A Time Projection Chamber with Optical Readout for Charged Particle Track Structure Imaging

A time projection chamber with optical readout for charged particle track structure imaging

U.Titt*+ , A.Breskin#, R.Chechik#, V.Dangendorf *, H.Schmidt-Böcking+, H.Schuhmacher* * Physikalisch-Technische Bundesanstalt, 38116 Braunschweig, Germany + Universität Frankfurt, 60486 Frankfurt, Germany # Weizmann Institute of Science, 76100 Rehovot, Israel

Abstract We report about a nuclear track imaging system which is designed to study in detail the ionization topology of charged particle tracks in a low-pressure gas. The detection method is based on a time projection chamber (TPC) filled with low-pressure triethylamine (TEA). Ionization electrons produced by energetic charged particles are three-dimensionally imaged by recording light from electron avalanches with an intensified CCD system. The detector permits to inves- tigate the spatial ionization distributions of particle tracks in gas, of equivalent length and resolution in tissue of 4 mm and 40 nm (RMS), respectively. We explain the relevance of this technique for dosimetry, describe the experimental method and the basic operation parameters. First results of the chamber response to protons and alpha particles are presented.

Keywords: microdosimetry, track imaging, optical read out, TPC, triethylamine, scintillation PACS codes: 8760M, 2940K, 2940M

Corresponding author: Volker Dangendorf email: volker.dangendorf @ ptb.de Physikalisch-Technische Bundesanstalt Tel: ++49 (0) 531 592 7525 Bundesallee 100 Fax: ++49 (0) 531 592 7015 38176 Braunschweig

1 1. Introduction or the use of a low-pressure tissue-equivalent pro- Microscopic information about the ionization den- portional counter (TEPC). The latter is able to sity along tracks of charged particles is required in simulate a tissue volume of about 1 mm in diameter various areas. Examples are microdosimetry, e.g. and to measure a quantity which is closely related for understanding radiation-induced effects in to the LET as long as the charged particles have organisms and semiconductor material, and radia- ranges of at least several micrometers in tissue. tion protection dosimetry. A generally accepted However, for short-ranged charged particles, e.g. model which can predict radiobiological effects those produced by low-energy neutrons, dose from the physics of radiation interaction does not equivalent is significantly underestimated [3]. exist to date. The models under discussion are based on microscopic information about radiation The particle track chamber may address these interaction with cells and relevant structures problems in various ways. On the one hand, the therein and therefore require interaction data down particle track chamber allows to identify the type to the nanometer level (examples are restricted and energy of charged particles produced in its linear energy transfer, energy imparted, proximity wall and gas and therefore enables the analysis of functions). These data are commonly provided by the primary radiation field. It also allows to meas- track structure calculations which have to rely on ure the neutron energy distribution directly, using a very scarce and poorly known atomic and molecu- 3He-gas filling. On the other hand, the chamber lar cross sections. allows a more accurate measurement of LET than a TEPC because the above-mentioned restrictions of In general, experimental methods for particle track a TEPC do not apply to the track chamber. analysis are based on the simulation of a microscopic volume in a solid or liquid by replacing it by In recent years, several attempts to apply charged a much larger cavity filled with gas of much lower particle track imaging in dosimetry, microdosime- density. Although the interaction data determined try and neutron spectroscopy were reported. Turner in gases might not directly be relevant for radiation et al. [4 - 6] employed a method, where the ioni- effects induced in liquids or solids, they represent zation electrons along a particle track are acceler- important benchmarks for track structure calcula- ated by a damped high voltage radio frequency and tions. Hence, any experimental method able to cause scintillation light. The scintillation is measure interaction data down to the nanometer viewed by two cameras from orthogonal sides and scale contributes to the understanding of radiation assembled to a three-dimensional picture. The action in matter. advantage of this method is that the ionization electrons produce their light immediately at the In radiation protection dosimetry the present con- place of their generation, thus avoiding possible cept is based on "protection quantities", used to inaccuracies due to electron diffusion. However, estimate the risk of an irradiated person, and due to the three-dimensional extension of the "operational quantities", used for measurements in charge distribution in the sensitive volume, the health physics [1, 2]. The definition of operational optical system requires a large depth of field quantities, which serve as conservative estimates (possibly several centimeters), and compromises for the protection quantities in the daily routine, between resolution and light sensitivity cannot be implies specific irradiation conditions with respect avoided. to the radiation field and to the receptor, i.e. the body or a so-called phantom. It takes into account Another approach was reported by Marshall et al. the biological effectiveness of the different types of [7,8]. Their method is based on the well-known radiation by quality factors which are defined as a cloud chamber technique to visualize particle function of the linear energy transfer, LET, of tracks in water vapour. An excellent spatial reso- charged particles at a position inside the phantom. lution of less than 5 nm (scaled to materials of a In contrast, the protection quantities are to be de- density of 1g/cm3) was obtained, making this termined for the irradiated person and the biologi- method the first choice in fundamental microdo- cal effectiveness of different types of radiation and simetric measurements, where high rate and real the radiation sensitivity of different organs is taken time capability are not required. The drawback of into account by radiation weighting factors this method is the extremely long duty cycle due to (defined as a function of the type and energy of the the long recovery time of the chamber. radiation outside the body) and tissue weighting More recent approaches have been reported by factors, respectively. Breskin et al, electronically recording the trails of No methods exist to date for determining these individual charges deposited by particles in low- dosimetric quantities directly according to their pressure gases. These methods comprise single definition. Instead, different and, at least partly, electron counters [9-11], and single ion detection complicated methods are employed. Examples are techniques [12]. The latter method should permit radiation field analysis by spectrometric methods

2 to measure equivalent resolutions in tissue of the locally deposited into the chamber gas. Moreover, order of less than one nanometer. the steady-field operation enables the optical time projection technique [17] to be used because the In the present work we will describe the concept, time development of the light pulse is always the instrumental setup and first experimental re- proportional to the amount of electrons arriving at sults of a newly designed OPtical Avalanche the amplification structure in a certain time inter- Chamber (OPAC) which will be applied as particle val. In the optical time projection chamber the track imaging device for dosimetric applica- light emission always occurs in the same layer. tions[13]. It is based on a time projection chamber Therefore no depth of field of the optical system is with a three-dimensional optical imaging capabil- required, and the maximum available optical ap- ity of particle-induced ionization patterns, by re- erture can be used. In addition, one obtains spatial cording the light emitted from electron avalanches information about the charge distribution perpen- with an intensified CCD camera. dicular to the amplification plane (see fig. 1. and chapter 3 for details of the instrumental setup). We report on the evaluation of the basic operation parameters such as: A key element in the optical readout technique is a · light and charge production in low-pressure gas mixture with sufficient light yield in the visible triethylamine (TEA) in parallel gap gas ampli- or near UV range to make the avalanche detect- fication structures, able with the IICCD. It was shown [18-21] that gas · single electron detection efficiencies under mixtures containing tetrakis[dimethyl- various amplification conditions, amino]ethylene (TMAE) and triethylamine (TEA) · drift and diffusion of electrons in low-pressure vapours enable operation of parallel grid chambers TEA. with large light yield in the proportional mode. Finally, preliminary results of the chamber re- TMAE-based gas mixtures have the advantage of sponse to well-defined particle irradiation are pre- emitting visible light at a peak wavelength of l = sented. 480 nm [22]. However, TMAE handling is rather delicate since its vapour pressure is small and TMAE partial pressure above several hPa usually 2. Concept requires heating of the detector and its gas supply Previous works in the field of optically read out facilities [23]. Furthermore, due to photon feed- avalanche counters were reported in the 1980s by back, the diameter of a single electron avalanche CERN and Weizmann Institute groups (see e.g. in low-pressure TMAE mixtures extends to a few [14,15] and related references therein). As op- mm [15]. TEA, on the other hand, emits in the posed to the conventional electronic detector read- near UV (peak emission at l = 280 nm [19, 22]) out, the optical method focuses on the capture of which can still be comfortably handled with regu- the UV or visible light emission, which in some lar quartz and Al-mirror optics. The light yield of gases copiously accompanies the formation of electron avalanches in TEA mixtures is about 10 charge avalanches. The successful implementation times larger than with TMAE and the single elec- of this readout scheme was facilitated by the avail- tron spot size is below 1 mm even at 10 hPa pres- ability of gateable, image-intensified CCD cameras sure (see below). Its vapour pressure at normal (IICCD) at moderate costs. temperature is sufficiently high (approximately 120 hPa at 300 K [20]) to operate the detector and the gas supply system at room temperature. For low Compared to electronic readout, the optical method pressure operation (up to 60 hPa), commercial has some advantages: techniques can be used to control the TEA flow · It offers low cost, high resolution, genuine two- and pressure. The IICCD readout provides the dimensional multi-hit readout capability. two-dimensional images of the electron distribu- · It is insensitive to electronic noise or RF-pickup tion. In fact, only the projection of the ionization signals in the detector environment. track onto the plane between the amplification · With properly chosen gases mesh electrodes grids is recorded. As was proposed by Charpak et can be applied instead of wires, which makes al [14], and later systematically developed by the chamber simpler and more rugged. Breskin et al [9-11], an additional spatial infor- The development of suitable gas mixtures emitting mation can be obtained by recording the temporal light in the near UV range and having a rather flow of charge into the amplification region. The large light yield per avalanche electron made it known drift velocity in the interaction volume of possible to operate those chambers not only in the the chamber and the measurement of the arrival saturated pulse mode [16] but also in a steady-field, times of electron clusters relative to an event trig- charge-proportional mode. Only in the latter op- ger provides the position of the cluster origin in the eration mode does the registered light pattern con- coordinate axis perpendicular to the CCD image tain the information about the ionization energy plane. In the proportional amplification area, this

3 charge flow is transformed into a time-dependent and the drift anode (DA) consist of stainless steel light emission with an intensity proportional to the wire meshes of 81% optical transmission. The charge. Thus, three-dimensional localization of parallel drift field is shaped by a series of guard each electron or electron cluster is achieved by electrodes made of stainless steel rings. combining the CCD-image and the time-resolved information of the light emission e.g. with pho- From the drift region, which defines the sensitive tomultiplier tubes [17]. volume of the chamber, the charges enter an amplification zone. The detector has a two-step ho- mogenous electric field amplification system, con- 3. Experimental Setup sisting of a stack of grids arranged at a spacing of 3.1 The detector 3.2 mm. Electrons which drift into the amplifica- The OPAC consists of a parallel-field drift cham- tion region are multiplied in a first proportional ber of 10 cm in diameter and length. It is charge amplification stage between DA and G1. mounted in a stainless steel vessel with a quartz This stage is followed by a light amplification window light-output port close to a parallel plate region, where a rather low reduced electric field of -1 -1 light amplification region. The housing provides E/p » 65 Vcm hPa , applied between G1 and G2, three beam ports to allow controlled irradiation of allows proportional scintillation induced by the preamplified electron swarm [23]. DC Uc

3.2 The Optical readout system e- charged With the technique of a parallel drift field and particle double-step charge and light amplification struc- E ture, a two-dimensional image of the track is pro- charge amplification jected onto the mesh of the light amplification DA stage. This image is recorded by a UV-sensitive optical imaging system, consisting of three main G 1 U A1 parts: G 2 U A2 A UV lens focuses the scintillation light pattern light onto the photocathode of an image-intensified hn amplification CCD camera (IICCD). The lens is an Al mirror lens (NYE, Lyman alpha) with a focal length f = 90 mm and an aperture of F = 1.1 . Despite the PM UV - lens disadvantage of a pillow-like distortion of the image by this lens it was selected due to its large F photo cathode value. With an object of 90 mm in diameter, cor- Image MCP responding to the sensitive part of the chamber, Intensified phosphor screen and an image size of 25 mm, an effective solid CCD Camera . -3 CCD angle of DW/4p = 2.44 10 is calculated. The conversion of UV light into photoelectrons in Fig.1 Schematic view of the OPtical Avalanche the image intensifier (Proxifier BV 2562 BcZ, [24]) Chamber (OPAC) with drift cathode (DC), drift is made by a bialkali photocathode, 25 mm in anode (DA) and amplification grids G1 and G2, diameter. A bialkali photocathode was chosen due and its optical readout elements UV lens, photo- to its lower thermal noise as compared to the usual cathode, multi-channelF igplate. 1 (MCP), charge- multialkali photocathodes. coupled device (CCD) and photomultiplier (PM). In the first setup only single step amplification The photocathode can be pulsed with short (down between DA and G1 was used. To achieve higher to 10 ns) high-voltage pulses. This allows con- light output a second amplification stage between trolled and very short exposure times of the CCD G1 and G2 was implemented. and avoids accumulation of noise as well as multi- ple exposures of the CCD. The electrons from the photocathode are multiplied by a single-step multi- the sensitive volume with charged particles at an- channel plate (MCP, BV2562QG) and accelerated gles of 90°, 45° and 0° with respect to the image in a parallel electric field. The accelerated elec- projection plane. The volume is filled with the trons hit a P-43 phosphor screen which emits vapour of (H5C2 )3 N (TEA) at a pressure of 10 hPa. visible light with a decay time of 1 ms. The Charged particles crossing this volume produce maximum light amplification of the image inten- free electrons along their path, which drift towards sifier is 2 . 105 . The phosphor is coupled by a coni- the amplification stage under a homogeneous cal fiber-optical faceplate to the CCD of a standard electric field (see fig. 1). The drift cathode (DC) slow-scan camera (EG&G, M4005).

4 The spatial resolution of the optical system is particles at different energies is studied to develop quoted to be 38 line pairs per mm, which trans- the data reduction algorithms for items 3, 5 and 6. forms to a resolution of 10.5 line pairs per mm in the object plane. This resolution does not limit the resolution of the instrument as larger effects are 3.4. The gas supply system contributed by the electron transport in the gas The OPAC has been operated at 10 hPa TEA in a (see chapter 4.2). proportional scintillation mode. To prevent deterio- ration of the gas quality due to leakes, outgassing To avoid the accumulation of dark noise between of detector materials and avalanche-induced gas two events, the CCD is continuously cleared after ageing the detector was continuously supplied with the readout of an event until the trigger of a new fresh gas. A dedicated gas supply system was de- event occurs. With this technique the noise induced veloped for this purpose and is schematically by the CCD dark current is negligible. When re- shown in fig. 2. It consists of a glass reservoir for ceiving an event trigger, the photocathode is pulsed the liquid TEA with the possibility of bubbling with a high-voltage pulse ( 200 V ) of about 3 ms carrier gases through the liquid. duration, corresponding to the maximum possible drift duration of the electrons from a particle track.

The secondary light emission as a function of time contains the spatial information of the ionization density distribution perpendicular to the drift projection plane. This light transient is observed by six photomultipliers (PHILIPS XP2018QB) whose anode outputs are connected in three groups. By analysing the time dependent signal ratios of the photomultipliers a rough information on the position in the projection plane can be derived. With this method the ambiguity in the sign of the angle between the particle track and the image projection plane is removed.

3.3. The data acquisition and analysis system The CCD images are transferred to a data acquisition system via an 8 bit variable scan frame- grab- ber card. The maximum frame rate of the present system is restricted by the camera to 15 s-1 . The anode signals of the photomultipliers are recorded with a digitizing oscilloscope and the data is transferred to a computer. Fig.2 Schematic drawing of the gas supply system. It is equipped with 3 gas flow controllers The data acquisition and analysis system has to (MFC) for producing gas mixtures of up to 4 com- perform the following tasks: ponents for approximating the atomic composi- 1. collection of the digitized CCD and photomul- tion of tissue (TE gas). tiplier data, 2. optional storage and display of both data sets, In this operating mode, the partial pressure of 3. reconstruction of the three-dimensional ioniza- TEA can be controlled by adjusting the tempera- tion distributions, ture of the TEA bubbler. However, for low- 4. track detection and histogramming of longitu- pressure operation (below 50 hPa) of the detector, dinal and lateral ionization density profiles, the TEA partial pressure can also be controlled by 5. identification of particle type and energy, and standard mass flow controllers (MFC, MKS generation of frequency distributions, 1259CC). For mixtures of TEA with other gases, 6. extraction of absorbed dose and track-structure two additional MFCs are available. This option data. will be required in the future for using tissue- In the current version of the acquisition system, equivalent (TE) gases. Pressure control is realized items 1, 2 and 4 have been implemented. Thus we by measuring the chamber pressure with a ca- are able to collect and store the complete three- pacitive pressure gauge (MKS 622) and a propor- dimensional information obtained by the sensor. tional valve (MKS 248) mounted behind the de- Presently, the response of the OPAC to various tector, downstream to the pumping line. To avoid poisoning of the pump oil and the environment

5 with TEA vapours and to prevent backdiffusion of compare the performance of pure low-pressure oil vapours into the detector a liquid nitrogen trap TEA with that of other gas mixtures and to find is mounted between the control valve and the suitable operation parameters, the ratio of light to roughing pump. charge (photons per avalanche electron) was measured. Details on the measurement procedure can be The operating gas pressure (p) of 10 hPa was cho- found in [13]. sen in order to achieve good position resolution as well as stable operation conditions at high 107 charge and light gain. The position resolution at tissue density (sh) is g determined by the scaling factor of the gas density q . -3 (rg) to tissue density (rh = 1 g cm ) and the position resolution of the detector (sg):

rg s h = s g × 6 rh L 10 y

Due to the proportionality of rg and gas pressure , q p, one should aim for the lowest possible pressure g in order to get maximum resolution. However, the y L resolution obtainable in the gas is limited by charge diffusion during the drift process and varies -1/2 like sg µ p . Therefore, the resolution sh obtain- 1/2 able in tissue varies as s µ p , and one should 5 h 10 aim at the lowest possible pressure in order to get the best resolution. 700 720 740 High gain operation of low pressure isobutane- U / V and methane-filled detectors was investigated by A1 Breskin et al [25, 26]. It was shown that at pres- Fig.3 Charge amplification gq and photon yield yL sures below about 10 -15 hPa the maximum attain- in a single 3.2 mm parallel amplification gap as a able gain drops due to feedback effects like photon- function of the applied Fig.3voltage UA1 , measured at and ion-induced secondary avalanches. We inves- 12 hPa TEA. The avalanche was induced by single tigated the maximum attainable gain for TEA in a photoelectrons pressure range between 5 and 20 hPa. For single primary electrons stable operating conditions at a gain of up to 106 in a single parallel gap amplifi- Fig. 3 shows the total photon yield per initial elec- cations stage was achieved at a pressure around 10 tron (yL), and the charge gain (gq) , as a function of hPa. Hence, this pressure was chosen as a reason- the applied voltage in a single parallel-plate am- able compromise between the desired resolution plification gap of 3.2 mm width in 12 hPa TEA. and sufficient amplification. TEA has a vapour Fig. 4 presents the light-per-charge yield as func- pressure of about 120 hPa at 298 K which is suffi- tion of the TEA pressure at two fixed reduced cient for operating the chamber at 10 hPa with the electric fields. It should be noticed that the light- mass flow controller and without the necessity of per-charge ratio is higher for lower reduced electric heating the gas supply components to prevent con- fields. This can be explained by the fact that, with densation. The scaling factor to 1 g . cm-3, which increasing field the average collision energy of the electrons with TEA molecules increases. With can be achieved at 10 hPa TEA (rg = 41.7 3 . 4 increasing collision energy the ionization prob- mg/cm ), is about 2.5 10 . Hence a resolution of ability of the TEA molecules increases more rap- 1 mm in the detector scales down to about 40 nm idly than the excitation probability [23]. Excitation in tissue. of TEA molecules by electron collisions is the main cause for the scintillation process. 4. Results and Discussion Along the central part of a non relativistic heavy 4.1. Charge and light amplification ion track the ionization density is generally suffi- In the past, only mixtures of noble gases and hy- ciently high so that even at a moderate light gain drocarbons with small concentrations of TEA were of the avalanche chamber the optical system is able investigated and applied [14,15]. For our work the to detect the track. However, tracks of minimum total light and charge yield in pure low-pressure ionizing particles or of fast delta electrons at some TEA had therefore to be investigated. In order to

6 source of slow electrons in the single- and double- 1,0 step amplification modes.

-1 -1 E/p = 124 V cm hPa To measure the drift parameters (drift velocity and E/p = 146 V cm-1 hPa-1 0,8 lateral diffusion) of single electrons, free electrons were ejected from a small spot on the drift cathode. The electrons were produced by a 3 ns long pulsed UV laser beam (nitrogen laser, emission wave- 0,6 length l = 337 nm) hitting a small aperture of 0.1 mm in diameter in an aluminium drift cathode.

The UV-light emission of the laser was registered 0,4 in the PM and used as time reference in all measurements. The photoelectrons produced by the laser in the Al wall of the aperture were transferred by photons per electron an electric guidance field into the drift zone. To 0,2 assure single electron injection, the laser intensity and the guidance field were adjusted such that only one out of ten UV pulses resulted in a free electron, visible as a pulse in the detector readout electron- 0,0 16 18 20 22 24 26 28 30 ics. p / hPa

Fig.4 Fig.4 Dependence of light per charge ratio on the TEA pressure in a single-step parallel gap of 20 3.2 mm in width at two fixed reduced electric fields 10 distance from the core of a heavy ion track mainly 8 consist of separated single electrons. A compari- 6 son of the experimental data with Monte Carlo simulations [13] showed that with a single amplification step the obtainable photon yield would not 4 be sufficient for efficient detection of these single electrons (see also chapter 4.5). Therefore a second amplification stage was implemented, which, de- 2 pending on the ionization density of the particles investigated, was operated at a reduced electric -1 -1

field between 22 and 81 Vcm hPa . Due to this Light output amplification factor element the light output could be augmented by a 1 factor of up to 20. Fig. 5 shows the light amplifi- 0,8 cation of the second 3.2 mm parallel gap as a 0 50 100 150 200 250 function of the applied voltage, measured in 10 hPa UA2 / V TEA. Fig.5 Dependence of lightFig.5 yield on the applied- voltage UA2 in the second parallel field-propor- 4.2. Electron drift in low-pressure TEA tional scintillation stage 3.2 mm in width in TEA The electron transport parameters in the drift cell at 10 hPa pressure determine the obtainable localization resolution of the detector. The transverse diffusion mainly a) Drift velocity causes the uncertainty in the position measurement The drift time as a function of the drift field of the projected image in the CCD object plane. strength was determined by measuring the delay Along the drift direction the localization measure- between the UV laser pulse in the PM and the ment is mainly influenced by the transition time of secondary light emission induced by the avalanche electrons through the second light amplification electrons in the amplification stage, for variable gap and by the longitudinal diffusion. Since no reduced electric drift field strength values. The data are available for the electron transport pa- pressure was kept at 10 hPa in all measurements. rameters in TEA we measured the drift velocity, The resulting electron drift velocity is shown in fig. the transverse and longitudinal diffusion and the 6. In the relevant E/p-range, the drift velocity time width of the light burst induced by a point varies almost linearly with E/p, with two different

7 slopes. Even at the highest field no saturation of the drift velocity is observed. The maximum ap- 1,0 plied field was limited here by spurious discharges within the detector. pressure of TEA : 10 hPa 0,8 drift length : 10 cm

5,6

0,6 5,4 sT

5,2 / cm T, P -1

s 0,4 s m 5,0 sP 0,2 / cm 4,8

Drift thermal limit v 4,6 0,0 0 2 4 6 8 10 12 14 16 4,4 E p-1 / V cm-1 hPa-1

Fig.7 Transverse(sT) andFig.7 projected (sP) diffusion 4,2 of electrons in 10 hPa TEA for 10 cm drift length 4 6 8 10 12 14 16 18 as function of the reduced electric field strength. Fig.6 Drift velocityE/p /of Velectrons cm-1 hPa in-1 10 hPa TEA as a function of the reducedFig.6 drift field strength. c) Longitudinal diffusion and time spread of light emission The spatial resolution in the direction parallel to b) Transverse diffusion (sT) To measure lateral diffusion, the images of single the drift field lines depends on the longitudinal electron induced scintillation in the amplification diffusion and on the time spread of the light emis- region were recorded with the IICCD. For different sion. To determine the two contributions sepa- drift field values the centre of gravity of each single rately, the width of the light and charge pulses electron spot in a series of several thousand events from the UV laser induced avalanches were measured. The laser was positioned in front of the were calculated. The standard deviation (s ) of the T quartz window in such a way, that an intense laser transverse diffusion for 10 cm drift in 10 hPa TEA beam hits the amplification grids and the drift is shown in fig. 7 as a function of the reduced drift cathode and causes simultaneously the emission of field strength. The transverse diffusion reaches a an electron cluster from each electrode. Hence, the flat minimum at a reduced electric field of about 5 -1 -1 charge signal, collected at G , provides two tempo- Vcm hPa . This corresponds to a resolution of 2 ral components: an instantaneous one caused by about 120 nm in tissue for an electron drift path the photoelectrons from the first amplification length of 10 cm. For dosimetric applications and grids, and a delayed one, caused by the photoelec- particle identification, this is a sufficiently good trons from the drift cathode, 10 cm away from the value. In microdosimetry, however, only resolu- amplification structure. The first pulse contains tions of a few tens of nm would constitute a sig- only the diffusion and other fluctuation effects of nificant improvement compared to existing tech- the avalanche formation while the time spread of nologies like TEPCs. A reduction of the drift the delayed pulse is additionally influenced by the length to 1 cm could improve the obtainable reso- longitudinal diffusion along the homogeneous drift lution in gas to 1 mm, corresponding to about 40 space. Thus, the longitudinal diffusion in time is nm in tissue density. derived from these measurements by quadratic subtraction of the width of the two components of The measurement of the lateral width of a particle the charge signal trail following each laser shot. track is determined by the vector component of the The contribution of the width of the laser pulse transversal diffusion, which points into the direc- (3ns) can be neglected. The geometrical longitudi- tion perpendicular to the particles’ flight direction nal diffusion is calculated using the measured drift and perpendicular to the drift field lines. This velocities (see section 3a). Fig. 8 shows the longi- component of the transversal diffusion is plotted as tudinal diffusion (sL) for various drift field values. sP in fig. 7.

8 0,28 a) 0,26 pressure of TEA : 10 hPa 100 drift length : 10 cm 0,24

0,22 10 / cm L 0,20 s

number of counts 1 0,18

0,16 0,1 0 1 2 3 4 5 0,14 CCD Integral (x 1000) 0 2 4 6 8 10 Fig.9 a E p -1 / V cm-1 hPa-1 30 Fig.8 Longitudinal diffusionFig.8 of electrons in 10 Noise b) hPa TEA for 10 cm drift length as function of the reduced electric field strength. 20

The combination of longitudinal diffusion and the time structure of the avalanche formation deter- mines the spatial resolution in the drift direction. For a 10 cm long drift path at a typically applied 10

-1 -1 number of counts reduced drift field of about 5.5 V cm hPa , an 4.4. Single electron detection efficiency RMS time spread of 60 ns [± 10 %] was measured. With a drift velocity of 4.4 cm ms-1, this leads to an overall spatial uncertainty of 2.7 mm in the drift direction, which corresponds to 112 nm at tissue 0 0 50 100 150 200 250 density. Pulse height / arb. units Fig.9 Light (a) and chargeFig.9b (b) pulse height distri- 4.3. Single electron spot size butions of avalanches from single primary elec- An advantage of TEA as a scintillating vapour is trons in 10 hPa TEA at UA1 = 530 V in the first its relatively large absorption efficiency for ava- gap and UA2 = 844 V in the second gap. lanche induced photons. These photons are isotropically emitted from the avalanche, giving rise to secondary avalanches surrounding the primary avalanche core and blurring the primary fusion, i.e. with single electrons photo-ionized by a avalanche image. This is the main cause for the UV-laser pulse from an Al photocathode. The relatively large lateral size of a single electron photocathode of the IICCD was gated by the signal avalanche spot in the CCD image when using of the laser pulse, recorded with the photomultipli- TMAE at room temperature [15]. We measured ers. the diameter of single electron spots in low- pressure TEA operated detectors with a single step The reference for the occurrence of a single elec- amplification system which resulted in a width of tron emission from the photocathode was provided about 0.4 mm (fwhm). by the electronic signal induced on the amplification anode. Fig. 9 shows light (9a) and charge (9b) pulse height distributions. The charge pulses are 4.4. Single electron detection efficiency clearly separated from the noise, thus a detection The detection efficiency for single electrons was efficiency of close to 100 % in the charge channel measured with a setup similar to that used for can be assumed. Therefore we can compare the measuring the drift velocity and the transverse dif- number of detected secondary light pulses through

9 the IICCD with the number of charge pulses above Most of these electrons are located around the the noise threshold. central core of the track of less than 1 millimeter lateral width. Therefore, in this central part of the track the recorded light pattern is composed of 100 many overlapping single electron avalanches which cannot be individually resolved by our system. Only at some distance from the track core, where the ionization pattern is determined by few sparsely ionizing d-electrons, single electrons can be resolved. 90 Fig. 12a shows the lateral projection of the ionization distribution of the image of fig. 11a. With one amplification step the detection efficiency for avalanches originating from single electrons is Efficiency / % 80 very low, as was explained in section 4.1. Only double-step amplification provides close to 100 % single electron detection efficiency. For comparison fig. 12b and 12c show the lateral and longitudinal projections of the event in fig. 11b with double step amplification. In 12b the "shoulders" 70 produced by fast delta electrons can clearly be 0 10 20 30 40 50 observed. The decreasing sensitivity towards the CCD threshold edge of the image, which is seen in the figures 11 and 12c is due to the distortion caused by the UV Fig.10 Fig.10 Single electron detection efficiency of the lens (see section 3.2). Nevertheless, from the un- optical system (p=10 hPa, UA1=530 V,UA2=844 distorted central part of the image it is obvious that V). The CCD threshold relates to the sum of all even for densely ionizing ions like a-particles the pixel contents. If a threshold value of 20 stochastic nature of energy deposition along the (compared to the maximum single pixel value of central particle track is still visible in a low-density medium. The ratio of the number of events detected optically to the number detected electronically directly gives We are presently investigating the response of the the single-electron detection efficiency of the OPAC to particles of known type and energy. OPAC at given amplification parameters. Fig. 10 Data for protons, a-particles, nitrogen, argon and shows the measured results as a function of the krypton ions with energies from 500 keV/u to 29 CCD threshold. The pixel sum (sum of the content MeV/u were obtained at different incidence angles. of all pixels) of the image intensifier and CCD for Fig. 13 shows some examples of particle tracks events induced by noise at normal temperature is with particularly interesting ionisation patterns due below 20 counts (at a lower threshold for the CCD to fast secondary particles (d-electrons, recoil ions). of 13 mV at 2 V full range ). Therefore, above a These particles are ejected by the primary ion and threshold of the CCD pixel sum of 20 counts the produce themselves extended ionization tracks. images do not contain noise while the single elec- For all measurements we recorded and stored the tron detection efficiency approaches 87 % . complete three-dimensional track-structure information for each event (about 300 kB per event) in order to analyse these data offline. In the first step 4.5 Track imaging results we will try to extract from these data the relevant Images of a-particle tracks from a 241Am source parameters for particle type and energy identifica- were recorded with the OPAC. a-particles crossed tion. The goal is to develop fast online data com- the drift volume perpendicular to the drift field pression algorithms which will enable the extrac- lines at a distance of 5 cm to the charge amplification and histogramming of the relevant data on- tion gap. A Si(Li) detector of 200 mm2 sensitive line. area, mounted on the opposite side of the drift cell, was used as an active collimator and as a trigger for data acquisition. Fig. 11 shows examples of a- 5. Conclusion and Outlook particle tracks obtained with the OPAC in a sin- In this article we have presented the basic design gle- (11a) and in a double-step (11b) amplification and operation parameters of the optical avalanche modes. 5 MeV a-particles produce about 800 elec- chamber (OPAC) - a particle track imaging device trons per centimeter path length in 10 hPa TEA. for dosimetry and dosimetry-related research. The device is able to measure the microscopic pattern of

10 a) b) t u

1cm

Fig.11 CCD image of a-particles in 10 hPa TEA with the OPAC obtained by a) single-step charge and light amplification at an amplification voltage of UA1 = 600V st b) double-step charge and light amplification.Operation parameters: 1 step UA1 = 600 V . 5 nd charge gain: gq= 5 10 , 2 proportional scintillation stage: UA2 = 800 V, . 5 light yield: yL = 2.5 10 photons).

charge deposition in its gas medium and will provide a variety of physical data on which models of radiation action on biological entities are based.

The spatial resolution (pixel size) of about 3 mm by 3 mm in 10 hPa TEA corresponds to a pixel of 120 nm edge size at tissue density in a total sensitive volume of 4 mm3. The position resolution can be improved at the expense of the size of the sensitive volume by reduction of the length of the drift path e.g. to 1 cm. At this drift path, the diffusion can be reduced by a factor of about three, and a lateral RMS resolution of less than 40 nm (scaled to tissue density) can be achieved. Such an operating mode is advantageous for microdosimetric measurements and investigations of ionization patterns relevant to radiation biology, where detection sensitivity is of minor importance.

In a further development stage, we foresee an improved detector design, where the chamber walls are made of tissue-equivalent material and the Fig.12 Ionization density distributions calcu- chamber gas composition is supplemented by lated from the images of fig.12. The coordinates u organic additives to approximate tissue stoi- and t are defined in fig 12 b. chiometry. For this modification the scintillation a) lateral ionization profile t with single-step properties of these new gas mixtures have to be amplification (correlated to fig. 12 a) carefully investigated. With such a detector we b) lateral ionization profile t with double-step could attempt to directly extract microdosimetric amplification (correlated to fig. 12b) spectra or calculate proximity functions and deliver c) longitudinal ionization profile u with double dose equivalent data in unknown radiation fields step amplification (correlated to fig.12b) without field characterization.

11 a Proton, Ep= 5,0 MeV b Proton, Ep= 5,0 MeV

2 cm 2 cm

c Proton, Ep= 5,0 MeV d Proton, Ep= 5,0 MeV

2 cm 2 cm

e Proton, Ep= 18,9 MeV f Proton, Ep= 18,9 MeV

2 cm 2 cm

g Alpha, Ea= 26 MeV h Alpha, Ea= 19 MeV

2 cm 2 cm

Fig.13 Images of proton and a-particle tracks in 10 hPa TEA. The images were obtained with particle injection parallel (a - e, g, h,) and 45° inclined (f) to the image projection plane. The flight direction of the particle is always from the right to the left side.As The mentioned colour code above, between neutron different spectrometry pictures varies.may Therefore no direct comparison of the ionisation densityprovide onanother the basis interesting of equal coloursfield of is possible.application.

12 The inelastic neutron reaction with 3He(n,p)t can References: be used [1] Recommendations of the International Com- to measure the spectral fluence of keV to several- mission on Radiological Protection; ICRP MeV neutrons with proportional chambers. In a Publication 60, Pergamon Press, Oxford conventional spherical or cylindrical proportional chamber, the recoil particles deposit their energy in [2] Determination of Dose Equivalents Resulting the chamber gas which contains several 105 Pa of from External Radiation Sources; 3He. The signal measured is proportional to the International Commission on Radiation Units sum of the nuclear reaction energy (Er=770 keV) and Measurements, Bethesda, MD, ICRU 39 and the kinetic energy of the neutron. In cases in [3] H. G. Menzel, L. Lindborg, T. Schmitz, H. which the neutron capture occurs close to the wall Schuhmacher, A. J. Waker; Radiat. Prot. or at high neutron energies, where the path length Dosim. 29 (1989) 55 of the recoils exceeds the stopping length of the detector, the measurement underestimates the neu- [4] S.R. Hunter; Nucl. Instrum. and Meth. A 260 tron energy (wall effect). Turner et al. proposed the (1987) 469 application of a nuclear track imaging device [27]. [5] J.E. Turner, R. Hunter, R.N. Hamm, H.A. A detector capable of measuring the track of both Wright, G.S. Hurst and W.A. Gibson; Radiat. recoils, proton and triton, could provide the coor- Prot. Dosim. 29 (1989) 9 dinates of the neutron-helium interaction locus and possible escapes of recoil ions from the sensitive [6] S.R. Hunter, W.A. Gibson, G.S. Hurst, J.E. volume (so that correction for the wall effect is Turner, R.N. Hamm and H.A. Wright; possible). For high neutron energies, where the Radiat. Prot. Dosim. 52 (1994) 323 recoil cannot be stopped in the gas, one could [7] T. Budd, M. Marshall and C.S. Kwok; Radiat. measure the angle between the two recoils and Res. 88 (1981) 228 calculate the neutron energy from the kinematic correlation between neutron, 3He and recoils. [8] M. Marshall, G.P. Stonell and P.D. Holt; Radiat. Prot. Dosim. 13 (1985) 41 For such an application the detector would need a [9] G. Malamud, A. Breskin and R. Chechik; Nucl. 3He gas filling of at least 105 Pa pressure to obtain Instrum. and Meth. A 307 (1991) 83 sufficient sensitivity. Light emission of noble gas mixtures with TEA (Ar, Kr, Xe) was investigated [10] A. Pansky,G. Malamud, A. Breskin and R. by Suzuki [19], Sauvage [20] and Anderson [28]. It Chechik; Nucl. Instrum & Meth. A323 (1992) was shown that the emission spectra are deter- 294 mined by the TEA emission. The excitation ener- [11] A. Breskin, R. Chechik, P. Colautti, V. Conte, gies and emission lines of the pure noble gases are A. Pansky, S. Shchemelinin, G. Talpo and G. in the vacuum UV range, thus TEA acts as a very Tornielli, Radat. Protec. Dosim. 61(1995)199. efficient wavelength shifter. We therefore expect this behaviour to occur also in He / TEA. It was [12] S. Shchemelinin, A. Breskin, R. Chechik, also shown [29] that the presence of He, with its A. Pansky, P. Colautti; in „Microdosimetry“, very high first excitation levels (20.5 eV), does not ed. by D.T. Goodhead, P. O’Neill, H.G. Men- alter significantly the field requirements for charge zel, ISBN 0-85404-737-9, (1997) 375 and light amplification. Therefore, the OPAC [13] U. Titt, A. Breskin, R. Chechik, V. Dangen could be used without altering the UV optics or the dorf, B. Großwendt and H. Schuhmacher; parallel-grid amplification structure. Radiat. Prot. Dosim. 70 (1996) 219 Acknowledgements: [14] G. Charpak, J.-P. Fabre, F. Sauli, M. Suzuki We gratefully acknowledge the contribution of K. and W. Dominik; Nucl. Instrum. and Meth. A Tittelmeier to the development of the OPAC and 258 (1987) 177 and references therein the measurements. We thank D. Mugai and [15] A. Breskin; Nucl. Phys. A 498 (1989) 457 and W.Wendt for their contributions to the experiment references therein electronics and J. Asher, W. Heinemann and M. Klin for their technical support. [16] D.F. Anderson, R. Bouclier, G. Charpak, S. Majewski and G. Kneller; Nucl. Instrum. and The work is supported by the European Commis- Meth. 217 (1983) 217 sion (Contract FI4P-CT95-0024) and by the Foun- dation Mordoh Mijan de Salonique. A.Breskin is [17]P. Fonte, A. Breskin, G. Charpak, W. Do the W.P. Reuther Professor in the peaceful use of minik, F. Sauli; Nucl. Instrum. and Meth. A283 atomic energy. (1989) 658

13 [18] G. Charpak; Proc. Int. Symposium on Lepton [25] A. Breskin, G. Charpak, S. Majewski; Nucl. and Photon Interactions at High Energies, Instrum. and Meth. A220 (1984) 349 Kyoto University (1985) 514 [26] A. Breskin, „Low Mass Detectors“, in: [19] M. Suzuki, P. Strock, F. Sauli, G. Charpak; Instrumentation for Heavy Ion Nuclear Nucl. Intrum. and Meth. A 254 (1987) 556 Research, Ed. D. Shapiro, Nuclear Science Research Conf. Series, Harwood Academic [20] D. Sauvage, A. Breskin and R. Chechik; Publishers 7,75 (1985) Nucl. Instrum. and Meth. A 275 (1989) 351 [27] J.E. Turner, R.N. Hamm, T.E. Houston, H.A. [21] V. Peskov, G .Charpak, W. Dominik, F. Sauli; Wright, S.A. Gibson, G.S. Hurst; Radiat. Prot. Nucl. Instrum. and Meth. A 277 (1989) 347 Dosim. 32 (1990) 157 [22] G. Charpak, W. Dominik, J.P. Fabre, J. [28] D.F. Anderson, S. Kwan, V. Peskov and B. Gaudean, V. Peskov, F. Sauli, M. Suzuki, Hoeneisen; Nucl. Instrum. and Meth. A323 A. Breskin, R. Chechik, D. Sauvage; IEEE (1992) 626 Trans. Nucl. Sci. NS-35, (1988) 483 [29] A. Breskin, R. Chechik, I. Frumkin; Nucl. [23] A. Breskin, R. Chechik, D. Sauvage; Nucl. Instrum. and Meth. A380 (1996) 562. Instrum. and Meth. A 286 (1989) 251 [24] H.W. Funk; Proxitronic GmbH & Co. KG, Robert Bosch Straße 34, D-64625 Bensheim

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