Development of slow positron beam lines and applications Nagendra Nath Mondal
Citation: AIP Conference Proceedings 1970, 040005 (2018); doi: 10.1063/1.5040217 View online: https://doi.org/10.1063/1.5040217 View Table of Contents: http://aip.scitation.org/toc/apc/1970/1 Published by the American Institute of Physics
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Development of Slow Positron Beam lines and Applications Nagendra Nath Mondal1, a) 1)
Department of Physics, Techno India Batanagar, B-7/360 Putkhali, Maheshtala, Kolkata 700141, West Bengal, India. a)
Corresponding author:
[email protected]
Abstract. A positron is an antiparticle of an electron that can be formed in diverse methods: natural or artificial E-decay process, fission and fusion reactions, and a pair production of electron-positron occurred in the reactor and the high energy accelerator centers. Usually a long-lifetime radio isotope is customized for the construction of a slow positron beam lines in many laboratories. The typical intensity of this beam depends upon the strength of the positron source, moderator efficiency, and guiding, pulsing, focusing and detecting systems. This article will review a few positron beam lines and their potential applications in research, especially in the Positronium Bose-Einstein Condensation.
INTRODUCTION (i) There are two E-decay process: ߚ ା -decay, when a proton (p) decays into a neutron (n); a positron (e+) and a neutrino (ν) are radiated, i.e., ՜ ݊ ݁ ା ߥ. Similarly in ߚ ି -decay, n decays into p, and e- and antineutrino ( ߥҧ ) are generated. Extracting those elementary charged particles accelerated beam lines are constructed with desirable intensities for various applications. (ii) Reactor based e+: Many fissile materials, e.g., Th232, U238 and Pu239 undergo fission reactions when bombarded by thermal neutrons resulting e+ radiation from the successive decay of daughter nuclei. A common fission reaction in atomic reactor is ଶଷ ଽସ ଵସ ݊ ଶଷହ ଽଶܷ ՜ ଽଶܷ ՜ ହସܺ݁ ଷ଼ܵ ݎ ʹ݊ Not only the charged particles but also energetic γ- rays (energy ≥ 1.022 MeV) are discharged from the decays of daughter nuclei and are forced to create pair productions by smashing the surface of a thin single crystal. Another process is the fusion reaction (naturally occurred in the Sun) stated as follows: ା ଵ ଵ ଶ ଵ ܪ ଵ ܪ՜ ଵ ܪ ݁ ߥ ଶ ଶ ଷ ଵ ܪ ଵ ܪ՜ ଶ ݁ܪ ߥ ଷ ݁ܪ ଷଶ ݁ܪ՜ ସଶ ݁ܪ ଵଵ ܪ ଵଵܪ ଶ After moderation this e+ can be utilized as a slow positron beam. Pair production can produce high intense e+ beam provided the intensity of mother beam, efficiencies of target and moderator in the reactor controlling system are very high. In the elementary particle physics e+ can be extracted from the decay of lepton too. For examples: ߨ ା ՜ ߤ ା ߥఓ ሬሬሬറ ߤ ା ՜ ݁ ା ߥ ߥ ఓ + ା Hadrons and Kaon undergo decays into e : ܭ՜ ߨ ݁ ା ߥ . (iii) Accelerator based e+: Innumerable scientists of particle accelerators are working in order to achieve the highest intense slow e+ beam. Some of the LINAC based centers are BNL (NY), CERN (Geneva), KEK (Tsukuba), SLAC (Stanford), Jefferson (VA) etc. TRIUMF (Vancouver), KFA (Julich), RIKEN (Saitama), JINR & FLNR (Dubna) International Workshop on Physics with Positrons at Jefferson Lab AIP Conf. Proc. 1970, 040005-1–040005-7; https://doi.org/10.1063/1.5040217 Published by AIP Publishing. 978-0-7354-1677-2/$30.00
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LBL (Berkeley), etc. are cyclotron based centers. Those particle accelerators provide energetic γ- rays (> 1.022 MeV) for the pair production. Details of these beam lines with applications are described in the following sections.
SLOW POSITRON BEAM LINES Radio isotope based slow positron beam lines Most of the slow positrons beam lines are developed on the basis of Radio Isotope (RI). The moderate lifetime (halflife ~ 2.6 years, measured by the formula, ܶଵȀଶ ൌ +
22
Ǥଽଷ ఒ
, where λ is called a decay constant.) and intensity (90% of
the decay is e ) of Na are made it an ideal runner for the construction of an intense slow e+ beam line. The typical intensity of this beam is about 104 – 107 e+/s. We had developed a slow positron beam line namely TOPS at Tokyo Metropolitan University, Tokyo, Japan in 1998 and studied many crystalline surfaces in order to justify the highest intense Positronium (Ps) production [1]. A schematic diagram of a “TOPS” is shown in Figure 1.
FIGURE 1: A slow positron beam line with laser facilities @ Tokyo Metropolitan University, Tokyo, Japan.
A few world-wide slow positron beam line facilities with intensities are summarized in Table-1. Bose-Einstein Condensation (BEC) is one of the moving areas of QED, atomic physics and statistical mechanics. In order to achieve the Ps-BEC (density is about 1015 Ps/cm3 at mK temperature), the highest intense Ps production on the surface of a target or inside the nano-pore materials are vital points. Hence suitable materials, intense source of continuous/pulsed slow e+s, advanced Laser cooling of ortho-Ps and Ps-BEC detection systems should be developed innovatively [2-14]. A slow e+ beam line is developed at the University of California, San Diego (UCSD), based on 22Na source whose strength is 50 mCi and is about 3 mm in diameter. The efficiency is measured about 30% with flux of 5×108 e+/s in the forward direction. With solid Neon moderator 6 – 9 ×106 slow e+/s is found under a 100 Gauss field with a beam diameter of 1.0 - 1.2 cm. Energy spread of the slow e+ is 1.9 eV (FWHM). They have developed a multi-staged buffer gas trapping system for collecting a large numbers of e+ [15]. Recently, 1015 e+ can be accumulated by using a Penning-Malmberg trap [16]. This highest intensity e+ burst allows them atomic physics experiments. Another RI based slow e+ beam line is developed by A.P. Mills, Jr., at the University of California, Riverside (UCR). The 22Na source strength is 25 mCi, where e+s are moderated with a solid Neon moderator engendering slow e+ beam of
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intensity ~ 6×106 e+/s. Those moderators are grown at a temperature of 7 K with ultra-pure 99.999% Neon within 7 minutes. The UCR group uses a trapping device to achieve low energy e+ pulses of intensity about 6×107 e+ with a width of 1 ns [17]. They have done tremendous efforts for the experiment of Ps-BEC [18]. TABLE 1. Summary of a few slow positron beams and applications Name and place Contact Positron Beam Energy persons source EPOS, Halle, Dresden LLNL, Livermore KEK-B Factory, Tsukuba TU-Delft reactor, Amsterdam MRR-FRM-II, Munich TOPS, Tokyo M. University GU, Tokyo Bonn University TUS, Tokyo SHI, Tokyo NCSU Jefferson
Prof. KrausRehberg Dr. R. H. Howell Dr. T. Kurihara Prof. P. J. Schultz Prof. G. Kogel
Beam Intensity e+/s
Applications Defects, AMOC, CDBS, PACS etc. Defects, CDBS, PACS etc. 2D-ACAR, TOF, Spin polarization 2D-ACAR, 2D-Doppler, Depth profile Positron microprobe, defect concentration BEC, Laser cooling, defects, polarization etc.
40 MeV e-Linac
0.2 – 100 keV
Pelletron, 3 MeV 2.5 GeV eLinac Reactor based
1 – 50 keV
Moderated: 109 and Pulse: 106 300, 20 MHz
10 – 100 keV
108
1 eV – 40 keV
108
Reactor based
100 eV
107 – 109
22
Dr. N. N. Mondal/ Dr. T. Kumita Dr. I. Kanazawa Dr. K. Maier
Na (150 mCi) source
1 eV – 250 keV
106
22
30 eV
10
22
150 eV
10
Dr. Y. Nagashima Dr. M. Hirose
22
100 eV
10
Compact Cyclotron
10 – 150 keV
10
Dr. Ayman Hawari Joe Grames
Reactor based
variable
6×108
LINAC
---
---
Na (3 mCi) Na (10 mCi) Na (740 MBq)
3
Vacancy-type defects
3
Surface and dislocation of materials Ps-, moderator, defects of materials. Commercial purpose, surface, interface, polarization. Defect studies of various materials Fundamental research
5 6
Reactor based slow positron beam lines There are only a few nuclear reactor-based e+ sources are available where e+s are created by pair production from the reactor core. The intensity of slow e+ not only depends on the power of the core, converter material, and moderator geometry but also on the criticality (keff), pulsing and guiding system. Reactor core based e+ source at PULSTAR is a 1-MW located at the North Carolina State University. Main interests of this center are in neutron diffraction, ultra-cold neutron studies, and deliver intense e+ beams to the users. Positrons are created from a converter-moderator assembly surrounded by cadmium blocks adjacent to the core [19]. Positrons are created by pair productions and also with neutron capture in the Cadmium (Cd). In this reaction 9 MeV radiation energies are released [20]: 113Cd + 1n Æ 114Cd + 9 MeV. The W target is placed closer to 30 cm of the core. The PULSTAR reactor beam currently uses two W arrays as e+ converters and moderators. Each array is 22 cm in diameter and 2.5 cm in length. An array is comprised of interlocking W strips of each thickness 250 μm can produce 5×108 slow e+/s. The NEutron induced POsitron source of MUniCh (NEPOMUC) [21] is a 20 MW reactor-based e+ beam facility. Reaction process is similar to PULSTAR and a structure of Platinum (Pt) and a stack of W foils are placed for converting the γ-rays into e+e- pairs. Platinum is used because e+e- production cross-section of Pt is higher than W. Inside the Cd cap, the mean-flux density is expected to be 4.1×1013 /cm2/s. About 15% of the γ-radiation of 6.2×1012 /cm2/s originates from the core. The maximum energy of e+ spectrum is about 800 keV. The W foils also act as a moderator assembly. After moderation e+ beams are accelerated by electrostatic lenses and guided by magnetic fields of 6 mT. The beam diameter is about 18 mm (FWHM) and intensity is close to 10 9 e+/s with average energy of 1 keV [22, 23].
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Positron beam line (POSH) at the Interfaculty Reactor Institute in Delft that provides a constant intensity of 0.8×108 e+/s to a 2D-ACAR target chamber [24, 25]. In the reactor core high energy γ-rays are generated when thermal neutrons are captured in Cd foils via the nuclear reaction 113Cd (n,γ) 114Cd that produce pair production in W foils. The POSH was designed installed at the Delft 2 MW research reactor [25] in 1998. An assembly of thin W foils, configured as 4 disks containing 10 × 10 mm square channels, serves to convert the γ-rays into slow down and thermalize the energetic e+s, and, finally to re-emit slow e+s (3 eV) from the tungsten surfaces into the vacuum tube containing the source. In March 1999 a beam intensity of 2×108 e+/s was demonstrated.
Accelerated based slow positron beam lines The highest intense slow positron beam with a variable time structures are developed by pair production facilities. Electron beam is used as a typical driving beam to produce the bremsstrahlung photon of energy ≥ 1.022 MeV that can create pair production on a suitable target. A Van-de-Graff generator can be used to accelerate p or deuteron (d) up to 4 MeV [26] in order to produce e+ alternatively. Accelerated d beam hits a graphite target and emits e+ via 12C (d,n)13N reaction. In BNL such a facility is exist. In Dresden, ELBE Super Conducting e- -LINAC (40 MeV) produces one of the highest intense slow e+ beam (EPOS) of energy 0.2 – 40 keV for materials studies [27]. The bunch structure of the driving beam is 77 ns with repetition cycle 13 MHz and 1 mA current [28]. The entrance diameter of e- beam is 5 mm and passes next to a stainless steel window (0.3 mm), 0.1 mm of water column, followed by a heap of 50 W foils of thickness 5 mm. The separation gap is 100 μm through which the cooling water runs and simulated intensity of e- beam is about 5 ×1013 /sec. The W moderator is placed near the converter and projected slow e+ intensity after moderation is about 5×108 – 109 e+/s. In Tsukuba (KEK) there is an e- accelerator (beam energy 70 MeV , 1 μs pulse with 3 μA average current of cycles 100 Hz) is developed by the group of Advanced Industrial Science and Technology (AIST). This facility is generating a slow e+ beam for material science experiments to identify the nano scale defects in various applied materials. A similar bremsstrahlung technique is utilized to produce e+ beam. The fast e+ s are immediately moderated through W films (25 μm) and using the magnetically guiding system (7 mT) those are transported to the experimental room which is about 20 m long in order to escape the background. The moderated e+ beam diameter and energy are about 10 eV and 10 mm respectively. The beam intensity at the experimental site is about 2-3×107 e+/s [29]. In order to reduce the spot size they followed the Brightness enhancement method where the beam was extracted from the magnetic field prior to reach the experimental hall. It was focused by a lens on a re-moderator (200 nm thick single crystal W) to enhance its brightness. After re-moderation, the spot size at the target place was measured ~30-100 μm [30]. The efficiency of the transmission in re-moderator is about 5%, which reduces the intensity to ~ 106 e+/s [31]. The primary source of positrons at LLNL is located at the end of a 100 MeV LINAC where e+ current is ~ 400 mA with repetition rate of 300 Hz. Duration of pulse is 3 μs is obtained by a penning trap system and average beam power is up to 45 kW. The energetic e-s were stopped in a water-cooled W target from where the shower of photons produced by bremsstrahlung method. The conversion of photons yields a pair of e+e-, an intense source of e+. It was slowed with Venetian blinds type moderator as described earlier. Again it was moderated by a single crystal W and attained the slow e+ beam (3×107/s) which is good for many material studies in conventional PALS systems [32].
Cyclotron based slow positron beam lines The pair production mechanisms produce a higher intense positron beam in many cyclotron centers world-wide. A high intensity (~ 7.8 ×1021 e+/s at peak position) pulsed e+ beam is obtained from the accelerated e- (energy is a few MeV) at the third generation cyclotron center in Shanghai [33]. The superconducting wiggler with 8-12 Tesla magnetic field of Spring-8 storage ring in RIKEN provides another highest intensity 10 12 (slow e+/sec). Sumitomo heavy industries in Japan has been producing different types of compact cyclotron for various purposes. Positron emitting cyclotron, proton therapy cyclotron, and Medical cyclotron for producing radio isotopes for clinical pupose are the most. For examples: 18F, 15O, 13N, 11C etc. are produced for the Positron Emission Tomography (PET) in order to image cancerous cells and tiny brain tumors in the body.
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APPLICATIONS OF SLOW POSITRON BEAM LINES Defect studies of materials Both the POSH and EPOS beam lines are utilized for performing the high resolution measurements of the electron momentum distribution (2D-ACAR) for depth localized defects, thin layer systems, interfaces and defect depth profiling. It helps to map “vacancy type” defects in a 3-D fashion. The lifetime spectrum can be described as a sum of decaying exponentials as follows: ூ ܰሺݐሻ ൌ σୀଵ ሺെݐȀ ߬ ሻ ఛ
Where f is a number of different annihilation and ߬ lifetime and ܫ intensities respectively are of the ith annihilation. In Doppler broadening measurement the detected energy spectrum of annihilation γ-rays will provide the information about the bulk and defects after analyzing the S (sharp) and W (wing) parameters. Those are defined by: ାி ܵൌ , and ܹ ൌ , respectively. ାା ାாାி The implantation of slow positron beam can be approximated by a Makhovian profile: ݉ ݖିଵ ݖ ܲሺݖǡ ܧሻ ൌ െ ൬ ൰ ൨ ݖ ݖ Where m is the shape parameter is set to be 2 and mean implantation depth: ݖറሾ݊݉ሿ ൌ
ξగ ݖ ଶ
ఈ
ൌ ܧ ఘ
Where ߩ represents the density of the material and ߙ, n are the material independent constants. Positron trapping rate at defect sites is given by: ܫଶ ͳ ͳ Ȟௗ ൌ ߤܥௗ ൌ ൬ െ ൰ ܫଵ ߬ ߬ௗ Where ߤ is trapping coefficient, ܥௗ defect concentration, ߬ is bulk lifetime and ߬ௗ is defect lifetime. It is also possible to know the defect size (volume) by the following formula: Ͷ ݒ ሺ߬ଷ ሻ ൌ ߨݎଷ ሺ߬ଷ ሻ ͵ Where, ߬ଷ is the third component of ortho-Positronium lifetime and ݎ is the radius of the volume of a defect. Hence e+ lifetime measurement is a unique and nondestructive spectroscopy of defect and structures of various materials including nanomaterial.
Medical diagnostics (positron therapy) An interesting diagnostic system, positron therapy similar to proton therapy has been launched in many institutes in advanced countries. There are four such facilities in Japan and one of them is in Tsukuba University. An intense e+ beam is extracted from the LINAC of 7 – 250 MeV beam transportation system. About 500 patients are given treatment during the year 2001 – 2004 and each patient is required 10 – 20 min/treatment [34].
Fundamental Research (BEC) Positronium is a quasi-bound atomic state comprises numerous fundamental researches in advanced atomic and molecular physics. In compare to hydrogen atom the lightest mass makes it possible to achieve of Ps-BEC. Thermal para-Ps and work-function Ps can’t be the subject of Ps-BEC because of their shorter life-time (125 ps) than the 1S – 2P transition period and higher kinetic energy (a few eV) respectively. On the other hand thermal ortho-Ps has a lifetime of 142 ns and lower kinetic energy (a few meV) is an ideal candidate only if phase space density 1015 Ps/cm3 can be extended. In order to realize the Ps-BEC many laboratories have been trying to reach the e+ beam intensity more than 1012 /sec, e+ storing (trapping) system, high intense laser (of wavelength 256 nm) and efficient detection system [35-37]. We had accomplished a laser cooling of ortho-Ps by constructing a slow e+ beam line (Figure-1), a Cr:LiSAF laser system, e+ pulsing system and advanced detection system. A typical spectrum of this measurement depicted in Figure-2. A few years back Mills and co-workers at UCR made tremendous efforts to achieve Ps-BEC and Ps molecule [18].
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FIGURE 2: Laser cooling of ortho-Ps measurement at different temperature. Spectra show the lifetime effect due to laser-Ps interactions.
IMAGING TECHNIQUES IN VARIOUS FIELDS OF RESEARCH An array of position-sensitive γ-ray detectors is one of the best detection and imaging system which is convenient for the visualization of Ps-BEC. The size and scintillation materials used in the detectors system play a vital role for the improvement of energy and time resolutions, detection efficiency and imaging without artifacts. Recently we have developed an image reconstruction algorithm (Position vector method) which enables us to take data and process the image in-situ treatment. In this precession measurement tiny brain tumors are imaged within the stipulated period of the diagnosis of the patient [38-40].
DISCUSSIONS AND CONCLUSIONS In order to achieve Ps-BEC and e+ therapy many laboratories have been developing higher intense slow e+ beam lines, efficient and advanced detection, Laser cooling, e+ accumulator, pulsing, guiding and polarization systems. High density nano-porous materials are relevant for accumulating the huge density Ps production which is advancing too.
ACKNOWLEDGEMENTS Author is pleased to acknowledge the financial support from his family and thanks the organizers (Prof. Joe Grames and Prof. Farida A Selim) for accepting the Talk in the conference.
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