for all tiles of size # 9. There is a clear correlation between the signals of the two PMTs for the tiles that present a response di erent smaller than the average. This was true for all such cases. The results show that the response of the tiles examined is uniform within a few percent. From the 154 tiles measured, only 8 were o by more than 3 from the average response of the corresponding tile type. 4

Tile 8
0.3

0.25

0.2

0.15

0.1

0.05

0

10

20

30

40

50

60

70

Figure 4: Signal output (au) from the central tile scan. The signal from the two bers together with
the sum are shown.

5 E ect of the Tile Masking
For the '97 batch, the masking was optimized based on the experience from the masking of the '96 batch 3]. We measured the uniformity of the tile response along the central line. The results are presented in gure 6. They show an improved uniformity compared to the '96 case (Module 0) 3], where only tile types from #6 to #11 were masked. A good uniformity of the plateau 1 (of the order of 1-2%, see table 4) is obtained with the masking for all tile sizes.

6 Light Yield vs w/A
Using the mean value from the scanned tiles of each type 2, we parametrized the light output as a function w/A (or equivalently the inverse of the central length of the tile, see
The plateau is de ned as the region where signal collected by each PMT is 5% smaller than the maximum measured signal, in order not to be sensitive to the e ect of the masking, provided that the source position is inside the region below the scanned tile. 2 the tiles with a light yield di ering by more than 3 from the mean have not been taken into account in this analysis
1

5

Tile nr Nr of Gaussian Tiles with values Total tiles spread (%) beyond 3 RMS(%) 5 6 1 2 3 4 5 6 7 8 9 10 11 30 30 8 8 8 9 7 8 9 9 10 9 9 3.6 1.5 3.9 3.2 3.0 1.8 1.8 1.9 2.2 2.9 3.3 2.7 2.7
Cell B12

0 1 0 0 0 2 1 0 0 2 1 1 0

3.6 3.4 3.9 3.2 3 4.5 7 1.9 2.2 4.6 5.2 4.4 2.7

EB0

Table 3: Tile-to-tile response uctuation. The spread in the tile response for each tile type, for the cell
B12 and for the full EB0 sample of the Barcelona module

Tile response vs source position
Tile nr. 1 2 3 4 5 6 7 8 9 10 11 RMS(%) 0.9 1.0 1.1 0.8 0.6 0.4 0.6 0.8 1.0 1.8 2.4

Table 4: RMS of the plateau. Response of the tile versus the source position along the length direction. gure 1), as suggested in reference 1]. Two methods of analysis have been applied. The rsts consists in taking as response of a tile the average over the region as displayed in gure 6. In this case, the resulting parameterization is ( - 0.1864+ 20.925 w/A), where w/A] is given in cm?1, and the light output is given in arbitrary units. The other method 6

tile.eq.9.

21000

20000

19000

18000

17000

16000

17000 17500 18000 18500 19000 19500 20000 20500 21000 21500

LEFINT

VS

RIGINT

Figure 5: Correlation between the measurements from the two PMT's for 10 tiles of size # 9: The
integrals are given in ADC counts.

consists of using the average light output of the three central points as the characteristic light output; this gives the parameterization, (-0.1854 + 20.96 w/A), consistent with the rst result. Both sets of results are shown in gure 7, where they are compared with the light yield of the 1996 production (Barrel Module 0); measured by exciting the tiles with the source at their centre 2]. The light yield parametrized with the second method (central points average) is also compared in gure 7, with the light yield of a sample of the 1997 tile production scanned at CERN 2]. From the comparison, it is observed that the CERN light yield measurements presents a rather atter dependence on the tile size. It must be noted that in the CERN measurements 2], the tile- bre coupling is slightly di erent: on the two lateral edges where the bres are connected, a small piece of aluminized Mylar was put ( xed with adhesive tape in the two sides) to keep the bers in place. The attenuation length has been measured for three tile types, #5, #8 and #11. The signal from one ber was tted by a single exponential, leaving out the masked region close to the edge. The measured attenuation lengths are 359, 364 and 373 mm ( 30 mm ) for tile type: #5, #8 and #11 respectively. This result is compatible with the value measured in 4] (350 mm for tile size # 11) and also with the measurement of the attenuation length of the largest tile (384 mm) of the Barrel Module 0 3]. 7

x 10

-3

Light output (a.u.)

0.8

0.7

0.6

0.5

0.4

0.3

0.2

-15

-10

-5

0

5

10

15

Distance (cm)

Figure 6: Light yield average vs source positions, for di erent tile types

7 Tile Response along the width
The motivation of such a scan is to measure the tile response in the region relevant fot the Cs source calibration and the data taken with muons impinging at 90 degrees at the center of the tile. The scans were done along three vertical lines (see gure 8). One passing exactly at the center of the tile across the holes, and the other two on each side of the central line. The distance between the lines is 1cm. The step between measurements in each line is 5 mm. A typical output from the PMTs for this type of scan (tile # 1) is shown in gure 9. The line # 2 passes through the center of the tile, across the two holes. They can be clearly seen (in gure 9) in the valleys of the response of the two channels. For the lines 1 and 3 the e ect of the holes depends whether the signal is collected on the same side (plateau with a small slope), or on the other side of the holes: some small structure due to the re ection of the light on the surface of the holes, appears in the plateau. The light yield in a tile is higher when the light is produced close to the longer edge of the trapezoid. The di erence, with the other edge, is about 10 %. The response of each tile type versus the source position has been tted by a straight line (using line 1 for PMT2 and line 3 for PMT1), and the average slopes for all tiles of each type were 8

0.8 0.6 0.4 0.2 0.028 0.03 0.032 0.034 0.036 0.038 0.04 0.042 0.044

0.8 0.6 0.4 0.2 0.028 0.03 0.032 0.034 0.036 0.038 0.04 0.042 0.044

0.8 0.6 0.4 0.2 0.028 0.03 0.032 0.034 0.036 0.038 0.04 0.042 0.044

Figure 7: Tile output vs w/A. For each size, the average response, calculated by two di erent methods

(see text) is given and compared to the result obtained for the batch of tiles produced in 1996 to equip the barrel module 0. For both sets of results, the light yield of the tile type #4 has been used to normalize the two methods to the results of the 1996 study. The 1997 (IFAE) measurements are compared with the 1996 (CERN) measurements, again using tile #4 for normalization.

calculated, they are shown in gure 10. The errors in gure 10 are obtained combining in quadrature the errors of the ts to the points of line 1 and line 3. Figure 10 shows that the slope tends to decrease as the tile size increases. The change of slope is not related to the increase of the thickness of the larger tiles towards the outer edge, which is reported in 4], because most of the 90 Sr electrons do not traverse the tile.

9

Figure 8: Vertical Scan

800 700 600 500 0 10 20

900 800 700 600 0 10 20

800 700 600 500 0 10 20

900 800 700 600 0 10 20

800 700 600 500 0 10 20

900 800 700 600 0 10 20

2250 2000 1750 1500 0 10 20

2750 2500 2250 2000 1750 0 10 20

Figure 9: Typical result of a vertical scan. Tile type # 1. 10

1.6

1.4

1.2

1

0.8

0.6 2 4 6 8 10

Figure 10: Tile response along the width. The average slopes and their errors for each tile type are
obtained as explained in the text.

11

8 Muons vs Cs Source Calibration Zone Response
To quantify the e ects of the tile non-uniformity on the two calorimeter calibration procedures (Cs source and muons), we have compared the light output from the zones around the holes, top and bottom (see gure 11), with the response at the center of the tile. The responses of each zone are averaged over the number of points scanned by the source (16 points around the bottom and top region, and 18 points in the central zone). The results are shown in table 5 and gure 12, were the the signal of each zone is given relatively to the signal at the center of the tile for each tile type. The results may be summarized as follows: The tile response around the holes is smaller than in the central zone for most of the tiles except for the largest ones. The light collection in the center of smaller tiles bene t from re ection on the hole surface, as also seen in gure 6. The di erence in response between the upper and lower hole regions is roughly 10%, independently of the tile size. This fact is useful in analysing \light budget" data obtained with Cs source scans 5].

Figure 11: Calibration Zones

12

Tile nr. 1 2 3 4 5 6 7 8 9 10 11

Response of di erent tile zones
Bottom .916 .035 .917 .014 .923 .034 .902 .049 .913 .055 .917 .016 .898 .024 .912 .053 .903 .055 .902 .072 .897 .024 Central 1. .040 1. .017 1. .037 1. .054 1. .067 1. .018 1. .031 1. .056 1. .054 1. .094 1. .024

Top .974 .040 .969 .018 .969 .039 1.005 .053 .984 .071 .982 .018 1.003 .032 .9873 .057 1.000 .053 1.042 .107 1.037 .025

Table 5: Cs versus muon calibration zones. The relative responses of the three calibration zones with

respect of the central zone (sampled by 90 degrees muons).The errors are the rms value of the distribution for each tile type.

1.4

1.3

1.2

1.1

1

0.9

0.8

0.7

0

2

4

6

8

10

12

Figure 12: Calibration Zones response comparison 13

9 Conclusions
The analysis of the tiles produced for the EB0 has shown that groups of tiles of the same size present only a few percent spread in their response. The tile response along the central tile line, when summing the response of both PMT's, is constant within a few percent, showing that the masking is well optimized. The decrease of light yield with the tile size is similar to what was observed in Barrel Module 0. Di erences of the tile response around the two holes and in the central zone of the tile are signi cant and should be taken into account in the calibration process (Cs and Muons).

References
1] J. Proudfoot and R. Stanek, ATLAS Internal Note, TILECAL-NO-066. 2] M. Cobal et al., ATLAS Internal Note, CAL-NO-81. 3] ATLAS Collaboration. ATLAS Technical Design Report, Calorimeter Performance, CERN/LHCC/96-40, (15 December 1996), pages 130-136. 4] Marina Cobal et al., ATLAS Internal Note, TILE-CAL-NO-124 . 5] S lvia Bravo, et. al., ATLAS Internal Note, TILE-CAL-NO-124, \Light yield of the 1997 Extended Barrels Module 0 from Cs data with nominal PMT settings" .

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