Test Beams of the LPI Accelerating Complex Pakhra

In this paper, we report the characteristics of the generated beams, the extracted electron beam and the beam of secondary electrons/positrons, of the LPI accelerator complex Pakhra as of November 2019 for testing and calibrating detectors used in large accelerator and astrophysical experiments. At energies of 250–500 MeV of the electron beam ejected into Hall 1 from the S-25R synchrotron, its energy resolution is δ ~ 1% and the intensity can be changed by collimators in the range of 10³–10¹⁰ s^–1. The quasi-monochromatic beam of secondary electrons produced in Hall 2 typically has an energy in the range of 50–300 MeV, a corresponding energy resolution of δ = 14–2%, and an intensity of ~ 10² s^–1. The performed simulation of the characteristics of the secondary positron beam allowed selecting the optimal geometry of the setup and showed good agreement with the experimental data obtained later.

Keywords: electron synchrotron; test beams; extracted electron beam; secondary electron beam; energy resolution; intensity; simulation

Copyright comment ISSN 1063-7788, Physics of Atomic Nuclei, 2020, Vol. 83, No. 12, pp. 1695–1699. © Pleiades Publishing, Ltd., 2020.

In experimental nuclear physics, particle physics, and astrophysics, it is often necessary to calibrate detectors and equipment. Medium-energy electron and photon beams are also required for solving a number of related problems, such as studying the structure of materials and radiation resistance of electronic components and measuring the cross sections of nuclear processes for civil and defense applications. For these purposes, beam-generating accelerators are needed.

The LPI electronic synchrotron S-25R in Troitsk (the accelerator complex Pakhra) was designed in the mid-1960s for a maximum electron energy of 1.2 GeV and launched by the mid-1970s. It was mainly intended for use in nuclear physics experiments using a bremsstrahlung gamma beam and an extracted (with slow extraction) electron beam in the energy range above the π-meson production threshold. At present, the direction of the research changed, partly because the implementation of large international projects (the GAMMA-400 astrophysical observatory, the SPD, MPD, and BMN setups of the NICA project, and experiments at the Nuclotron in Dubna) required inexpensive testing tools based on existing medium-energy accelerators.

The S-25R synchrotron is currently practically the only constantly operating accelerator in Russia that generates beams of electrons, positrons, and photons with energies up to 850 MeV. In accordance with the existing needs, the task arose to revive and modernize the once existing magneto-optical channel of the extracted high-intensity electron beam and to create again a less intense test beam of secondary electrons (positrons) based on the bremsstrahlung photon beam.

The S-25R synchrotron has four sections of turning magnets with rectilinear intervals between them. The radius of the equilibrium orbit in the turning sections is 400 cm, and the length of each rectilinear interval is 190 cm. The injector of the synchrotron is a microtron with an output energy of 7.4 MeV. After being extracted from the microtron, the beam is formed by an electron-optical path and, with the help of a magnetic inflector, is introduced into the synchrotron chamber. The synchrotron magnetic field change frequency is 50 Hz. The injection of electrons into the synchrotron is carried out at a magnetic field induction in the turning magnets of Bi = 0.0060 T. The maximum electron energy is reached at Bmax = 1 T. The accelerator resonator is excited at a frequency of 55 MHz.

To test detectors and equipment of large modern setups, a calibration (test) zone was created in Hall 1. Created in the 1980s, the channel for the slow extraction of electrons of the S-25R accelerator [[1]] (Fig. 1) had to be geometrically rebuilt and adjusted after installing a special compensator in order to reduce the effect of the edge field of the synchrotron. The main advantages of the test zone in Hall 1 (based on the slow extraction channel) compared to the test zone in Hall 2 (based on a secondary electron or positron beam) described below are the low-background operating conditions of experimental facilities, better energy resolution, and, if necessary, high intensities of the electron beam.

Graph: Fig. 1. Schematic of the LPI accelerator complex Pakhra. M1 and M4 are the turning magnets; М2 and М3 are the spectrometric magnets.

The electron beam extracted from the S-25R synchrotron is formed by a slow extraction system and an extended (~25 m) magneto-optical channel (MOC).

Slow extraction of electrons using the resonance of radial betatronic oscillations of the fourth order (νx = 3/4) is implemented using two pole windings of the accelerator and two extraction septum magnets [[1]]. The gap of the magnetic cores of both septum magnets has a height of 1.2 cm and a width of 3.5 cm. The magnetic field induction in the gap of the first septum magnet is 0.08 T and reaches 0.4 T in the gap of the second septum magnet. The position of both septum magnets relative to the central orbit can be changed using the displacement system in the range of 0–5 cm.

Through the output window of the accelerator (aluminum plate with a thickness of 0.2 mm) and an air gap with length of 0.7 m, an electron beam is introduced into the MOC channel (Fig. 2). The MOC electronic channel is evacuated, and the channel diameter is 38 mm at the lens locations and 80 mm in the interlens gaps. The total length of the channel from the output window of the accelerator in the accelerator hall to the SP-57 magnet in Hall 1 is ~25 m. The channel includes four lenses and one SP-3 turning magnet in the accelerator hall with a pole size of 500 × 200 mm2 and an interpole distance of 50 mm. The required beam angle of 18.5° is achieved at the magnetic field induction of BSP−3 ≈ 0.6 T for electrons with energy of E0 = 350 MeV. Beam proportional chambers, as well as installed radiators and video cameras, perform the correctness control of the beam passage.

Graph: Fig. 2. Schematic of the slow extraction channel of the S-25R accelerator. MFC is the edge magnetic field compensator, L1–L4 are the quadrupole lenses of the magneto-optical channel, and SP-3 is the turning magnet.

One of the key elements of the channel is a magnetic field compensator (MFC), a device designed to correct the initially developed extraction scheme, namely, to exclude the effect of the edge magnetic field of the accelerator magnet on the electron beam. After exiting the accelerator, the beam travels 70 cm near the pole of the accelerator magnet, whose edge alternating field (Bmax ~ 0.4 T) strongly "stretches" the beam in the horizontal plane. When the beam enters the MFC, the effect of the field ceases, and it is transported without distortion to the first lens and then to Hall 1. The MFC is a part of the vacuum channel as its first element. The basis of the MFC is a stainless steel cylinder welded from three pipes of various diameters with a thin wire of soft annealed iron tightly wound. The inlet and outlet diameters of the MFC are 1.5 and 2.5 cm, respectively. The length of the device is 110 cm.

At an electron energy of E0 = 350 MeV, the beam size at the output window of the accelerator is 7–10 mm both horizontally and vertically at an extracted beam intensity of ~5 × 1010 s–1. In the test zone of Hall 1, the beam is focused by the L4 lens of the MOC onto the frontal plane of the detector under study into a round spot with a diameter of ~10 mm. The beam intensity at the accelerator exit is determined by tuning the parameters of the accelerator and the slow extraction system. A decrease in the intensity during beam transport is determined by its "spreading" by the scattered field of the accelerator magnet in the gap between the output window and the MFC, multiple scattering in air in the same gap, and scattering by the residual gas in a long (up to 25 m) transport channel. Combined with the high MOC selectivity, these factors reduce the intensities in the test zone to 5 × 109–1010 s–1. Thus, at least half an order of magnitude of the initial intensity is lost during transport. However, even this intensity on a scale of ~1010 s–1 has to be reduced by installing collimators with holes with a diameter of 2–5 mm on the output channel in front of the L2 lens (in front of the SP-3 turning magnet) and be reduced to a level of 103–106 s–1, which is convenient for testing and calibrating detectors. The energy range of the extracted electrons is Ee = 200–500 MeV with an energy spread of δe ~ 1%.

Implemented on the basis of the SP-57 spectrometric magnet, a test zone γ1 is equipped in Hall 2. It includes (a) a 40-m channel for transporting a beam of bremsstrahlung gamma quanta with a maximum energy of 300–500 MeV generated by interactions of the edges of the bunches of accelerated electrons with an internal tungsten target with a thickness of 0.22X0 in the synchrotron vacuum chamber (X0 is the radiation length) [[2]], and (b) a beam of secondary electrons (or positrons) with an energy range of Ee = 30–300 MeV at an energy resolution of δ = ΔЕe/Ee = 14–2%, respectively (Fig. 3) [[3]]. The intensity of the secondary electron beam (positrons) with the 30-mm C3 collimator in the lead shield is about 102 s–1.

Graph: Fig. 3. Schematic of the test channel γ1 of the Pakhra accelerator: EGc—internal tungsten target; Tv—vacuum terminal with an aluminum window with a thickness of 0.2 mm; C1—the first collimator of the γ1 channel with an outlet diameter of 13 mm; Wl—reinforced concrete wall of the accelerator hall with a thickness of ~3 m; T—output of channel γ1 into Hall 2 (steel pipe with a diameter of ~16 cm); C2—the second scriber-type channel collimator (outlet diameter is 3 cm); M1—cleaning magnet SP-03; GEc—copper converter with dimensions of 0.5 × 5 × 0.1 cm at the edge of the M2 magnet poles; M2—SP-57 spectrometric magnet (the center of the magnet is located at a distance of ~40 m from the inner target of the γ1 channel); C3—collimator of the left shoulder of the test channel (diameter is 3–30 mm); PbWall—protective lead wall of the left shoulder of the test channel (thickness is 10 cm, collimator diameter in the wall is 3–30 mm); tD—location of the studied detector.

Before the final installation of the test channel γ1 of the Pakhra accelerator, the characteristics of the secondary positron beam formed by the "photon beam–converter–magnet–collimator" system were simulated. Numerical calculations were performed using the GEANT4 package, ver. 10.0 with the inclusion of models of the main physical processes corresponding to one of the standard sets, i.e., Physics List QGSP_BERT. The simulation was mainly aimed to determine the optimal geometry of the facility and to estimate the expected energy spread of particles in the secondary beam, taking into account the finite size of the initial photon beam, converter, collimator, edge magnetic field, and scattering in the converter and in air.

Figure 4a shows the distribution of the positron flux in projection onto the median plane of the SP-57 magnet. The results were obtained for a copper converter 2 with dimensions of 0.5 × 5 × 0.1 cm3 located at the cut of the magnet poles 3 on the trajectory of the photon beam passing from bottom to top in the figure. The magnitude of the magnetic field induction at the center of the interpole gap with a width of 6 cm was 0.75 T, the beginning of the field decay was at 6 cm from the cut inward, and the field decay constant was 6.7 cm. The centerline of the collimator of the left (5) protective lead wall with dimensions of 150 × 150 × 10 cm3 made an angle of 36° with the axis of the primary photon beam. The calculation was carried out with the introduction of a technical cylindrical region with a diameter of 600 cm and a height of 30 cm (7 in Fig. 4a), outside of which (excluding the collimator region) the particles were not traced (fell into the "black hole").

Graph: Fig. 4. Simulation results using GEANT4 of the generation of a secondary positron beam at the SP-57 magnet converter. (a) top view of the median plane of the magnet: (1) technical cylinder, outside of which particles cannot be traced; (2) converter mounted on the edge of the magnet pole (small circle in the center); (3) pole of the SP-57 magnet; (4, 5) right and left lead protective walls; (6) collimator and sampled particle flow; (7) (light area) the main flow of positrons. (b) Positron emission angle θ depending on the energy of the produced particle Ee at several values of the induction in the magnet center Bmax: (1) 0.1, (2) 0.2, (3) 0.25, (4) 0.5, and (5) 0.75 T.

Figure 4b shows the result of numerical simulation of the dependence of the emission angle θ of positrons on their energy and of the positron beam intensity on the magnetic field induction at the center of the magnet of Bmax = 0.1, 0.2, 0.25, 0.5, and 0.75 T. The figure shows that, with the above parameters of the collimator, positrons with energies up to ~300 MeV pass through it. The intensity of the positron beam with energies ranging from ~50 to ~130 MeV is maximal, which was further confirmed by direct measurements.

The GEANT4 was also used to simulate the passage of positrons from the birth point in a copper converter with a thickness of 1 mm through a 10-mm collimator to the location of the studied detectors in magnetic fields in a wide range of Bmax values (Fig. 5). The calculated dependence of the energy of positrons passing through the collimator and the experimentally measured dependence of the same energy coincided within ~3% [[4]], which indicates the reliability of the approximate magnetic field map used in the calculations.

Graph: Fig. 5. Simulation results using GEANT4 of the positron beam energy, as well as the average energy of positrons determined experimentally depending on the maximum value of the field of the SP-57 magnet.

The paper presents the characteristics of the test γ and e± beams of the LPI accelerator complex Pakhra. The beams are intended for calibrations of detectors used in large modern accelerator and astrophysical experiments.

The channel for the slow extraction of electrons in Hall 1 produces an electron beam with the maximum intensity of up to ~1010 s–1. The use of additional collimation on the magneto-optical channel allows reducing the beam intensity to a more convenient level of 103–106 s–1. In this case, the energy range of electrons removed from the accelerator is 250–500 MeV at an electron-beam energy resolution of δ ~ 1%.

A low-intensity quasi-monochromatic beam of secondary electrons (positrons) in Hall 2 has the following main characteristics: the range of energies of electrons (positrons) is from 30 to 300 MeV with a corresponding energy spread δ = ΔE/E from 14 to 2%. The intensity of the secondary electron (positron) beam in the case of using a 30-mm collimator in a lead shield is about 102 s–1.

Modeling the generation of a secondary positron beam by a bremsstrahlung photon beam showed that, in the range of magnetic induction values of 0.1–0.75 T, positrons with energies of up to ~300 MeV pass into the 30-mm collimator located in a lead shield at an angle of 36° relative to the photon beam trajectory. The maximum positron intensity was determined to be in the energy range of ~50–130 MeV, which was further confirmed by direct measurements. The calculated dependence of the energy value of positrons passing through the collimator within ~3% coincided with the experimental dependence, which confirmed the reliability of the simulation.

In the future, the simulation is supposed to continue and study, in particular, the question of the feasibility of installing helium bags to eliminate the particle scattering in the air and reduce their energy spread after passing through the collimator.

This work was supported by the Russian Foundation for Basic Research-NICA, grant nos. 18-02-40061 and 18-02-40079, and reported at the ICTEF conference (November 2019).

The authors declare that they have no conflicts of interest.

Translated by A. Ivanov