Micromegas Detector Using 55 Fe X-ray Source

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1 Micromega Detector Uing 55 Fe X-ray Source Hamid Mounir #, Seddik Bri* 2 # Spectrometry Laboratory of Material and Archaeomaterial (SLAMAR), Faculty of Science, Moulay Imail Univerity, Mekne Morocco * Electrical Engineering Department, High Scool of Technology, ESTM, Moulay Imail Univerity, Mekne- Morocco ABSTRACT The Multi-Wire Proportional Chamber (MWPC) and MICROMEGAS (Micro-Meh Ga Structure) detector are the major familie of poition detector in High Energy Phyic, introduced in the late ixtie, detect and localize energy depoit by charged particle over large area, are widely ued in particle. The goal of thi work i to imulate the potential and electric field ditribution in (MWPC) detector, to clarify the phenomenon of induction of ignal in the electrode of the MICROMEGAS detector, in particular, in avalanche and micro-meh. Initially, we focu on electronic and ionic current induced, then; we will calculate the total charge induced. We chooe Argon-iobutane(Ar-iC4H0) and Argon-dimethyl-ether (DME) ga mixture, uing 55 Fe radioactive 5.9keV X-ray ource. In the normal condition, (T=298k, p=atm), we will imulate the temporal tructure of the current ignal, and the contribution of the total charge induced, then we will imulate the pre-amplified ignal. Thee reult are compared to other reult publihed. Finally, we will dicu the dependence of the output ignal at the parameter of MICROMEGAS detector noting ome remark on improving the performance of MICROMEGAS. Key word: MWPC, MICROMEGAS, Modeling and Simulation, MATLAB Program, Avalanche phenomenon, Primary Ionization, Townend coefficient, Mixture Ga, Electric and Amplification field, Micromeh, Signal. Correponding Author: Hamid. Mounir. INTRODUCTION Initially, all detector were trace of an optical nature, and each event wa analyzed individually. Any electronic intrument wa, therefore, particularly awaited. One poibility wa to aemble the erie of proportional tube but thi wa not practical in term of mechanic. A revolution ha occurred with the invention of the Multi wire proportional chamber G. CHARPAK (WPC) []. Indeed, G. CHARPAK ha hown that the matrix of regularly paced wire anode in the ame chamber function a a proportional tube aembly. Moreover, with recent development in electronic-baed tranitor, each thread can have it own amplifier built in the chamber. Wire chamber wa quickly adopted in particle phyic and timulated new experimental generation. The recovered ignal determine the poition of the interaction in a patial coordinate. A calculation of ma center from the ignal with adjacent wire may help to clarify the place of interaction. The patial reolution of thee device i directly dependent on the pacing between the wire. For mechanical and electrotatic reaon, they cannot be reconciled; however, the Page 67

2 patial reolution of thi type detector wa quickly capped. Depite thi handicap, the MWPC ha been widely exploited, becaue they can cover large area at low cot. They played a major role in the development of modern detector [2]. Ga proportional chamber baed on the MICROMEGAS (MICRO Meh Ga Structure) [3] [4] technology i an example of a new generation of detector that exploit narrow anode-cathode gap, rather than thin wire, to create ga gain. Thee detector are inherently pixel detector that can be made at a large ize for reaonable cot. Becaue of their intrinic gain and room temperature operation, they can be intrumented at very low power per area unit, making them valuable for a variety of particle phyic application where large-area particle tracking i required. The MICROMEGAS i a new gaeou detector initially developed for track-in high-rate, highenergy, phyic experiment ince 990. It how higher counting rate capacity up to 0 8 mm 2 -, poition-enitive with patial reolution better than 00 µm and good performance of radiation hardne[5]-[6], which ha been developed ince 996 at Saclay, France[7]. MICROMEGAS i a Parallel Plate Detector (PPD) with three electrode, cathode, micromeh and anode and a narrow amplification pace, typically micron, between the micromeh and the anode. The detector gain depend directly on the ditance between the micromeh and anode; therefore, controlling the geometry of the amplification region i crucial in order to achieve good energy reolution [4]. The concept, developed for tracking application, ha hown great promie for handling high data rate with a rather low cot tructure. The working principle i baed on the double tage parallel plate avalanche chamber. MICROMEGAS i widely ued in particle phyic and i a uitable candidate in high-intenity X-ray detection and tomography. In order to improve the performance of MICROMEGAS detector by optimizing ome parameter, we will, therefore, tudy the effect of all thee parameter and find compromie that allow u to operate the MICROMEGAS in condition where it will be 00% effective for particle at leat in ionization and be able to find the poition of the paage of the particle with great preciion. Firt, we will etablih the baic equation that underpin our work. Our imulation model baed on a numerical calculation code developed by MATLAB Program, uing the ga mixture Ar-iobutane (Ar-C 4 H 9 ) and Ar-dimethyl-ether (Ar-CH 2 -O-CH 3 ) that decreae the diffuion coefficient, we ue alo, 55 Fe ource that produce x-ray photon of 5.9KeV. The temporal evolution of electron and ion current were imulated. The output ignal of micromeh and the temporal evolution of total charge were alo calculated. Thee reult are compared with that obtained in author literature with author ga mixture. MODELING AND PHYSICAL PROCESS The (MWPC) detector, hown chematically in figure, conit of grid of parallel wire (anode) equiditant (the order i about one millimeter) between two cathodic plan heated to a high voltage and defining a volume filled with ga. During the paage of a charged particle, the ionization electron produced in the ga drifted toward the anode under the effect of electric field and multiplied in their environment. An electrical pule appear on the wire anode. In the MWPC, the patial reolution i cloely related to the pacing between the wire anode. It i technically difficult to reduce that ditance under one millimeter. Page 672

3 Fig : Diagram of multi-wire proportional chamber (MWPC) detector The phyical procee that determine the functioning of the ga detector are tatitical in nature: the ionization of ga molecule, the diffuion of free electron, the ga amplification, etc. Thee tatitical procee can be meaured and undertood, however, if all the poible function governing the detector are known, it will be very difficult to find analytical expreion to calculate the performance of detector uch a the efficiency or patial reolution. When many of probabilitic phenomena mut be taken into account imultaneouly, an alternative approach to analytical olution i the imulation. It i a imulation on a computer, a phyical phenomenon: giving an initial condition, all procee are evaluated, and if a probabilitic phenomenon come into play in the proce, then a random choice i made at the probabilitic ditribution of the phenomenon in quetion. The interpretation of reult require great caution becaue prediction are baed on theoretical or empirical model, the validity of which i limited. Therefore, the imulation reult mut be verified whenever poible by a comparion with appropriate experience. Thi i a multi-wire chamber detector characterized by the ize hown on the figure below, the mathematical expreion of the potential i uually calculated analytically uing conformal mapping or tranformation Chritoffel-Schwartz [8]. The configuration that we want to tudy i hown in the diagram below, it i a matrix that repreent a grid (wire anode) of length L, thee parallel wire are placed equiditant from =mm including a plan inerted into a gap of thickne Δ. The dimenion characterizing the detector are hown in figure 2. Fig 2: Front view and dimenion of (MWPC) detector Page 673

4 Potential [V] International Journal of Advanced Scientific and Technical Reearch Iue 3 volume, January-February 203 POTENTIAL AND ELECTRIC FIELD DISTRIBUTION The ditribution of potential and electric field in a multi wire proportional chamber or drift chamber can generally be calculated in good preciion of the exact formula for the potential around a et of line of charge q along the axi oz and located in: y = 0, x = 0, ±, ± 2,. V x, y = q ln 4 in πx 2 4πε 0 + inh 2 πy () The profile of potential ditribution i repreented in the figure below, imulated by MATLAB Program x , Y [cm] ,6-0,8-0,6-0,4-0,2 0 0,4 0,2 X [cm] 0,6 0,8 Fig 3: Potential ditribution in MWPC detector Finally, we can get the total field: E x, y = q 4ε 0 in 2 2πx in 2 πx + inh 2 2πy + inh 2 πy 2 (2) Figure 4 how the profile of Electric Field ditribution, calculated uing the MATLAB Program Page 674

5 Electric Field [V/cm] International Journal of Advanced Scientific and Technical Reearch Iue 3 volume, January-February 203 x , Y [cm] ,8-0,8-0,6-0,4-0,2 0 0,4 0,2 X [cm] 0,6 0.8 The equation of equipotential urface are Fig 4: Electric Field ditribution in MWPC detector coh 2 πy = co 2 πx (3) The field line are determined by the equation: tanh πy = k tan πx (4) Where k i an integration contant. The profile of the field line i hown in figure 5, calculated uing the Garfield program [9]. Fig 5: Field line in a MWPC detector, calculated by Garfield Program Page 675

6 DESCRIPTION AND BASIC PRINCIPLE OF MICROMEGAS DETECTOR In more recent deign (985), MICROMEGAS (Micromeh Gaeou Structure) ugget that new higher performance technique previouly developed. It operate on the principle of multiwire proportional chamber (MWPC), but with better patial reolution ( 50 micron) and a more rapid formation of the ignal. MICROMEGAS i a gaeou detector trongly aymmetric parallel face. The baic principle i to decouple the drift region of the amplification gap, either by a grid wire, but by a very fine metal grid (3 micron thick). The trick lie in the ue thi micromeh pierced by a multitude of hole (in le than or equal to 50µm). MICROMEGAS principle i illutrated in figure 6. It conit of a drift gap (3 mm) and a thin amplification gap (00 µm). The propertie of the electric field in two gap have been tudied by MC imulation with Garfield [9].The voltage of drift electrode and meh i HV2 and HV, repectively, and MICROMIGAS i operated in ga mixture of Argon-iobutane (Ar-iob) and Argon-dimethyl ether (Ar-DME), at a temperature of 300 K and a preure of atm. The electric field in the amplification region i very high ( 40 kv/ cm), and the one in the converion region i quite low. Therefore, the ratio between the electric field in the amplification gap and that in the drift gap can be tuned to large value. The MICROMEGAS i baed on the planar electrode. The drift electrode i made from 5 5 cm 2 500LPI (line per inch) teel meh pated on an epoxy frame, and the ame meh i ued a the amplification electrode. The drift gap i alo called converion gap becaue the ionization of the ga by incoming radiation occur mainly in it. The field in the amplification gap catche the free electron from the drift gap to forcefully accelerate them through the mall amplification gap. They, then, acquire enough energy to ionize urrounding ga. The newly ionized electron gain a well a ufficient energy to ionize the ga again and again, thu forming an avalanche and leading to a meaurable electric ignal on the anode and on the meh. Fig 6: Schematic view of MICROMEGAS: dimenion and operating principle Table preent ome parameter configuration of MICROMEGAS detector. Page 676

7 Table. Parameter configuration of MICROMEGAS detector parameter Symbol Example of value Ga Mixture Ray-ource Drift gap h 3mm Amplification gap d 00µm Track pitch P p 37µm Width of a track w 250µm Step of the grid P g 52µm voltage drift V d 700V Voltage meh V g 400V Drift field E d kv.cm - Amplification field E a 40kV.cm - Argon-iobutane (Ar-C 4 H 9 ) Argon-dimethyl-ether (Ar-CH 2 -O-CH 3 ) X-ray ource 55 Fe (5,9keV) ELECTRIC FIELD CONFIGURATION A detailed decription of the electric field configuration and the principle of operation are given in [0] - []. Ionization electron, created by the energy depoition of an incident charged particle in the converion gap, drift and can be tranferred through the cathode micromeh; they are amplified in the mall gap, between anode and cathode, under the action of electric-field, which i high in thi region. The electron could i finally collected by the anode micro trip, while the poitive ion are drifting in oppoite direction and are collected on the micromeh. The electric-field mut be uniform in both converion and amplification pace. Thi i eaily obtained by uing the micromeh a the middle electrode. The electric-field hape i, however, diturbed cloe to the hole of the micromeh. The knowledge of the hape of the field line cloe to the micromeh i a fundamental iue for the operation for our detector, and epecially for efficiency of the paage of electron through the micromeh, a well a, for the fat evacuation of the poitive-ion build-up. MICROMEGAS SIGNAL The idea of a ignal obtained by charge collection in the electrode can be confuing. Dice their appearance, the electron and ion created the ignal charge in the electrode of the detector. For decribe the formation of the ignal in the detector, follow the electron a they pa through the meh of the micromeh and then during the avalanche. Thi i not the collection of charge on the electrode, the lope for electron and ion to the micromeh, which i the ource of the ignal but it i the moving charge induce a ignal in the electrode [2]. The electron avalanche occurring in the amplification gap produce a high number of ionization. The average of thi number i called the multiplication factor or ga gain of the chamber. The Page 677

8 ize of the avalanche how large variation around it average value. A model of ga amplification, etablihed in [3], give the number of electron n (t) in an avalanche. The current generated by the moving charge in the amplification gap ha two contribution: - A brief and fat ignal due to the electron generated motly cloe to the anode plan and quickly collected ( 2 n). The drift velocity of electron in ga i of everal cmμ - A long ignal due to the ion moving lowly toward the meh ( n), their drift velocity i at leat two order of magnitude lower than the electron one. Baic Equation Conider the movement of n 0 electron from the micro-grid to the anode. Where t e i the time of the electron collection t e = d v (5) Where d i the width of the amplification gap and v- i electron velocity, we oberve a current: I e t = n 0q t e, 0 t t e (6) We follow the development of the avalanche and the increaing number of electron, we obtain: I e t = n e t q t e, 0 t t e (7) The number of electron in the avalanche follow the law: n e t = n 0 exp T w v t (8) Where T w i the Firt Townend coefficient (amplification factor) i expreed a: T w = p A exp B p E (9) Where p i the atmopheric preure, E the amplification field and A, B are parameter of the ga mixture [5, 4, 5]. I e t = n 0 q t e exp T w d t e t, 0 t t e (0) The ion produced during the avalanche on vat majority, will move toward the micro-meh with a velocity v + much lower than the velocity of the electron. The ionic current will be in two Page 678

9 term, the number of ion created by the development of the avalanche decreaed by the number of ion that have been collected in the micromeh. The ion current i expreed thu, if v + i le than v - : I ion t = n 0 q d d exp T t w t exp T ion t w t, 0 t t e t e ion () n 0 q d exp T t w d exp T w t, t ion t e t t e + t ion ion The time of ion collection i a: t ion = d v + (2) Deducting the total current in avalanche a: I t = n 0 q t e exp T w d t e t + n 0q n 0 q t ion t ion exp T w d exp T w exp T w d t e t exp T w d d t ion t t ion, t e t t e + t ion t, 0 t t e (3) From equation (3), we can deduce the total charge collected in avalanche: Q t = n 0 q T w d exp T d w t, 0 t t t e e n 0 q exp T t w d t, t e t t ion ion 4 We ee that the ratio of the ignal produced by electron and ion i in the report of / (T w d), thi i the movement of ion repreenting the main contribution of the ignal over the detector. Simulation Model The analytical tudy of our ytem lead u to extract ome equation that lead u to model our detector decribed above. The imulation model of the phyical procee occurring within MICROMEGAS i baed on the MATLAB Program that i the tool of imulation, and more effective for our work. In the general cae, There are three type of parameter affecting the detector: the parameter related to the chamber (geometry), the ga parameter (diffuion, gain, Mixing coefficient), the parameter of the trace (angle, energy, ignal, type of particle). From equation (9) and (3), uing the table II hown below [5], we can etimate accumulated ignal in the avalanche for different proportion of each ga mixture ued; the ame program remain what we need to change are the value of the coefficient A i and B i [4] [5]. Thee parameter are adjuted by experience. Page 679

10 Table2. Gain and Firt Townend coefficient for different mixture of an amplification gap of 00 μ m wide Mixture Percentage A i (cm - ) B i (kv.cm - ) T Wi (cm - ) Amplification Gain(G i ) Ar-iobutane Ar-DME The calculated reult of the Firt Townend coefficient (T wi, i=, 2, 3, 4, 5) and the amplification gain (G i, i =, 2, 3, 4, 5) for different proportion of gaeou mixture uing equation (9) and n 0 =, are obtained in [6], but in thi work, we et the electric field amplification at 50 KV / cm, and ee the imulated value for different proportion of the mixture in Table II above. t e are in the order of nanoecond and t ion of the order of hundred of nanoecond [7]. The Global Signal Simulation in the Micromeh The total ignal i collected in avalanche calculated uing the equation (3), and by applying the ame calculation procedure. Figure 7 how the reult obtained. Fig7: Global Signal in avalanche for different proportion of Argon-iobutane (a) and for Argon- DME (b) We can ee the hape of the calculated total ignal MICROMEGAS for both ga mixture. Alo, we note the very high peed collection of ion in MICROMEGAS. An important finding in thi imulation i that the ratio of electron and ion current in the order of the ratio of electron and ion drift velocitie. Page 680

11 Simulation of Charge collected in Micromeh Equation (4) allow u to calculate all the charge collected in micro-meh for both Ar-iobutane and Ar-DME ga mixture following the ame procedure a thoe applied in the imulation of the current ignal. The reult are hown in Figure 8 Fig8: Global charge in Micromeh for different proportion of Argon-iobutane (a) and for Argon-DME (b) Firt, we note that the temporal evolution of the total charge for both ga mixture keep the ame evolution qualitatively. On the other hand, the charge decreae with the increae in the proportion of the quencher, and that the variation for Ar-iobutane i greater than that of Ar- DME. It i intereting to note that the total charge i imply the gain multiplied by the primary charge, and that in fact the amplified electron do not contribute much to the final detected charge. The main contribution i clearly due to ion [8]. The proportion between the two contribution relate directly to the gain of the chamber. The fraction f e of the ignal due to electron i given by: f e = Q e = Q T T w d e T w d = Log (G) (5a) Similarly we can calculate the proportion of charge induced by ion: f ion = f e = T w d e T w d (5b) Where Q e, i the charged induced by electron, Q T, the total induced charge, T w, the firt Townend coefficient, d the gap width and G the ga gain. The Amplification Signal After Thi imulation and it reult, we conclude that our MICROMEGAS i equivalent to a current generator low ignal [7]. That i why; we need an amplification device to be able to clarify the quantitative calculu. Figure 9 how the block equivalency MICROMEGAS connect to an amplifier. Where I D (t) repreent the total current calculated (equation (3)), C D i the equivalent capacitance between the micro-meh and the electrode. R And C are repectively the Page 68

12 capacitor (depend on the length of the track of the detector and the urface of the grid) and the dicharge reitor of the ignal integrator which thee value that are adjuted depending on the type of the preamplifier ued (ORTEC 42A, 42B ORTEC, ORTEC 42C ). Our imulation require value to have remarkable reult acceptable and meaningful, we ued an ORTEC 42A preamplifier which give u the value of C belong to (0pF, 30pF), and the dicharge time τ = RC. Fig9: Equivalent circuit of the MICROMEGAS detector connected to the preamplifier The ignal obtained after amplification i calculated uing the above mounting and the variou equation that contribute to the evolution of the ignal v (t), the global formula of the ignal i: v t = n 0 q C dt W + te τ + τ t ion exp T w d t dt ion W + t ion t e τ d. T w exp t e t τ exp T w d t e t + dt W + t ion τ dt W + t ion τ exp t τ exp T d w t + exp t t ion dt W + te τ, 0 t t e < 0 τ (6) d + t ion τ t e ). exp t τ exp T d w t, t t e t t e + t ion ion exp ((T w The imulation of thi parameter allow u to do bet in the Functioning of MICROMEGAS and improving it performance mot reponding to ytem and put it in a privileged tate, in particular, the two main parameter: the patial and temporal reolution. For thi work, we apply the ame procedure that we have een in the temporal current imulation and that of the charge collected in the micro-meh taking into account all parameter and ga mixture reported above baed on the table, 2 and equation (6). We conider v i (t) (i =, 2, 3, 4) and v j (t) (j =, 2, 3, 4.5) the ignal obtained after amplification in micromeh, where each index i correpond to the percentage of the ga mixture Ar-iobutane (6, 0, 20, 30)%, and each index j correpond to the proportion of thoe of Ar-DME (6, 0, 5, 20, 30)%. The reult-baed Ar-iobutane and thoe of Ar-DME are hown in Figure 0. Page 682

13 Fig0: Preamplifier output ignal of MICROMEGAS for different proportion of Ar-iobutane(a) and Ar-DME(b) The Figure Ar-iobutane (a) and Ar-DME (b) repreent the evolution of the preamplifier ignal in a logarithmic cale to better clarify the evolution of the ignal for each percentage of the ga mixture and to facilitate the interpretation of temporal variation to recover remark accomplihed during thi imulation. CONCLUSION In um up, the charge induced by the electron i much larger for MICOMEGAS that in the cae of a multi-wire proportional chamber (MWPC). Thi property i related to the hape of the electric field. Indeed, in (MWPC), the electric field varie a ( / r). The majority of electron-ion pair are produced cloe to the fil.le electron travel a hort ditance before being collected by the anode and the charge induced by the electron i a very mall fraction of the total charge. In the cae of MICROMEGAS, the avalanche tart much cloer to the cathode due to the uniform electric field in the amplification gap. Thi property allow MICROMEGAS to obtain fater and more intene ignal. In thi paper, the imulated reult of the MICROMEGAS detector developed are preented. The reult indicate that our ytem with a micro-meh (500LPI) can reach rather good operational tate. However, uing 55 Fe radioactive 5.9 kev X-ray ource in an Ar-iobutane (%) and Ar- DME (%) ga mixture in normal condition. All the imulated reult obtained indicate that the performance of the detector depend on many parameter, which need to be optimized for a particular purpoe. In the ummary, the imulation reult are nearly conitent with the data that are publihed in other reference, and provide important information in the MICROMEGAS deign, making and operation. Our imulation predict that further improvement are till poible. We expect the ue of a 500LPI would give a bet patial and temporal reolution for MICROMEGAS detector. Page 683

14 REFERENCE [] G Charpak and al. The Ue of Multiwire Proportional Counter to Select and Localize Charged Particle. Nucl.Intr. and Meth. A62, P235, 968. [2] Roger Fourme. Poition-enitive ga detector: MWPC and their gifted decendant. Nucl. Intrument and Method in Phy. Re. A392:-, 997. [3] Y. Giomatari et al. Nuclear Intrument and Method, A376, 29, 996. [4] Y. Giomatari, Development and propect of the new Gaeou detector MICROMEGAS, Nucl. Intrum Meth. A49, 239, 998. [5] Sauli F. Nucl. Intrum. Method. A 477, -7, 2002 [6] G. Charpak et al. Nucl. Intrum. Method, A42, 47-60, 998. [7] Giomataria Y et al. Nucl. Intrum. Method. A376, 29-35, 996. [8] E. Durand, Electrotatic, Tome 2, Pari Maon (963). [9] R. Veenhof, GARFIELD program: imulation of gaeou detector, verion 6.32, CERN Program Library Pool W999 (W5050). Tranport Equation, Nucl. Intr. and Meth. 273, , 975. [0] S. A. Korff, Electron and Nuclear Counter, Van Notrand, Princeton (955). [] D. Mazed and M. Baaliouamer. A emi-microcopic derivation of ga gain formula for Proportional Counter.Nucl. Intr. and Meth. In Phy. Re. A437: , 999. [2] A. Zatawny. Standardization of ga amplification, decription in proportional Counter. Nucl. Intr. And Meth. In Phy. Re, A385, , 997. [3] Irene Pere: Kinetic modeling of electron and ion in the cylindrical proportional counter. PhD thei, PAUL SABATIER Univerity, TOULOUSE (990). [4] A. Sharma et F. Sauli, Nuclear Intrument and Method, A334, , 993. [5] M. PUIL Geal: Development of Micromega, a new ga detector for poition micromeh. PhD thei, Univerity of Caen. (2000). [6] H. Mounir, M. Haddad, S. Bri, International Journal of Science and Advanced Technology A 305, 89 96, 20. [7] Nuclear Intrument and Method in Phyic Reearch, A572, , Page 684