MEG-TN013
R&D report on the drift chamber prototype at Univ. of Tokyo
Hajime Nishiguchi and Satoshi Mihara
ICEPP, Univ. of Tokyo, Japan
Mar 2001
Abstract

Study with the drift chamber prototype is in progress in Tokyo. The prototype has been constructed for optimizing basic parameters such as gas mixture and applied HV, and for studying the vernier pattern.
Introduction
The prototype has been constructed to study operating conditions such as gas mixture and applied HV, and to investigate vernier pattern response using the same structure of the drift cell shown in the proposal. The gas mixtures used in this study were Ar:C2;H6=50:50, He:C2H6=50:50, 60:40, and 70:30. Amplification gain dependences of the anode signal on gas mixture and on applied HV were measured together with the drift velocity. The position resolutions along the direction perpendicular to the wire (x-direction) were measured by using 2.283 MeV electrons form a beta source of 90Sr. The hit position along the wire direction (z-direction) was determined in 2 steps as shown in the proposal, by the charge division method and then with vernier patterns. For designing the vernier pattern, it is quite important to confirm that the simulation predicts the ratio of observed charges by 3 pads (the left, center, and right pads). In the following sections, the experimental setup is described in Sec. 2, analysis procedures and results are in Sec. 3, and in the final section the result and a summary of this study is presented.
Experimental Setup
The prototype used in this study has a simple mechanical structure as shown in Fig. 1. It consists of 2 layers of drift cells staggered with each other by a half cell pitch as shown in Fig 2. Each layer consists of 8 drift cells that structure is presented in the proposal. The vernier foils are put on G10 plates which is sandwiched by aluminum plates where sense and potential wires are streatched. At the outermost sides of the detector, cathode foils are placed and are kept at the ground electrically.
top view side view
Fig 1: Schematical views of the drift chamber prototype, the top (left) and side(right) views.
cell structure
Fig 2: Cell structure and the vernier pads configuration of the prototype.
The anode wires are made of Tungsten coated with Au and the potential wire of Mo coated with Au. The diameters of the sense and potentiaal wires are 30µm and 125µm respectively. Applied wire tentions are 150g and 50g respetively for potential and anode wires. The vernier and cathode foils are made of 12.5µm thick kapton and aluminum layer of 500Å. The pattern is formed by the printing circuit technology with an accuracy of 3µm. Fig. 3 shows the dimension of the vernier pattern. The pattern has a 6cm periodic structur along the z-direction. Induced charges on the 3 individual pads were measured at one end of the pattern. HV is applied on the anode wires keeping the potential wires, cathode planes, and vernier foils at the grand electrically. Tests were performed applying HV of 1.5kV to 2.0kV on the anode wires.
pad design
Fig 3: Dimension of the vernier pattern.

  • Signal Readout scheme
    For reading signals out from anode wires and vernier pads, preamplifiers originally developed for CDF and used in BESS experiments were employed. The details of the performance of this amplifier is reported in Ref [1]. The scheme for applying HV and preamplifier readout is schemtaticall drawn in Fig 4.
    frontend circuit
    Fig 4: HV applying and anode signal readout scheme.
    The anode signals amplified by the preamplifier were amplified again by the post-amplifier whose circuit diagram is shown in Fig 5. After amplification, signals were discriminated for drift time measurement and their arrival time was measured by TDC that has a least count of 100nsec.
    post amp for anode signal
    Fig. 5: Circuit diagram of the post-amplifier for the anode signal..
    Total charge induced on anode wires measured at both ends were recorded for the z-postion measurement by the charge division method. Signals from the vernier pads wre amplified by two stages, too. The preamplifier is the same one used for the sense wire signal, and the post-amplifier has slower response than that for the sense wires and inverts the output so that induced charge can be measured with conventional ADCs(LRS 2249W).
    post amp for cathode signal
    Fig. 6: Circuit diagram of the post-amplifier for the cathode signal..
    To study the drift chamber reponses using the prototype, a beta source of 90Sr was used. Spot size on the active volume was collimated by a collimator made of 10mm thick Teflon with a hole of 0.6mm in diameter. The configuration of the beta source, collimator, triggering scintillation counter is shown in Fig. 7. The prototype was located between the collimator and the triggering counter. The triggering counter consists of a scintillator block of 6mm×6mm×2mm viewd by a half inch PMT (HAMAMATSU H3165) through a light guide. The beta source and collimator were set on a stage which can be moved with the trigger counter in the x-direction with an accuracy of 1µm. The stage was moved in z-direction on a rail for changing the incident z positions to the chamber.
    xstage
    Fig. 7: Configuration of the beta source, collimator, trigering counter on a movable stage.

    The photograph of the whole setup is shown in Fig. 8.
    whole setup
    Fig. 8: Whole setup of the study..

  • Gas flow controlling system
    Four kinds of gas mixtures of He:C2H6=50:50, 60:40, and 70:30 and Ar:C2H6=50:50 were studied. The gas mixture was controlled by two sets of mass flow meters [2] which can controle the individual flow rate of He and C2H6 with an accuracy of 0.02 cc/min at 0° and 1 atom. Whole system to controll the gas mixture and flow is schematically presented in Fig. 9. For the gas mixture of Ar:C2H6=50:50, a bottle of pre-mixed gas was used. The inner pressure of the prototype was regulated at 1.01 atom during the study. Alarm level for the inner pressure inside the chamber was set at 1.1 atom for safety. For replacing the inner volume with any gas, the flow was continued for at least one day before the study with a flow rate of 25 cc/min.
    gas line
    Fig. 9: Diagram of the gas flow controlling system.
    Data analysis
  • Amplification gain and drift velocities
    Relative amplification gain for various conditions were measured with placing the beta source at the postion 2mm away from an anode wire. Fig. 10(left) shows the gain dependnce of the anode signal on the applied HV. It can be seen that we can obtain higher gain with larger percentge of He. Drift velocities were also measured for each gas mixture at the center of the drift cell. Fig. 10(right) shows the drift velocties for each gas mixtre as functions of applied HV. It can be seen that the drift velocities in He based gases are slower than that of Ar:C2H6=50:50, and that higher percentage of He results in slower drift velocities.
    relative gas gain drift velocity
    Fig. 10: (left) Amplification gain dependence on the gas mixture and applied HV. The vertical axis is arbitrary. (right)Drift velocity measured at the center of the drift cell as a function of applied HV.
  • Analysis of anode wire signal
    By moving the beta source in the x-direction with 1mm step, the x-t correlation was evaluated. In this procedure for determing the precise x postion in the upper (lower) layer where the particle went through, information of hit position in the lower (upper) layer was utlized taking into account the geometry of the collimator and the drift chamber. The x-t relation was updated step by ste and this iteration procedure was repeated until the x-t relations of both layers converges. Fig.11(left) shows an example of x-t relation for the gas mixture of He:C2H6=60:40 with applying 2000V on the anode wires. The distribution was fitted with a polinomial function and the residuals at positions of (a) 4.6mm, (b)3.6mm, (c)2.6mm, and (d)1.6mm from the anode wire are presented in Fig.11(right).
    x-t relation residual
    Fig. 11: (left) x-t relation obtained for the gas mixture of He:C2H6=60:40 with applying 2000V on the anode wires. (right)Distributions of residuals at positions (a) 4.6mm, (b)3.6mm, (c)2.6mm, and (d)1.6mm from the anode wire.
    Distributions of residuls shown in Fig11 (right) contains following other effects in addition to the itrinsic position resolution.
    • position resolution on another layer used for the prediction of the hit position.
    • size of the illuminated area on the anode layer.
    • effect of the multiple scattering.
    Because the position resolution on the another layer is reasonablly supposed to be same, the sigmas of the distributions were divided by square root of 2 before subtructing the other effects. The second effect was estimated to be 176.5±10µm considering the geometry. The third effect was calculated to be 26.2 µm taking into accout the radiation length of materials which electrons had to cross before reaching active reagion. For other gas mixtures, those are summarized in Table 1 together with properties of used gases.
    Gas Density] Radiation Length
    He 0.125[g/l] 94.32[g/cm]2
    Ar 1.396[g/l] 19.55[g/cm]2
    C2H6 0.509[g/l] 45.47[g/cm]2
    Gas Mixture Radiation Length Multiple Scattering
    He/C2H6(50:50) 50.64[g/cm2] 29.9[µm]
    He/C2H6(60:40) 52.84[g/cm2] 26.2[µm]
    He/C2H6(70:30) 56.04[g/cm2] 24.2[µm]
    Ar/C2H6(50:50) 23.08[g/cm2] 87.1[µm]
    Table 1: Propaties of used gases and radiation length of employed gas mixtures.

    The second and third effects were subtructed to obtain the intrinsic position resolutions. Fig. 12 (left) shows the position resoution as a function of the distance from the anode wire for the gas mixture of He:C2H6=60:40. The best resolution was obtained around the center of the drift cell while it deteriorates to 230µm near the anode wire and ends of the drift cell. Resolution dependence on HV applied on the anode wires is summarized in Fig. 12(right) where resolutions at the center of the drift cell are plotted for different gases.
    resolution resolution
    Fig. 12: Position resolution in x-direction as a fuction of the distance from the anode wire for the gas mixutre of He:C2H6=60:40 (left). Position resolution at the center of a drift cll as a function of appllied HV for different gases.

  • Analysis of vernier pad signal
    Since the vernier pattern is designed to repeat the periodic structure by 6 cm, it is required that the hit position should be estimated before analysing its signals with accuracy better than 6cm for resolving which period the hit positon belogns to. For this purpose the charge divistion technique was employed. Total charges at the both ends of anode wires were measured simultaneously with drift time. The beta source with a collimator was utilized again to study the resolution. The source and collimator was moved along the z-direction. Fig. 13 shows the reconstructed positions as a function of the source position for the gas mixture of Ar:C2H6=50:50. Fitting this relation with a liner function results in less than 2mm resolutions in sigma, which is sufficient to give predictions for the analysis of vernier pad signal.
    charge division
    Fig. 13: Reconstructed z position by the charge division technique as a fuction of the source position for the gas mixutre of Ar:C2H6=50:50.

    Fig. 14 (left) shows the ADC spectra observed by 3 indivisual pads for different source positions in case of using the gas mixture of He:C2H6=60:40 and applying 2000 V on the anode wires. In Fig. 14 (right) the peak values of the spectra are compared with prediction by the simulation.
    ADC spectra oberved by pads ADC spectra oberved by pads
    Fig. 14: ADC spectra observed by 3 indivisual pads for different source positions (left) and the peak values of the spectra as a function of the source positions(rigth). The solid lines show predictions by the simulation, which are normarized to fit with data points globally.

    The simulation calculates the induced charges on each pad by integrating the following formula over each pad region.
    charge distribution

    where q is induced chage, a is distance between an anode wire and a cathode plane, and (x,y) represents the coordinate on the cathode plane. Using the observed charge by 3 indivisual pads, ratios of them were calculaed in the following way.
    • R1=(C1-C2)/(C1+C2)
    • R2=(C3-C2)/(C3+C2)
    where C1, C2, and C2 are observed charges at each pad. Scatter plots of R1 and R2 are presentd in Fig. 15.
    scatter plot of vernier signals scatter plot of vernier signals
    Fig. 15: Relation between R1 and R2 for different source positions (left). The expanded view of the left figure is shown in right together with those for other gas mixtures. (a) for He:C2H6=50:50, (b) for 60:40, (c) for 70:30 and (d) for Ar:C2H6=50:50.

    Fitting this shape with 4 continuous functions, the response function was obtained and the resolution was estimated. The results are summarized in Table 2. Note that the effects of multiple scattering and illuminated size on the effective volume as mentiond in the previous section for the anode wire signal aanalysis have not yet been subtracted.
    Gas Mixture Position Resolutions
    He/C2H6(50:50) 949.9±14µm
    He/C2H6(60:40) 508.4±8.0µm
    He/C2H6(70:30) 425.4±6.9µm
    Ar/C2H6(50:50) 1750±43µm
    Table 2: Position resolutions in z-direction obtained by using vernier pad signals.

    The applied voltage on anode wires was same for all gas mixtures (2000V). It is clear that increasing He percentage results in better z resolution because observed charges by each pad increases. However it shoud be mentioned that increasing He percentage could lead to higher multiplcity of anode wires due to lack of queancher. Adding small amount of iso-C4H10 or CF4 is expected to reduce the multiplicity. This effect to the track reconstruction should be studied carefully using the full simulator.
    Summary
    We studied the drift chamber response using a simple prototype with several gas mixtures. Signal gain, drift velocity, and position resolutions were studied for different settings of appiied HV on anode wires and gas mixture. Position resolution perpendicular to the anode wire of 93.1±10µm in simga was obtained with the gas mixtre of He:C2H6=70:30 and HV of 2000V on anode wires. We performed also systematic study on a vernier pattern using a beta source of Sr with a collimator. Using vernier pad signals position resolution of 425.4±6.9µm along the wire direction has been achieved with a gas mixture of He:C2H6=70:30. Relations between the ratios of observed charges (R1 and R2) are found to be well predicted by the simuation, which means that the simulation can be used for further optimiztion of the pattern. It was confirmed that increasing the percentage of He results in better resolution because observed charge by each pad increases. However multiplicity of hit wires in one layer for each event will also increase with higher percentage of He. In addition that will make the drift velocity slower, which is not suitable in high rate operation. This might cause unexpected problems for track reconstruction.
    References
    [1] Test of the 'Bess-Experiment' Fujitsu Preamplifiers using a Charge Injector, Oct. 2000, P.-R.Kettle
    [2] KOFLEC Model-3660