- Email: [email protected]
A beam diagnostic multisensor for low energy radioactive beams
Nuclear Instruments and Methods in Physics Research A 622 (2010) 512–517
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
A beam diagnostic multisensor for low energy radioactive beams Luigi Cosentino n, Paolo Finocchiaro, Alfio Pappalardo Laboratori Nazionali del Sud, INFN, Via S.Sofia 62, 95125 Catania, Italy
a r t i c l e in f o
a b s t r a c t
Article history: Received 22 March 2010 Received in revised form 16 July 2010 Accepted 29 July 2010 Available online 6 August 2010
During the last years the EXotics with CYclotron and Tandem (EXCYT) facility has been developed at LNS—INFN in Catania, in order to produce radioactive ion beams in the energy range accessible to the 15 MV Tandem Van de Graaf accelerator, with intensity around 105 particles per second (pps). In order to guarantee quick and precise beam transport procedures, the facility has been equipped with a high sensitivity beam diagnostics, well designed for very low intensity beams. In this paper we describe the features and the experimental results of the device developed for the low energy beam line (10–300 keV) of the facility. Such a multisensor device is mainly based on particle detectors and allows to collect the needed parameters in real time. In particular it can measure the X–Y transverse beam profile and intensity, and can identify the radioisotopes present in the beam. & 2010 Elsevier B.V. All rights reserved.
Keywords: Beam diagnostics Radioactive ion beams Low intensity CsI(Tl) Beam Imaging Very low energy
1. Introduction Low and intermediate energy radioactive beams, far from the valley of beta stability, offer new opportunities in nuclear physics and nuclear astrophysics research, as they allow a deeper exploration of nuclear structure and reaction mechanisms. At LNS Catania we have developed an Isotope Separator On Line (ISOL) facility, known as EXCYT [1], that allows to produce radioactive ion beams (RIBs) with energy up to 7 MeV/A. Both LNS accelerators are involved in such a purpose (Fig. 1). The Superconducting Cyclotron (Emax ¼ 80 MeV/A) primary beam is stopped inside a thick target, in order to produce nuclear species of interest as a consequence of nuclear reactions. These are suitably selected, by means of a two-stage high-resolving-power mass separator (DM/M ¼1/20000), in order to produce a beam with just one nuclear species. This beam is finally injected into the second accelerator (Tandem Van De Graaf, Vmax ¼15 MV), and transported down to a suitable experimental apparatus. The primary beam stopper, i.e. thick target, is located inside the so called ‘‘target-ion-source complex’’. It is installed inside a well shielded bunker, in order to confine the abundantly produced radioactivity. The first part of the beam line, from the source complex to the first stage of the mass separator, is mounted on two high voltage platforms biased at a typical voltage of 100–200 kV. The rest of the beam line, from the second stage of the mass separator to the Tandem injection, is at ground potential. In
n
Corresponding author. Tel.: +39095542279; fax: +39095542252. E-mail address: [email protected] (L. Cosentino).
0168-9002/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2010.07.085
such a configuration the beam is extracted from the source with a kinetic energy of the order of 10 keV, by means of a corresponding extraction voltage. This kinetic energy value is maintained while on the platforms, and then it suddenly increases when the beam crosses the high voltage gap between the platform and the second stage of the mass separator, being accelerated to 100–200 keV. The final beam intensity is strongly bound to the chosen nuclear species. In particular it depends on the production crosssection, but also on the overall physical/chemical extraction efficiency from the source and on many other matching/transport efficiencies involved, along the beam line and down to the experimental apparatus. One of the main bottlenecks (efficiency well below 10%) is represented by the charge exchange cell (CEC), that is a piece of equipment required just after the extraction to convert the ion charge state from + 1 to 1, in order to allow the acceleration with the Tandem. The beam intensities measured so far with 8Li, 9Li and 21Na beams are of the order of 103–105 particles per second (pps). Obviously the facility needs an efficient beam diagnostics, distributed along the beam line to optimize the beam tuning operations. It must allow to transport the beam as easily as possible in a reasonably short time, keeping its intensity as high as possible and offering at the same time an efficient debugging tool for the beam line elements. The very low beam current (of the order of femtoAmps) and energy do not allow to use the typical diagnostic devices for ‘‘standard’’ beams. Devices as Faraday cups, wire scanners and harps, imaging alumina screens, etc., cannot attain a sufficient signal to noise ratio for the EXCYT beam needs [2]. Particle detectors offer instead the needed sensitivity for such applications, therefore we decided to adopt them in order to
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Fig. 1. Layout of the EXCYT facility. The first high voltage platform containing the ion-source, is inside a well shielded bunker. The two stages of the mass separator are located on the ground floor, whilst the Tandem is on the upper floor. A vertical beam line allows the radioactive beam to be transported on the Tandem floor.
achieve enough sensitivity for the facility requirements. Basic issues were also robustness and reliability of the devices, and this is why we decided to discard solutions based on electron multipliers, such as microchannel plates (MCP), since our earlier tests we immediately realized they were not robust and operatorfriendly enough. We chose to make prevalent use of scintillators and solid state detectors, that offer a high sensitivity down to the single particles and so far have never shown particular problems or failures.
2. LEBI device The diagnostic system we have developed for the low-energy beam line is called LEBI: Low Energy Beam Imager/Identifier, and a very preliminary prototype was described in Ref. [3]. Such a compact multisensor allows to cover the needs of the EXCYT beam transport procedures, just excluding the first high voltage platform because of the high radioactivity level close to the target-ion-source complex. LEBI allows to acquire in real time the 2D transversal beam profile of the stable (pilot) and radioactive beams, to measure the beam intensity and to identify the nuclear species present in the radioactive beam. It is basically made of two components (Fig. 2). The first one is used for beam imaging and is based on a scintillating screen (5 5 cm2 area, 1–2 mm thick) made of cesium iodide doped with thallium, CsI(Tl), chosen since its light yield (6 104 photons/MeV) is far higher than the ordinarily used phosphor screens. Such a screen is placed at an angle of 451 with respect to the beam axis and is watched by a high sensitivity CCD camera (WAT 902 H, sensitivity of 10 4 lux). The second component is used to detect beta particles emitted as a consequence of the radioactive decay. It can measure the beam rate and can identify the beta-emitting radioisotopes by means of their half-life and their beta energy spectrum. The detector is a plastic scintillator BC408 (BICRON, tdecay ¼ 2.1 ns, size 6 6 5 cm3), optically coupled to a short photomultiplier tube (PMT, Hamamatsu R1924A), that is placed inside the vacuum chamber and is powered by an active voltage divider (Electron Tubes—PS 1807/5, Vpower ¼6 V) installed outside.
Fig. 2. LEBI device. (a) Photomultiplier, (b) plastic scintillator, (c) CsI(Tl) screen, (d) mylar tape, (e) axis of the stepper motor.
The CsI(Tl) and the plastic scintillator are mounted on the same holder, mechanically fixed to a steel rod that can slide vertically by means of a pneumatic actuator. Depending on the application (stable beam imaging, radioactive beam imaging, beam identification/counting), the device can be inserted in beam in three different operating positions or removed (Fig. 3).
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Fig. 3. Working position of LEBI at the three different heigths (a) radioactive beam imaging, (b) stable beam imaging, (c) beam counting/identification. The dashed line represents the beam axis, the arrows and the circles indicate the region where the beam impinges on the detector.
2.1. Beam imaging LEBI is firstly used to acquire the transversal 2D profile of a stable pilot beam, used for setting up the beam line parameters at a magnetic rigidity close to that of the weaker radioactive beam. For such an operating mode the actuator is slid in, so that the CsI(Tl) screen can directly intercept the beam (Fig. 3b). Part of the energy released by each particle inside the screen is converted into scintillation light, thus the beam produces a light spot with a shape corresponding to its transverse profile. A shielding grid, made of thin metallic wires with diameter of 20 mm and set at ground potential, has been installed on top of the scintillator front face: in case of higher beam intensity the screen surface could charge up to a few kV, and grounding the screen prevents the possible distortion of trajectories of the incoming particles as well as abrupt electrical discharges. The adopted CCD camera, located outside the beam pipe, is small enough to be inserted into a suitable cup equipped with a glass window (viewport). This allows to place the lens as close as possible to the scintillating screen, at a distance of about 7 cm from its center. A short distance between screen and lens, and consequently large solid angle, is needed in order to maximize the collection of scintillating light, thus yielding a better sensitivity for the imaging system. The optics have been chosen for a suitable magnification that ensures the correct viewing angle of the screen, with a spatial resolution well below 1 mm. We have adopted a standard 35 mm objective lens coupled with a 40 mm extension ring, thus achieving the needed macro configuration. A computer-driven video multiplexer allows to select one video output among those provided by the several LEBI units installed along the beam lines. The beam image viewed by the selected CCD camera is acquired by means of an 8 bit-deep frame grabber, installed on a computer placed in the accelerator console. Such a configuration allows the operators to acquire realtime videos of the beam profiles; moreover, at the same time they can perform on-line image analysis in order to extract parameters useful for the beam transport, such as beam size and centroid position. For each LEBI the reference of the true geometrical beam axis was recorded at the time of the mechanical installation on the beam pipe, following an accurate alignment and spatial calibration. With the aid of a video pattern generator module, a graphical pattern centered on the beam line axis can be superimposed to the video image of each device, allowing to compare
the actual beam centroid to its nominal position in real time for adjustments. When operating with radioactive beams, and in particular if the beam or some contaminant has a somewhat longer decay time (several seconds or more), one would like to prevent its implantation onto the screen that would result in a long and impractical afterglow. For this reason an additional position of the screen was foreseen (Fig. 3a), allowing the beam to be implanted on a thin (6 mm) aluminized mylar tape that covers the lower half of the CsI(Tl) screen surface. Whenever needed, the tape can be rolled horizontally by means of a stepper motor, thus providing the screen with a fresh radiation-free portion of tape. The radiation emitted by the decay of the implanted beam particles (mainly b and g rays), crosses the scintillating screen thus producing the beam image. One could argue that such an image would be blurred, because of the non-local energy release of beta and gamma radiation. However, as the beam implanted onto the tape behaves like a radioactive source distributed over the transverse beam profile, a simple simulation has shown that a sort of proximity focusing occurs, which gives rise to an image with space resolution of the order of the screen thickness. In particular we tested this imaging principle using a b source (90Sr, Emax ¼546 and 2280 keV) [4]. In Fig. 4 a light spot acquired with the LEBI is shown, using a b collimated source 2 mm wide and 104 pps intensity.
2.2. Beam identification and count rate measurement To work with the plastic scintillator, the LEBI support rod is placed in the third position (Fig. 3c). The radioactive beam is implanted onto the thin aluminum reflecting layer used to wrap the plastic scintillator. Roughly 50% of the beta particles emitted by the radioactive species decay interact with the scintillator, due to solid angle reasons. Each interaction produces a fast light pulse that is converted into an electrical pulse by means of the PMT. Its output is split along two paths; one goes to a discriminator for the pulse counting, the other to a spectroscopy amplifier for the acquisition of the energy spectrum. A National Instruments 6601 multichannel scaler PCI board allows to count the pulses crossing the threshold of the discriminator, that is set to a suitable value in order to suppress the background noise. In order to express the radioactive beam intensity in particles per second, it is necessary
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involved gamma levels and the relative detector efficiency (70% for our detectors), the beam intensity can be inferred. Two germanium detectors are quite useful for radionuclides emitting gamma cascades, as the background can be strongly suppressed by imposing a coincidence. By means of such a procedure selected gamma levels can be enhanced in the resulting spectrum. The management of all the LEBIs and their associate equipment is performed by means of rack computers, that are installed both inside and outside the second high voltage platform. They drive the mechanical actuators, the motors for winding the mylar tape, the power supplies, the PMT data acquisition, the video signals from CCD cameras. All of this equipment is controlled by software code developed in LabView environment, which implements a friendly Graphical User Interface (GUI) currently operated by the accelerator crew without any special training.
3. Experimental results Fig. 4. Picture of a 90Sr radioactive source. The particle flux is 3 103 pps/mm2; the average energy released in the scintillating screen by the beta particles is 1 MeV.
to correct the count rate for the detection efficiency of the plastic scintillator. This was calculated by means of the PENELOPE Monte Carlo code [5], using as input the beta energy spectra of the radioisotopes calculated by means of the Fermi law. The dynamic range of the counting system is up to 106 particles per second (pps), for beyond this value the PMT linearity is lost. The behaviour of the radioactive population and decay process is exponential, and it follows respectively (Eqs.(1) and (2)). Rp ðtÞ ¼ rð1et=t Þ ¼ rð1etln 2=T1=2 Þ
ð1Þ
Rd ðtÞ ¼ ret=t ¼ retln 2=T1=2
ð2Þ
Eq. (1) gives the activity of the implanted radioisotopes during the beam-on phase, whereas Eq.(2) gives the activity during the beam-off phase. r is the true beam particle rate (in pps) and T1/2 ¼ t ln 2 is the characteristic half-life of the implanted radionuclide. The population/decay curves of the radioisotopes can be constructed using the multichannel scaler. We count the decay rate in suitably short time bins as a function of time, under the conditions of beam-on (Eq.(1)) and beam-off (Eq.(2)), obtained by opening and closing a fast beam valve. The duration of each measurement can be limited to about 5 times the decay constant t, as after such a time span the difference from the true asymptotic final value is below 1%. A fit of the experimental curves allows to calculate T1/2, thus allowing to quickly identify the radionuclide (this holds for simple cases where only one or two decays are present). Trouble can arise if the beam is contaminated by a non-negligible contribution of isobaric radioactive species, and in those cases one has to disentangle the different contribution of each radionuclide. Such cases have to be evaluated carefully, setting up appropriate data analysis procedures in order to extract useful information from the energy spectra and the decay curves. For beam species involving gamma decay channels, LEBI can be supplemented with one or two high purity coaxial germanium detectors (HPGe). Indeed, two hollow cups with thin windows, made from Ergal, allow to insert the HPGe heads down to 7 cm from the tape region where the beam is implanted, yet staying in air outside the beam pipe. This allows to perform gamma-ray spectroscopy in order to fingerprint the nuclear species present in the beam, thus providing a better characterization of the beam itself. Moreover, by knowing the branching ratios related to the
So far EXCYT has produced and reaccelerated a 8Li radioactive beam, successfully exploited for some experiments at LNS [6]. In order to start with the commissioning, the first operation consisted of transporting a 10 keV 7Li pilot beam on the two platforms. The following step was to cross the high voltage gap between platforms and ground, transporting the 120 keV beam down to the second stage of the mass separator. Such a session was necessary to tune the beam line instrumentation (electrostatic quadrupoles, magnetic dipoles, diagnostics, etc.) with a beam of E100 pA, before starting with the weaker 8Li beam. Later on, in order to shift from 7Li to 8Li, only the magnetic rigidity Br for each dipole had to be rescaled. The imaging sensitivity for stable beams was measured by decreasing the beam energy at the lowest allowed value, i.e. 5 keV, and then decreasing the intensity until the beam spot was barely detectable. The measured sensitivity limit at 5 keV of 7Li roughly corresponds to a flux of 104 pps/mm2, that is a power released in the screen of about 10 pW/mm2. However, after several hours of continuous irradiation a slight decrease of the CsI(Tl) light yield was observed, due to ageing of the scintillating screen face exposed to the beam. This effect gradually reduces the sensitivity of the imaging system, but the light efficiency can be completely recovered by cleaning the very thin irradiated layer (tens to hundreds of nm) with a wet cloth (water or alcohol). Such an issue is currently under investigation at LNS, as we want to study the dependence of the light efficiency on the integrated dose, by irradiating CsI(Tl) plates with several ion species in the energy range 10–100 keV. All the LEBIs installed along the low energy beam line allowed a real time display of position and beam profile for a wide range of energies and intensities. In Fig. 5 a typical beam profile is shown, whose shape was squeezed horizontally in order to inject the beam into the mass separator dipoles. The graphical patterns superimposed to the on-line images allowed a quick beam alignment as well as an overall test and debug of the installed equipment and of the whole complex facility. Finally, we were able to optimize the setting of all the beam transport elements, in order to maximize the transmission step by step from the ion source up to the Tandem. 3.1. Results with the 8Li radioactive beam b
The 8Li radioactive beam (8 Li-8 Be, Emax ¼12.96 MeV) was produced using 13C at 45 MeV/A as a primary beam [7]. The 8Li ions produced inside the thick target (graphite in this case) were extracted by means of a 10 kV voltage and preselected by means of a suitable magnetic dipole on the first platform. After setting
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the beam line transport parameters for the specifications of 8Li, the LEBIs have immediately allowed to measure the beam intensity, to display its profile and to identify the beam as 8Li. In Fig. 6 the counting rate profile is shown, as acquired with a beam on/off sequence, obtained by opening and closing a fast line valve ahead of a LEBI. The measurement was done with the multichannel scaler by counting PMT pulses in time intervals of 100 ms. After opening the valve it is possible to observe the exponential increase of the counting rate (Eq.(1)), whose value becomes stable after roughly 5 s, in agreement with approximately 5 times the decay constant, that is tdecay ffi 1.2 sec. The exponential fit allows to deduce the half-life T1/2 ¼0.8470.01 s, in perfect agreement with the known value of the 8Li half-life (T1/2 ¼ ln 2 tdecay ¼0.83870.006 s) [8]. By integrating the counts for a time interval of 1 s and correcting for the efficiency of 4871%, as calculated with PENELOPE, the value of the beam rate could be measured on each device in real time. Concerning the beam imaging, we have been able to display 8 Li beam profiles whenever the overall energy released by the beta electrons inside the screen was above the LEBI sensitivity threshold, that in this case was corresponded to the order of 102–103 pps/mm2. In Fig. 7 the profile of a beam with a particle
flux of 6 103 pps/mm2 is shown. As can be observed, a difference with respect to the stable beam is that for the radioactive beam the spot boundaries are faded, since the beta radiation is emitted isotropically and the corresponding image is thus proximity focused. As a consequence the space resolution in such a case is of the order of the screen thickness (1–2 mm), for the MeV-range beta particles are capable of crossing it.
Fig. 5. 7Li stable beam profile, having a width of about 3 mm. The shadows of the thin tungsten wires are visible.
Fig. 7. 8Li beam profile. The kinetic energy is 10 keV, the intensity is 100 fA (6 105 pps).
4. Conclusions With the help of the eleven LEBIs installed along the low energy beam line, we have been able to easily transport the beam across the mass separator with close to 100% efficiency. In light of the complexity of the facility, the advantage of having a multisensor in a single device has allowed to speed up the beam transport procedures and to increase the overall reliability of the beam diagnostics system. The beam crew have routinely operated the device to acquire the needed parameters along the whole beam line, in every point critical for the beam transport. In Table 1 a list of future beam candidates is reported with their respective half-life T1/2. It can be observed that in some cases (e.g. 15O,
valve closed
7000 valve opened
6000
counts
5000 4000 3000 2000 1000 0
0
2
4
6
8
10 time (sec)
12
14
16
18
20
Fig. 6. Temporal profile of the counting rate after closing (beam-off) and opening (beam-on) a line valve. The exponential behaviour is fitted in order to calculate the T1/2.
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Table 1 Possible beams to be produced with the EXCYT facility. Isotope
half-life
Isotope
half-life
8
0.838 s 0.178 s 19.255 s 20.39 min 2.449 s 112.24 s 26.9 s 13.51 s 64.49 s
18
109.77 min 11.16 s 7.183 s 6.56 min 2.511 s 1.526 s (0 + ) 32.00 min (3 + ) 37.24 min 55.6 min 1.35 min
Li Li 10 C 11 C 15 C 15 O 19 O 20 O 17 F 9
F F 25 Al 29 Al 33 Cl 34 Cl 38 Cl 39 Cl 40 Cl 20
11 C, 18F) the time needed to reach the saturation activity value can be as long as several minutes. For the expected future beams, accurate preparation of all the diagnostics has therefore to be performed, taking into account the peculiarity of the radionuclides to be analyzed, in particular the half-life T1/2 and the endpoint energy of the beta decay.
517
Acknowledgments The authors wish to thank the accelerator staff and the electronics service of LNS, for the significant contribution to the development and test of the LEBI device. In particular Mr. A. Amato, Mr. P. Litrico, Mr. V. Scuderi and Mr. F. Tudisco deserve a special mention. References [1] D. Rifuggiato, et al., Nucl. Instr. and Meth. Phys. Res. B 261 (2007) 1040. [2] P. Finocchiaro, in: Proceedings of the 15th CAARI November 1998, Denton, Texas, USA. [3] S Cappello, et al., Nucl. Instr. and Meth. A479 (2002) 243. [4] L Cosentino, et al., IEEE Transac. Nucl. Sci. NS-48 (4) (2001) 1132. [5] Nucl. Instr. and Meth. B 132 (1997) 377 /http://www.nea.fr/abs/html/ nea-1525.htmlS. [6] A Del Zoppo, et al., Nucl. Instr. and Meth A 581 (2007) 783; M. La Cognata, et al., Phys. Lett. B 664 (3) (2008) 157; A. Musumarra, et al., Nucl. Instr. and Meth. A 612 (2010) 399. [7] G. Cuttone, et al., Nucl. Instr. and Meth. B 266 (2008) 4108. [8] F. Ajezenberg-Selove, Nucl. Phys. A 490 (1988) 1.
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
A beam diagnostic multisensor for low energy radioactive beams Luigi Cosentino n, Paolo Finocchiaro, Alfio Pappalardo Laboratori Nazionali del Sud, INFN, Via S.Sofia 62, 95125 Catania, Italy
a r t i c l e in f o
a b s t r a c t
Article history: Received 22 March 2010 Received in revised form 16 July 2010 Accepted 29 July 2010 Available online 6 August 2010
During the last years the EXotics with CYclotron and Tandem (EXCYT) facility has been developed at LNS—INFN in Catania, in order to produce radioactive ion beams in the energy range accessible to the 15 MV Tandem Van de Graaf accelerator, with intensity around 105 particles per second (pps). In order to guarantee quick and precise beam transport procedures, the facility has been equipped with a high sensitivity beam diagnostics, well designed for very low intensity beams. In this paper we describe the features and the experimental results of the device developed for the low energy beam line (10–300 keV) of the facility. Such a multisensor device is mainly based on particle detectors and allows to collect the needed parameters in real time. In particular it can measure the X–Y transverse beam profile and intensity, and can identify the radioisotopes present in the beam. & 2010 Elsevier B.V. All rights reserved.
Keywords: Beam diagnostics Radioactive ion beams Low intensity CsI(Tl) Beam Imaging Very low energy
1. Introduction Low and intermediate energy radioactive beams, far from the valley of beta stability, offer new opportunities in nuclear physics and nuclear astrophysics research, as they allow a deeper exploration of nuclear structure and reaction mechanisms. At LNS Catania we have developed an Isotope Separator On Line (ISOL) facility, known as EXCYT [1], that allows to produce radioactive ion beams (RIBs) with energy up to 7 MeV/A. Both LNS accelerators are involved in such a purpose (Fig. 1). The Superconducting Cyclotron (Emax ¼ 80 MeV/A) primary beam is stopped inside a thick target, in order to produce nuclear species of interest as a consequence of nuclear reactions. These are suitably selected, by means of a two-stage high-resolving-power mass separator (DM/M ¼1/20000), in order to produce a beam with just one nuclear species. This beam is finally injected into the second accelerator (Tandem Van De Graaf, Vmax ¼15 MV), and transported down to a suitable experimental apparatus. The primary beam stopper, i.e. thick target, is located inside the so called ‘‘target-ion-source complex’’. It is installed inside a well shielded bunker, in order to confine the abundantly produced radioactivity. The first part of the beam line, from the source complex to the first stage of the mass separator, is mounted on two high voltage platforms biased at a typical voltage of 100–200 kV. The rest of the beam line, from the second stage of the mass separator to the Tandem injection, is at ground potential. In
n
Corresponding author. Tel.: +39095542279; fax: +39095542252. E-mail address: [email protected] (L. Cosentino).
0168-9002/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2010.07.085
such a configuration the beam is extracted from the source with a kinetic energy of the order of 10 keV, by means of a corresponding extraction voltage. This kinetic energy value is maintained while on the platforms, and then it suddenly increases when the beam crosses the high voltage gap between the platform and the second stage of the mass separator, being accelerated to 100–200 keV. The final beam intensity is strongly bound to the chosen nuclear species. In particular it depends on the production crosssection, but also on the overall physical/chemical extraction efficiency from the source and on many other matching/transport efficiencies involved, along the beam line and down to the experimental apparatus. One of the main bottlenecks (efficiency well below 10%) is represented by the charge exchange cell (CEC), that is a piece of equipment required just after the extraction to convert the ion charge state from + 1 to 1, in order to allow the acceleration with the Tandem. The beam intensities measured so far with 8Li, 9Li and 21Na beams are of the order of 103–105 particles per second (pps). Obviously the facility needs an efficient beam diagnostics, distributed along the beam line to optimize the beam tuning operations. It must allow to transport the beam as easily as possible in a reasonably short time, keeping its intensity as high as possible and offering at the same time an efficient debugging tool for the beam line elements. The very low beam current (of the order of femtoAmps) and energy do not allow to use the typical diagnostic devices for ‘‘standard’’ beams. Devices as Faraday cups, wire scanners and harps, imaging alumina screens, etc., cannot attain a sufficient signal to noise ratio for the EXCYT beam needs [2]. Particle detectors offer instead the needed sensitivity for such applications, therefore we decided to adopt them in order to
L. Cosentino et al. / Nuclear Instruments and Methods in Physics Research A 622 (2010) 512–517
513
Fig. 1. Layout of the EXCYT facility. The first high voltage platform containing the ion-source, is inside a well shielded bunker. The two stages of the mass separator are located on the ground floor, whilst the Tandem is on the upper floor. A vertical beam line allows the radioactive beam to be transported on the Tandem floor.
achieve enough sensitivity for the facility requirements. Basic issues were also robustness and reliability of the devices, and this is why we decided to discard solutions based on electron multipliers, such as microchannel plates (MCP), since our earlier tests we immediately realized they were not robust and operatorfriendly enough. We chose to make prevalent use of scintillators and solid state detectors, that offer a high sensitivity down to the single particles and so far have never shown particular problems or failures.
2. LEBI device The diagnostic system we have developed for the low-energy beam line is called LEBI: Low Energy Beam Imager/Identifier, and a very preliminary prototype was described in Ref. [3]. Such a compact multisensor allows to cover the needs of the EXCYT beam transport procedures, just excluding the first high voltage platform because of the high radioactivity level close to the target-ion-source complex. LEBI allows to acquire in real time the 2D transversal beam profile of the stable (pilot) and radioactive beams, to measure the beam intensity and to identify the nuclear species present in the radioactive beam. It is basically made of two components (Fig. 2). The first one is used for beam imaging and is based on a scintillating screen (5 5 cm2 area, 1–2 mm thick) made of cesium iodide doped with thallium, CsI(Tl), chosen since its light yield (6 104 photons/MeV) is far higher than the ordinarily used phosphor screens. Such a screen is placed at an angle of 451 with respect to the beam axis and is watched by a high sensitivity CCD camera (WAT 902 H, sensitivity of 10 4 lux). The second component is used to detect beta particles emitted as a consequence of the radioactive decay. It can measure the beam rate and can identify the beta-emitting radioisotopes by means of their half-life and their beta energy spectrum. The detector is a plastic scintillator BC408 (BICRON, tdecay ¼ 2.1 ns, size 6 6 5 cm3), optically coupled to a short photomultiplier tube (PMT, Hamamatsu R1924A), that is placed inside the vacuum chamber and is powered by an active voltage divider (Electron Tubes—PS 1807/5, Vpower ¼6 V) installed outside.
Fig. 2. LEBI device. (a) Photomultiplier, (b) plastic scintillator, (c) CsI(Tl) screen, (d) mylar tape, (e) axis of the stepper motor.
The CsI(Tl) and the plastic scintillator are mounted on the same holder, mechanically fixed to a steel rod that can slide vertically by means of a pneumatic actuator. Depending on the application (stable beam imaging, radioactive beam imaging, beam identification/counting), the device can be inserted in beam in three different operating positions or removed (Fig. 3).
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Fig. 3. Working position of LEBI at the three different heigths (a) radioactive beam imaging, (b) stable beam imaging, (c) beam counting/identification. The dashed line represents the beam axis, the arrows and the circles indicate the region where the beam impinges on the detector.
2.1. Beam imaging LEBI is firstly used to acquire the transversal 2D profile of a stable pilot beam, used for setting up the beam line parameters at a magnetic rigidity close to that of the weaker radioactive beam. For such an operating mode the actuator is slid in, so that the CsI(Tl) screen can directly intercept the beam (Fig. 3b). Part of the energy released by each particle inside the screen is converted into scintillation light, thus the beam produces a light spot with a shape corresponding to its transverse profile. A shielding grid, made of thin metallic wires with diameter of 20 mm and set at ground potential, has been installed on top of the scintillator front face: in case of higher beam intensity the screen surface could charge up to a few kV, and grounding the screen prevents the possible distortion of trajectories of the incoming particles as well as abrupt electrical discharges. The adopted CCD camera, located outside the beam pipe, is small enough to be inserted into a suitable cup equipped with a glass window (viewport). This allows to place the lens as close as possible to the scintillating screen, at a distance of about 7 cm from its center. A short distance between screen and lens, and consequently large solid angle, is needed in order to maximize the collection of scintillating light, thus yielding a better sensitivity for the imaging system. The optics have been chosen for a suitable magnification that ensures the correct viewing angle of the screen, with a spatial resolution well below 1 mm. We have adopted a standard 35 mm objective lens coupled with a 40 mm extension ring, thus achieving the needed macro configuration. A computer-driven video multiplexer allows to select one video output among those provided by the several LEBI units installed along the beam lines. The beam image viewed by the selected CCD camera is acquired by means of an 8 bit-deep frame grabber, installed on a computer placed in the accelerator console. Such a configuration allows the operators to acquire realtime videos of the beam profiles; moreover, at the same time they can perform on-line image analysis in order to extract parameters useful for the beam transport, such as beam size and centroid position. For each LEBI the reference of the true geometrical beam axis was recorded at the time of the mechanical installation on the beam pipe, following an accurate alignment and spatial calibration. With the aid of a video pattern generator module, a graphical pattern centered on the beam line axis can be superimposed to the video image of each device, allowing to compare
the actual beam centroid to its nominal position in real time for adjustments. When operating with radioactive beams, and in particular if the beam or some contaminant has a somewhat longer decay time (several seconds or more), one would like to prevent its implantation onto the screen that would result in a long and impractical afterglow. For this reason an additional position of the screen was foreseen (Fig. 3a), allowing the beam to be implanted on a thin (6 mm) aluminized mylar tape that covers the lower half of the CsI(Tl) screen surface. Whenever needed, the tape can be rolled horizontally by means of a stepper motor, thus providing the screen with a fresh radiation-free portion of tape. The radiation emitted by the decay of the implanted beam particles (mainly b and g rays), crosses the scintillating screen thus producing the beam image. One could argue that such an image would be blurred, because of the non-local energy release of beta and gamma radiation. However, as the beam implanted onto the tape behaves like a radioactive source distributed over the transverse beam profile, a simple simulation has shown that a sort of proximity focusing occurs, which gives rise to an image with space resolution of the order of the screen thickness. In particular we tested this imaging principle using a b source (90Sr, Emax ¼546 and 2280 keV) [4]. In Fig. 4 a light spot acquired with the LEBI is shown, using a b collimated source 2 mm wide and 104 pps intensity.
2.2. Beam identification and count rate measurement To work with the plastic scintillator, the LEBI support rod is placed in the third position (Fig. 3c). The radioactive beam is implanted onto the thin aluminum reflecting layer used to wrap the plastic scintillator. Roughly 50% of the beta particles emitted by the radioactive species decay interact with the scintillator, due to solid angle reasons. Each interaction produces a fast light pulse that is converted into an electrical pulse by means of the PMT. Its output is split along two paths; one goes to a discriminator for the pulse counting, the other to a spectroscopy amplifier for the acquisition of the energy spectrum. A National Instruments 6601 multichannel scaler PCI board allows to count the pulses crossing the threshold of the discriminator, that is set to a suitable value in order to suppress the background noise. In order to express the radioactive beam intensity in particles per second, it is necessary
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involved gamma levels and the relative detector efficiency (70% for our detectors), the beam intensity can be inferred. Two germanium detectors are quite useful for radionuclides emitting gamma cascades, as the background can be strongly suppressed by imposing a coincidence. By means of such a procedure selected gamma levels can be enhanced in the resulting spectrum. The management of all the LEBIs and their associate equipment is performed by means of rack computers, that are installed both inside and outside the second high voltage platform. They drive the mechanical actuators, the motors for winding the mylar tape, the power supplies, the PMT data acquisition, the video signals from CCD cameras. All of this equipment is controlled by software code developed in LabView environment, which implements a friendly Graphical User Interface (GUI) currently operated by the accelerator crew without any special training.
3. Experimental results Fig. 4. Picture of a 90Sr radioactive source. The particle flux is 3 103 pps/mm2; the average energy released in the scintillating screen by the beta particles is 1 MeV.
to correct the count rate for the detection efficiency of the plastic scintillator. This was calculated by means of the PENELOPE Monte Carlo code [5], using as input the beta energy spectra of the radioisotopes calculated by means of the Fermi law. The dynamic range of the counting system is up to 106 particles per second (pps), for beyond this value the PMT linearity is lost. The behaviour of the radioactive population and decay process is exponential, and it follows respectively (Eqs.(1) and (2)). Rp ðtÞ ¼ rð1et=t Þ ¼ rð1etln 2=T1=2 Þ
ð1Þ
Rd ðtÞ ¼ ret=t ¼ retln 2=T1=2
ð2Þ
Eq. (1) gives the activity of the implanted radioisotopes during the beam-on phase, whereas Eq.(2) gives the activity during the beam-off phase. r is the true beam particle rate (in pps) and T1/2 ¼ t ln 2 is the characteristic half-life of the implanted radionuclide. The population/decay curves of the radioisotopes can be constructed using the multichannel scaler. We count the decay rate in suitably short time bins as a function of time, under the conditions of beam-on (Eq.(1)) and beam-off (Eq.(2)), obtained by opening and closing a fast beam valve. The duration of each measurement can be limited to about 5 times the decay constant t, as after such a time span the difference from the true asymptotic final value is below 1%. A fit of the experimental curves allows to calculate T1/2, thus allowing to quickly identify the radionuclide (this holds for simple cases where only one or two decays are present). Trouble can arise if the beam is contaminated by a non-negligible contribution of isobaric radioactive species, and in those cases one has to disentangle the different contribution of each radionuclide. Such cases have to be evaluated carefully, setting up appropriate data analysis procedures in order to extract useful information from the energy spectra and the decay curves. For beam species involving gamma decay channels, LEBI can be supplemented with one or two high purity coaxial germanium detectors (HPGe). Indeed, two hollow cups with thin windows, made from Ergal, allow to insert the HPGe heads down to 7 cm from the tape region where the beam is implanted, yet staying in air outside the beam pipe. This allows to perform gamma-ray spectroscopy in order to fingerprint the nuclear species present in the beam, thus providing a better characterization of the beam itself. Moreover, by knowing the branching ratios related to the
So far EXCYT has produced and reaccelerated a 8Li radioactive beam, successfully exploited for some experiments at LNS [6]. In order to start with the commissioning, the first operation consisted of transporting a 10 keV 7Li pilot beam on the two platforms. The following step was to cross the high voltage gap between platforms and ground, transporting the 120 keV beam down to the second stage of the mass separator. Such a session was necessary to tune the beam line instrumentation (electrostatic quadrupoles, magnetic dipoles, diagnostics, etc.) with a beam of E100 pA, before starting with the weaker 8Li beam. Later on, in order to shift from 7Li to 8Li, only the magnetic rigidity Br for each dipole had to be rescaled. The imaging sensitivity for stable beams was measured by decreasing the beam energy at the lowest allowed value, i.e. 5 keV, and then decreasing the intensity until the beam spot was barely detectable. The measured sensitivity limit at 5 keV of 7Li roughly corresponds to a flux of 104 pps/mm2, that is a power released in the screen of about 10 pW/mm2. However, after several hours of continuous irradiation a slight decrease of the CsI(Tl) light yield was observed, due to ageing of the scintillating screen face exposed to the beam. This effect gradually reduces the sensitivity of the imaging system, but the light efficiency can be completely recovered by cleaning the very thin irradiated layer (tens to hundreds of nm) with a wet cloth (water or alcohol). Such an issue is currently under investigation at LNS, as we want to study the dependence of the light efficiency on the integrated dose, by irradiating CsI(Tl) plates with several ion species in the energy range 10–100 keV. All the LEBIs installed along the low energy beam line allowed a real time display of position and beam profile for a wide range of energies and intensities. In Fig. 5 a typical beam profile is shown, whose shape was squeezed horizontally in order to inject the beam into the mass separator dipoles. The graphical patterns superimposed to the on-line images allowed a quick beam alignment as well as an overall test and debug of the installed equipment and of the whole complex facility. Finally, we were able to optimize the setting of all the beam transport elements, in order to maximize the transmission step by step from the ion source up to the Tandem. 3.1. Results with the 8Li radioactive beam b
The 8Li radioactive beam (8 Li-8 Be, Emax ¼12.96 MeV) was produced using 13C at 45 MeV/A as a primary beam [7]. The 8Li ions produced inside the thick target (graphite in this case) were extracted by means of a 10 kV voltage and preselected by means of a suitable magnetic dipole on the first platform. After setting
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the beam line transport parameters for the specifications of 8Li, the LEBIs have immediately allowed to measure the beam intensity, to display its profile and to identify the beam as 8Li. In Fig. 6 the counting rate profile is shown, as acquired with a beam on/off sequence, obtained by opening and closing a fast line valve ahead of a LEBI. The measurement was done with the multichannel scaler by counting PMT pulses in time intervals of 100 ms. After opening the valve it is possible to observe the exponential increase of the counting rate (Eq.(1)), whose value becomes stable after roughly 5 s, in agreement with approximately 5 times the decay constant, that is tdecay ffi 1.2 sec. The exponential fit allows to deduce the half-life T1/2 ¼0.8470.01 s, in perfect agreement with the known value of the 8Li half-life (T1/2 ¼ ln 2 tdecay ¼0.83870.006 s) [8]. By integrating the counts for a time interval of 1 s and correcting for the efficiency of 4871%, as calculated with PENELOPE, the value of the beam rate could be measured on each device in real time. Concerning the beam imaging, we have been able to display 8 Li beam profiles whenever the overall energy released by the beta electrons inside the screen was above the LEBI sensitivity threshold, that in this case was corresponded to the order of 102–103 pps/mm2. In Fig. 7 the profile of a beam with a particle
flux of 6 103 pps/mm2 is shown. As can be observed, a difference with respect to the stable beam is that for the radioactive beam the spot boundaries are faded, since the beta radiation is emitted isotropically and the corresponding image is thus proximity focused. As a consequence the space resolution in such a case is of the order of the screen thickness (1–2 mm), for the MeV-range beta particles are capable of crossing it.
Fig. 5. 7Li stable beam profile, having a width of about 3 mm. The shadows of the thin tungsten wires are visible.
Fig. 7. 8Li beam profile. The kinetic energy is 10 keV, the intensity is 100 fA (6 105 pps).
4. Conclusions With the help of the eleven LEBIs installed along the low energy beam line, we have been able to easily transport the beam across the mass separator with close to 100% efficiency. In light of the complexity of the facility, the advantage of having a multisensor in a single device has allowed to speed up the beam transport procedures and to increase the overall reliability of the beam diagnostics system. The beam crew have routinely operated the device to acquire the needed parameters along the whole beam line, in every point critical for the beam transport. In Table 1 a list of future beam candidates is reported with their respective half-life T1/2. It can be observed that in some cases (e.g. 15O,
valve closed
7000 valve opened
6000
counts
5000 4000 3000 2000 1000 0
0
2
4
6
8
10 time (sec)
12
14
16
18
20
Fig. 6. Temporal profile of the counting rate after closing (beam-off) and opening (beam-on) a line valve. The exponential behaviour is fitted in order to calculate the T1/2.
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Table 1 Possible beams to be produced with the EXCYT facility. Isotope
half-life
Isotope
half-life
8
0.838 s 0.178 s 19.255 s 20.39 min 2.449 s 112.24 s 26.9 s 13.51 s 64.49 s
18
109.77 min 11.16 s 7.183 s 6.56 min 2.511 s 1.526 s (0 + ) 32.00 min (3 + ) 37.24 min 55.6 min 1.35 min
Li Li 10 C 11 C 15 C 15 O 19 O 20 O 17 F 9
F F 25 Al 29 Al 33 Cl 34 Cl 38 Cl 39 Cl 40 Cl 20
11 C, 18F) the time needed to reach the saturation activity value can be as long as several minutes. For the expected future beams, accurate preparation of all the diagnostics has therefore to be performed, taking into account the peculiarity of the radionuclides to be analyzed, in particular the half-life T1/2 and the endpoint energy of the beta decay.
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Acknowledgments The authors wish to thank the accelerator staff and the electronics service of LNS, for the significant contribution to the development and test of the LEBI device. In particular Mr. A. Amato, Mr. P. Litrico, Mr. V. Scuderi and Mr. F. Tudisco deserve a special mention. References [1] D. Rifuggiato, et al., Nucl. Instr. and Meth. Phys. Res. B 261 (2007) 1040. [2] P. Finocchiaro, in: Proceedings of the 15th CAARI November 1998, Denton, Texas, USA. [3] S Cappello, et al., Nucl. Instr. and Meth. A479 (2002) 243. [4] L Cosentino, et al., IEEE Transac. Nucl. Sci. NS-48 (4) (2001) 1132. [5] Nucl. Instr. and Meth. B 132 (1997) 377 /http://www.nea.fr/abs/html/ nea-1525.htmlS. [6] A Del Zoppo, et al., Nucl. Instr. and Meth A 581 (2007) 783; M. La Cognata, et al., Phys. Lett. B 664 (3) (2008) 157; A. Musumarra, et al., Nucl. Instr. and Meth. A 612 (2010) 399. [7] G. Cuttone, et al., Nucl. Instr. and Meth. B 266 (2008) 4108. [8] F. Ajezenberg-Selove, Nucl. Phys. A 490 (1988) 1.