RECENTLY, a new type of radioactive decay was observed. Optical time projection chamber for imaging of two-proton decay of 45 Fe nucleus

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1 Optical time projection chamber for imaging of two-proton decay of 45 Fe nucleus M. Ćwiok,W.Dominik, Z. Janas, A. Korgul, K. Miernik,M.Pfützner,M.Sawicka and A. Wasilewski Institute of Experimental Physics, Warsaw University, Hoża 69, PL -68, Warsaw, Poland Sołtan Institute for Nuclear Studies, PL 5-4, Otwock-Świerk, Poland Abstract We present a design of the gaseous detector for 3-D measurement of the topology of a recently discovered radioactive decay of 45 Fe nucleus which involves simultaneous emission of two protons from the ground state. Our apparatus, called Optical Time Projection Chamber, consists of several parallel wire-mesh electrodes inside a gaseous medium which form the conversion region and the multistage charge amplification structure. Selected gas mixture enables strong emission of photons during avalanche process. photons from charge avalanches are converted into visible light by means of a wavelength shifter foil. A CCD camera located outside the detection volume records 2-D image of the decay process. Drift time of primary ionisation charge towards the amplification stage provides the third coordinate. Correlation of 2-D image with drift-time structure allows 3-D reconstruction. We present results on: the light yield, the charge gain and the electron drift velocity for several gas mixtures consisting of: Ar, He, N 2 and triethylamine vapour. A feasibility of the event topology reconstruction and the energy measurement is addressed. Index Terms gas detectors, TPC, track imaging, optical read out, triethylamine, life-time I. INTRODUCTION RECENTLY, a new type of radioactive decay was observed in which an extremely neutron-deficient atomic nucleus emits simultaneously two protons from the ground state. Such mode of decay, although predicted already in 96, could have been experimentally confirmed because of recent progress in the nuclear spectroscopy techniques based on fragmentation reactions of relativistic heavy ions and very sensitive separation methods allowing studies of single atoms of exotic species. The first evidence for the two-proton radioactivity has been obtained for the proton drip-line nucleus 45 Fe in two experiments, one performed at the GSI Darmstadt [] and the second at the GANIL Caen [2]. In both these experiments selected and identified in-flight ions of 45 Fe were implanted into a silicon detector in which the total energy released in the subsequent decay and its time were measured. The decay energy was found to be.4(5) MeV and the half-life T /2 = ms. It is important to note that the emitted particles could not have been recorded individually and the interpretation of the experimental findings relies on theoretical arguments. Thus, it is of vital This work was supported by the European Commission under contract HPRI- CT Corresponding author is Mikołaj Ćwiok ( cwiok@fuw.edu.pl, Fax: ). interest to prove experimentally that indeed two protons are ejected from the 45 Fe nucleus. Even more important would be precise measurement of energies and angular correlations of the decay products. It will shed light on the correlations between protons inside the atomic nucleus and may help to answer an intriguing question whether a virtual diproton state ( 2 He particle) plays a role in the decay process. The full kinematical reconstruction of the decay event represents a serious technical problem. As a consequence of the production and separation methods, the ions of 45 Fe arrive to the detection set-up with energy of about MeV/ nucleon. The ions have to be slowed down and implanted into the detection volume where they create large ionisation charge density. After the time interval corresponding to the 45 Fe halflife two low-energy protons (about 5 kev each) are emitted. Their tracks in space and energies have to be recorded. In the present paper we propose a detector for 3-D measurement of the decay topology. II. DETECTION TECHNIQUE The detection medium of low density allows one to expand in space the tracks of low energy decay products to measure precisely the kinematics of the decay process with moderate resolution of a recording system. The technique of proportional gaseous detectors operated at atmospheric pressure is adequate for the physical goal. Since tracks of the decay products should be fully contained in the detection volume the density of the medium results from a trade-off between the projectile stopping efficiency and the length of the proton tracks. The range of 5 kev protons is about 2 mm in a detection volume filled with 5% Ar + 5% He, what facilitates the event topology detection by means of a recording system with moderate granularity. A technique of Multi-Step Avalanche Chambers (MSAC) [3] with optical imaging may be implemented [4]. During charge avalanche process in gas in uniform electric field photons are emitted, mainly in V range. Addition of small amount of N 2 or triethylamine (TEA) to the nobel gas results in the efficient emission of photons in near spectral range. The majority of photons is emitted during the last multiplication stage near the very last collecting electrode. Emerging photons can be converted to a visible light by a thin layer of wavelength shifter () and recorded by a CCD camera and a photomultiplier. A capability of optical detection of particle tracks /4/$2. (C) 24 IEEE

2 preamplification conversion (2 cm 2cm 3 cm) transfer 2-step charge amplification Fig.. p Ion p gating electrode visible light glass A scheme of the Optical Time Projection Chamber. PMT imaging camera has been demonstrated by G. Charpak et al. for gas mixtures with TEA vapour [5]. Similar detection technique using pure TEA vapour at low pressure has been exploited by U. Titt et al. [6]. Our detector, called Optical Time Projection Chamber (OTPC), consists of parallel wire-mesh electrodes arranged in layers to form several uniform electric field regions (see Fig. ). The detector is operated in a flow mode and under atmospheric pressure. Charged particles entering the conversion volume produce ionisation electrons along their tracks. The primary ionisation electrons drift toward the multistage multiplication structure where the avalanche process occurs in high electric field. The process of electron multiplication occurs consecutively in two-stages: pre-amplification and final amplification. These stages are separated by a transfer gap of weak electric field. The final multiplication stage is done in two steps. The ratio of electric field strength applied to the gaps of the final stage defines the light yield for a given charge gain [7]. Such structure allows one to achieve gains up to 7 for single primary charges [3]. An optical image of two-dimensional projection of particle tracks onto a plane perpendicular to the electric field will be shot by a CCD camera coupled to an image intensifier. Electric signals from the photo-multiplier will be recorded. Its pulse-structure carries information about the timing of the decay process and, due to drift time in gas, about the coordinate of local ionisation in the direction of the electric field. The combination of the event projection with the time structure allows 3-D reconstruction. The recording time of the electronic signal and the integration time of the 2-D image should correspond to 5 half-lives of 45 Fe nucleus. Since the ionisation density due to 45 Fe nucleus stopped in the conversion volume is about two orders of magnitude larger than the one induced by 5 kev protons, gating capability of the charge transfer to the amplification stage is essential. This task is a major experimental challenge for successful detector operation. By controlling the voltage of the gate electrode one can define the charge transfer efficiency to the amplification structure. Dynamic tuning of the gate electrode potential allows significant reduction of variation of charge density entering Drift velocity (cm/µs) Fig %Ar+ TEA( C) 5%Ar+ 5%He + TEA(.5 C) 49%Ar+ 49%He + 2%N 2 + TEA( C) %He + TEA( C) Drift field (V/cm) The electron drift velocity measured for selected gas mixtures. the amplification stage, thus, it prevents the chamber from discharging when operated at high gain. III. STUDY OF MIXTURES In this paper we present results on electron drift velocity and on correlations between the light yield and the charge gain for selected gas mixtures of Ar, He, N 2 and TEA vapour. A. Electron drift velocity The electron drift velocity has been measured for gas mixtures containing TEA vapour for electric fields ranging from.6 V/ cm/ Torr to.6 V/ cm/ Torr. All measurements were performed under atmospheric pressure ( to 5 hpa) and at room temperature (24 C). Pure nobel gases (or their mixtures) were bubbled through liquid TEA at C. The results for 4 selected gas mixtures are shown in Fig. 2. Two gas mixtures containing equal partial pressures of Ar and He (open and solid squares) are only slightly faster than the gas mixture of pure He with TEA vapour (open circles). The mixture of 5% Ar + 5% He is expected to represent a good compromise by providing enough stopping power for heavy ions, like 45 Fe, and long enough tracks for low-energy protons. In such mixtures the position of a primary ionisation cluster in the conversion volume can be determined with precision of about 5 µm in the direction of drift field assuming drift velocity of.5 cm/µs and MHz sampling rate of the photo-multiplier s signal. B. Correlation of light yield and charge gain The experimental setup consisted of a cylindrical aluminium vessel housing a set of three parallel metal electrodes of /4/$2. (C) 24 IEEE

3 conversion (4 mm) ff collimator of ff-particles (ο4.5 MeV) aluminium container amplification (3.5 mm) quartz PMT signal digitizer integrating preamplifiers fi ο 5 μs Fig. 3. The experimental set-up for measuring the correlation between the light yield and the electron charge gain. Charge gain %Ar + TEA( C) 5%Ar + 5%He + TEA( C) 4 + TEA( C) 49.5%Ar %He + %N Electric field (V/cm) Fig. 4. The electron charge gain as a function of the electric field applied to 3.5 mm-thick gap for selected gas mixtures. cm cm area. The detector was operated in the flow gas mode. A quartz of 4 mm diameter and 5 mm thickness on the top of the vessel allowed to couple a photomultiplier tube for the light yield measurement. The three parallel electrodes formed a drift region and amplification gaps of 4 mm and 3.5 mm thickness, respectively (Fig. 3). Collimated alpha particles from 24 Am source entered the drift volume at a rate of about Hz through a thin hole in the copper cathode plate. A middle wire mesh electrode, at ground potential, served to pick-up the avalanche charge. The photons emitted during electron multiplication were converted into visible light by a Number of photons (arb.units) %Ar + TEA( C) 5%Ar + 5%He + TEA( C) + TEA( C) 49.5%Ar %He + %N Electric field (V/cm) Fig. 5. The light yield from electron avalanches as a function of the electric field applied to 3.5 mm-thick gap for selected gas mixtures. Number of photons / electron (arb.units) %Ar + TEA( C) 5%Ar + 5%He + TEA( C) + TEA( C) 49.5%Ar %He + %N Charge gain Fig. 6. The number of photons per single electron (a.u.) as a function of the electron charge gain in 3.5 mm-thick gap for selected gas mixtures. thin foil pressed between the last wire mesh electrode and the quartz. Signals induced from the pick-up electrode and the photomultiplier were integrated by means of pre-amplifiers of 5 µs decay constant and mv/fc charge sensitivity /4/$2. (C) 24 IEEE

4 For comparison we performed our measurements for 4 gas mixtures having equal concentration of Ar and He and small addition of TEA vapour or N 2. The measurement for pure argon with TEA vapour served as a reference. The electron charge gain and the light yield from the photomultiplier are depicted as a function of the amplification field in Fig. 4 and Fig. 5, respectively. The solid lines are result of a polynomial fit to the data. All lines end at the electric field value corresponding to a discharge limit. In Fig. 6 the number of photons per single electron in arbitrary units is shown as a function of the charge gain for each gas mixture. The shape is similar for all gas mixtures and exhibits maximum for gas gains between 2 and 5. It can be seen that gas mixture of 49.5% Ar % He + % N 2 (full triangles) produces more light from electron avalanches for a given charge gain than pure Argon with TEA vapour (open stars) and 5% Ar + 5%He with TEA vapour (open squares). By comparison of gas mixtures corresponding to nitrogen concentration of 2% and % (full circles and full triangles, respectively) one can expect that an optimal value of N 2 concentration may be below %. IV. IMAGING OF α TRACKS The reduced version of the OTPC detector was constructed in order to verify the imaging capability with Ar / He / N 2 gas mixture and selected CCD cameras. The chamber of 2 cm 2 cm active area consists of double amplification structure preceded by 2 mm conversion volume as is shown in Fig. 7. Two consecutive gaps of charge amplification structure are of 3 mm and 5 mm width, respectively. All electrodes are made of stainlesssteel meshes. The grid forming the last electrode has optical transparency of 8%. The foil converting photons to narrow-band spectrum around 43 nm is placed behind the charge collecting electrode and in contact with the grid. Gas tightness of the chamber is assured by the optically transparent glass facing the optical readout device and by 2 µmthick aluminised mylar foil on the opposite side. A collimated 24 Am source of low intensity (effective energy due to internal absorption was around 4.5 MeV) was inserted into conversion gap in such a way that the radiation was emitted in the direction parallel to the field shaping electrodes. Ionisation density of 4.5 MeV α is only 3 times larger than the one induced by a 5 kev proton. The detector filled with 49.5% Ar % He +%N 2 gas mixture was operated in the flow mode under atmospheric pressure. Imaging capability was tested using two different camera systems: Camera-: MCP intensified gated 8-bit CCD [8], Camera-2: Peltier cooled low-noise 5-bit CCD [9]. An example of single α track imaged with Camera- during 3 s exposure time is presented in Fig. 8. The electric field strengths applied to drift and amplification gaps were chosen to maximise the light yield, namely, 88 V/ cm, 767 V/ cm and 66 V/ cm for 2 mm, 3 mm and 5 mm gap, respectively. The ambient pressure and temperature were, respectively, hpa and 2.4 C. The light signal projected onto direction of motion gated Micro- Channel Plate conversion 2-step charge amplification (2 mm gap) (3 mm and 5 mm gaps) ff collimator of ff-particles (ο4.5 MeV) glass CCD sensor ( pixels) Fig. 7. A reduced version of the OTPC detector for imaging α tracks with MCP intensified CCD camera (Camera-). Row (pixels) Column (pixels) Fig. 8. The 2-D image of a single α track obtained with Camera- after millisecond exposure time. of the α particle is shown in Fig. 9. An increase of the ionisation density at the end of the track is clearly visible, what demonstrates the operation of the chamber in proportional mode. In the case of Camera-2 the minimal exposure time was limited to. s due to speed of a mechanical shutter. Thus, for a given intensity of the 24 Am source, few α tracks were superimposed on each image as can be seen in Fig.. Nevertheless, single tracks can be clearly distinguished from the background noise. The image corresponds to electric field Intensity (arb.units) /4/$2. (C) 24 IEEE

5 Intensity (arb.units) Projection of 8 pixel rows two-step electron amplification structure is capable to image α particles having moderate energies of about 4.5 MeV. Chemically inert gas mixture composed of Ar, He, N 2 and, eventually, trace of water vapour allows high charge multiplication in PPAC structure for heavily ionizing radiation. Large light yield from avalanches in this gas mixture allows one to visualise α tracks by means of low-noise CCD camera even without pre-amplification stage in a MSAC detector. The partial results confirm feasibility of the envisaged technique of Multi-Step Avalanche Chambers with optical readout for three dimensional kinematical reconstruction of the decay of long-lived nuclei, such as 45 Fe Column (pixels) Fig. 9. A projection of the light signal of the α track from Fig. 8 on the particle s direction (from right to left). Row (5-pixel bins) Single alpha 5x5 binning Column (5-pixel bins) Fig.. The 2-D image of few α tracks obtained with Camera-2 after millisecond exposure time. strengths of 88 V/ cm, 7 V/ cm and 656 V/ cm for 2 mm, 3 mm and 5 mm gap, respectively. The ambient pressure and temperature were, respectively, 8 hpa and 24.2 C. During our measurements we have observed that the light yield and the maximal available gas gain strongly depend on impurities in the gas system. For example, addition of.8% of water vapour results in decrease of the light yield by a factor of two, but prevents electron avalanches from spreading out in space what results in more localized track images Intensity (arb.units) ACKNOWLEDGMENT The authors would like to thank H. Czyrkowski, R. Dabrowski, W. Kuśmierz and Z. Sałapa (Warsaw University) for their excellent technical support. We also appreciate very much help of Dr. G. Wrochna (Sołtan Institute for Nuclear Studies) and Prof. T. Stacewicz (Warsaw University) for providing CCD cameras and the lenses. REFERENCES [] M. Pfützner, E. Badura, C. Bingham, B. Blank, M. Chartier, H. Geissel et al., Eur. Phys. J. A 4 (22) 289. [2] J. Giovinazzo, B. Blank, M. Chartier, S. Czajkowski, A. Fleury, M. J. Lopez Jimenez et al., Phys. Rev. Lett. 89 (22) 25. [3] G. Charpak and F. Sauli, Phys. Lett. 78 B (978) 523. [4] M. Suzuki, A. Breskin, G. Charpak, E. Daubie, W. Dominik, J. P. Farbe et al., Nucl. Instrum. Methods A 263 (988) 237. [5] G. Charpak, W. Dominik, J. P. Farbe, J. Gaudaen, F. Sauli and M. Suzuki, Nucl. Instrum. Methods A 269 (988) 42. [6] U. Titt, V. Dangendorf, H. Schuhmacher, H. Schmidt-Böcking, A. Breskin and R. Chechik, Nucl. Instrum. Methods A 46 (998) 85. [7] A. Breskin, R. Chechik, Z. Fraenkel, D. Sauvage, V. Steiner, I. Tserruya et al., Nucl. Instrum. Methods A 273 (988) 798. [8] DICAM-3 camera by PCO Computer Optics GmbH equipped with: lenses of F=5 mm and.95 aperture, Micro Channel Plate of 25 mm diameter, gated 8-bit CCD sensor having pixels. Available: [9] GENESIS astronomical digital camera equipped with: lenses of F=25 mm and.95 aperture, 5-bit CCD sensor having pixels (Kodak KAF-4E), Peltier cooling and mechanical shutter. Available: V. CONCLUSION The drift velocity and the light-to-charge ratio have been measured for several gas mixtures of noble gases, nitrogen and TEA vapour. Satisfactory results for imaging of highly ionizing particles were obtained for the gas mixture consisting of 49.5% Ar % He + % N 2. A prototype detector with /4/$2. (C) 24 IEEE