ARTICLE IN PRESS Nuclear Instruments and Methods in Physics Research A 518 (2004) 448–451 Imaging with the leak microstructures: preliminary results C. Baiocchia, G Balbinotb, A. Battistellaa, G. Galeazzic, F.S. Lombardid, M. Lombardia,*, G. Pretea, A. Simona a INFN Laboratori Nazionali di Legnaro, viale dell’Universita" 2, Legnaro, Padova, Italy b Italstructures, via Monte Misone 11/d, Riva del Garda, Trento, Italy c Universita" degli studi di Padova, Dipartimento di Fisica Galileo Galilei, Padova, Italy d Osservatorio Astronomico S. Lucia Stroncone, Terni, Italy Abstract We report on some images obtained with the Leak Microstructures (LM), which are elements for the detection of gas ionizing particles based on needle-point anodes. X-ray images of a mask with seven holes, 300 mm in diameter and 100 mm separated, drilled on a tantalum sheet were obtained with a very good spatial resolution. Varying the potential to the drifting electrode we put in evidence the possibility to perform a ‘‘zoom’’ of the image. X-ray images of a tantalum sheet of about 20
20 mm2 were obtained using a matrix of 9
9 LMs read by only 6 electronic chains (6 ADCs). r 2003 Elsevier B.V. All rights reserved. PACS: 29.40.Cs; 85.60.Gz; 87.59.e Keywords: Gaseous detector 1. Introduction We would like to remember that the Leak Microstructures (LM), widely reported elsewhere [1–7], are elements for the detection of gas ionizing particles based on the point of needles as anodes. For every single ionizing radiation detected they give a pair of ‘‘induced’’ charges of the same amount (pulses of the same amplitude), with opposite sign, the same collection time and, essentially, in time coincidence that are proportional to the primary ionization collected. Gas *Corresponding author. Osservatorio Astronomico S. Lucia Stroncone, Terni, Italy. Fax: +39-49641925. E-mail address: mariano.lombardi@lnl.infn.it (M. Lombardi). multiplication more than 105 with more than 108 electrons in the primary avalanches, working in proportional region, are possible [3]. The complete lack of insulating materials in the active volume of these microstructures avoids problems of charging-up and makes their behaviour stable and repeatable. In our attempts to get images with the LMs we had well in mind the works reported in Ref. [8,9]. 2. Imaging with alpha particles To begin with imaging using LM and to set in order our electronic set-up and data acquisition system we used an alpha particles source 241Am. 0168-9002/$ - see front matter r 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2003.11.043 ARTICLE IN PRESS C. Baiocchi et al. / Nuclear Instruments and Methods in Physics Research A 518 (2004) 448–451 449 Fig. 1. Electronic set-up. Fig. 3. Image of two wires. Fig. 2. Experimental set-up (not to scale). The electronic and experimental set-up used are reported in Figs. 1 and 2. The needle (anode) and the four pads (10
10 mm2 each) of the detector were each equipped with an inverting current preamplifier (input impedance Z ¼ 100 O, Vout =Vin ¼ 3 or 0.3 mV/mA) [3], followed by main amplifier (Silena 7612) and by ADC (Silena 4419/ V). A trigger pulse has been generated from anodic signal in order to start with the acquisition. The height of the point, to respect the plane of the coplanar pads, was 100 mm. For every detected particle there are five digital numbers that depend on the amplitude of the signals; four of these numbers must be elaborated with the charge-ratio method in order to calculate the avalanche position. The algorithm we used is the following: PX ¼ X1  X2 ; X1 þ X2 PY ¼ Y1  Y2 ; Y1 þ Y2 where X1 ; X2 ; Y1 ; Y2 are the four ‘‘induced’’ charges (pulses) on the pads. After having equalized the five electronic chains we got our first data with the experimental set up reported in Fig. 2. Two copper wires, set at Vdrift ; 1 mm in diameter and 1 mm spaced, were placed between the detector and the source at 2 mm from pads. Fig. 3 reports the distribution of data (4110 detected alpha particles) on XY plane when VLM was 1050 V and Vdrift 300 V in 760 Torr of isobutane. Alpha particles from the extended 241 Am source that hit wires are certainly adsorbed while others, which ionize the gas under them, make a map of electric field and so of the wires. These results permit to conclude that it is possible to make images with a needle and four coplanar pads, that is a LM, with a spatial resolution which is certainly better than 1 mm. Considering that the distance between the two axes of the wires is 2 mm, the field of vision results to be of several millimetres in diameter. 3. Imaging with a 55 Fe source The experimental set-up consisted in a window drift electrode set at 3 mm from the pads and above it a tantalum sheet, thickness 0.2 mm, on which seven holes 300 mm in diameter and separated by 100 mm were drilled, was leaned as mask. Above this mask was set the 55Fe source. The LM used and the electronic set-up were still the same of Fig. 1. Because the source we used is punctiform, to reduce parallax error, we put it at 1 mm above the mask. ARTICLE IN PRESS 450 C. Baiocchi et al. / Nuclear Instruments and Methods in Physics Research A 518 (2004) 448–451 Fig. 4. Zoom effect due to different drifting electric field; starting from left, the drifting electrode was at 50, 100, 200 and 500 V, respectively. The working conditions were VLM 1150 V in 760 Torr of isobutane. Fig. 4 reports results when Vdrift was 50, 100, 200 and 500 V respectively and the other working conditions were the same for all the images. The evaluated mean spatial resolution across the edges of images is 118 mm FWHM. It should be noted that increasing the drifting electric field a zoom effect is obtainable. Fig. 5. Electronic set-up to get the image of Fig. 6. 4. Imaging with a X-ray generator With a printed board for 256 LMs, reported in Ref. [7], we started to make images with an extended X-ray generator. Each LM of this board has 4 pads, two for the X position (left and right) and two for the Y position (Up and Down). We found that, connecting (electrically) together all the 256 Left pads, as well as the 256 Right, Up and Down pads to be all read with only four channels (4 Peamp., 4 Main Amp. and 4 ADCs) and making again the image of Fig. 4, the mean spatial resolution was still of the order of 100 mm FWHM. To perform a two-dimensional image, with a multi-LM matrix, it remained to read the anodepoints one by one and to give them an address. This was accomplished reading each point through two diodes, Fig. 5, one for the X position and one for the Y ; supplying two delay lines. In this manner it was enough to use only six electronic chains, four for the pads as said before and another two to read the two time-to-amplitude converters for the addresses, to get the image of Fig. 6. This was achieved in 760 Torr of isobutane (C4H10), at VLM ¼ 1900 V (this voltage is common to all the points, diodes and delay lines) when to Fig. 6. Shadowgram of a tantalum sheet of about 20
20 mm2. the window drift electrode, 3 mm above the pads, was applied 200 V. In this image, in which there is the shadowgram of a sheet of tantalum of about 20
20 mm2 and 300 mm thick, only a matrix of 81 LMs were instrumented. The image was taken using the primary beam of a tungsten-anode X-ray generator setting the HV at 20 kV. In this figure 81 domains (9
9 LMs) are well distinguishable. The pitch of the points was 5.08 mm. Unfortunately we were not able to light the points in such a manner that they would have a field of vision of 5.08
5.08 mm2 but only about 3
3 mm2. To ARTICLE IN PRESS C. Baiocchi et al. / Nuclear Instruments and Methods in Physics Research A 518 (2004) 448–451 overcome this problem now we are evaluating the proper pitch to be used to obtain a continuous image. References [1] M. Lombardi, et al., Proceedings of the International Conference-Brolo (Messina) 15–19 Oct., World Scientific, Singapore, 1996, pp. 459–465. [2] M. Lombardi, F.S. Lombardi, Nucl. Instr. and Meth. A 392 (1997) 23. 451 [3] M. Lombardi, et al., Nucl. Instr. and Meth. A 388 (1997) 186. [4] M. Lombardi, et al., Nucl. Instr. and Meth. A 409 (1998) 65. [5] M. Lombardi, et al., Nucl. Instr. and Meth. A 461 (2001) 91. [6] M. Lombardi, et al., PRAMANA (Indian Acad. Sci.) 57 (1) (2001) 115. [7] M. Lombardi, et al., Nucl. Instr. and Meth. A 477 (2002) 64. [8] J.E. Bateman, Nucl. Instr. and Meth. A 240 (1985) 177. [9] M. Lampton, R.F. Malina, Rev. Sci. Instrum. 47 (11) (1976) 1360.