In accordance with the goals and tasks of the ISTC within the frameworks of the present Project it is suggested to create a research group from scientists and engineers mainly of YerPhI who previously were engaged in realization of defense programs, for research and development of new type neutron flux monitors based on vibrating wire with precise spatial resolution, wide range of operation, robust against radiation. These monitors aimed to use for neutron beams in guides before usage in numerous neutron instruments. This task is urgent for accelerators and reactors aimed to provide neutron beams, nuclear plants diagnostics in field of nuclear safety and Neutron capture therapy for cancer treatment.
Many research centers in the world use neutrons as probes to investigate diverse properties of a wide range of materials. Neutrons have found interesting uses in medicine, both in the treatment and in the development of ultra-sensitive analytical techniques [1]. Neutron scattering gives detailed information about atomic level structure and dynamics. Neutrons used in the scattering experiments have wavelengths similar to the atomic spacing, allowing the structures of materials to be studied by diffraction on the length scales from atomic dimensions to macromolecular scales. There are few centers specialized in neutron therapy [2], where neutrons are particularly generated from low energy cyclotrons. Control of the spatial distribution of the beam and its intensity in this field of usage is vital. So there is need for devices, capable of real-time aquisition of the beam intensity.
Most of the existing neutron sources are based on nuclear reactors or spallation facilities [3], where neutrons are produced by bombardment of heavy nucleus target by high-power proton beam. Of great importance is to form neutrons in a beam with high flux, and well-identified and precisely-controlled parameters of intensity, divergence, geometrical coordinates and sizes. To reach these requirements, a new generation of neutron sources has been developed based on particle accelerators and spallation technology. So far, neutron beam’s flux intensity reaches to 1013-1015 n/cm2/s. The new task is to increase the intensity of the neutron beams further. Instrumentation for the neutron beams needs a variety of detectors and monitors for measurements and controls. All known detectors for slow neutrons today are based on the conversion of neutrons into charged particles. After this conversion, the following technologies are normally used: gas proportional counters, ionization chambers, scintillation detectors, and semiconductor detectors.
In this proposal for neutron beam measurements we suggest new method where used heat release of neutrons in the wire. Proposed new detectors with simple engineering design have fine spatial resolution defined by wire diameter. We intend to combine two unique opportunities – the unprecedented sensitivity of the natural frequency of a clamped vibrating wire to the wire temperature, and remarkable ability of some gadolinium isotopes to the neutron capture. The 157Gd has the highest thermal neutron capture cross section of all the stable isotopes in the periodic table. The capture reaction initiates prompt gamma-rays and complex inner-shell transitions that generate emission displacing an inner core electron. We propose to measure temperature increase of the wire containing gadolinium isotopes, which occurs when neutrons penetrate the wire and deposit some energy into the wire. To match efficient neutron capture with corresponding release of prompt gamma-ray and emission electrons as a heat into the wire we offer to use composite wires with few layers.
So, Vibrating wire neutron monitor (VWNM) with composite wires and wide dynamic range aimed to high level spatial resolution profiling of high flux and large cross-section neutron beams of specialized neutron sources (research reactors and spallation source) on the inlet of neutron based instruments will be developed. As distinct from existing neutron measurement principles proposed method has high spatial resolution, depends on wire diameter. We propose to develop two types of VWNM with different scales of wire length, which defines the resolution and response time. These types will cover different dynamic ranges of neutron flux intensities and can be used in different applications from high-flux neutron beam profiling to high-speed environmental monitoring in nuclear safety field.
In the second method we propose to use vibrating wire as a target occupied different positions in the space during the oscillation process. If the measured beam flux density varies on distances about oscillations amplitude the difference on scattering process in extreme positions can provide information on flux gradient. For neutron beam we intend to use the same gadolinium covered vibrating wires. Difference of measurements of prompt gamma-rays arise at neutron capture in gadolinium layer in synchronism with the wire’s oscillation frequency allow to obtain exclusively the signal of capture events and discriminate usually presented homogeneous background of gamma-rays with wide spectrum of energies. This takes opportunity to use simple nonselective gamma-ray detectors. Usage of sensitive gamma-ray detectors can improve the resolution of the neutron beam profiling compared with the measurements of vibrating wire frequency changes that need considerable heating of the wire. Response time of method is defined by vibrating wire frequency and estimated of order of 1-0.1 ms. Transverse beam profile measurement by this method was tested with lightening the oscillating wire by a laser beam [4]. This type of monitors we call Resonant target vibrating wire neutron monitor (RT-VWNM).
Proposed monitors (VWNM and RT-VWNM) should be a new modification of previously developed by us vibrating wire sensors with wide dynamic range, inherent long-term stability, high precision, and resolution, good reproducibility, minimum zero drift, and small hysteresis. Usage of few simultaneously operating vibrating wires allows correspondingly increase the response speed of monitor and exclude the complicated mechanical system of scanning.
Developed as a result of project fulfillment monitors can be widely used for all applications of neutron beams. VWNM as a precise monitor with excellent spatial resolution for high flux neutron beams of specialized neutron sources with multibranch infrastructure of numerous instruments for material research. Small sizes scale VWNM with 10-100 ms response time may be used in field of nuclear safety as a portable, high-speed environmental monitor. As a portable device VWNM can be used as radiation robust detector at neutron weapon usage.
RT-VWNM as a reliable instrument with also excellent spatial resolution can be applied for low flux neutron beam diagnostics (e.g. for centers of neutron therapy). Specialized multiwire monitors with possibility of rotation along the beam axis can be used to recovery of complicated 2D profiles of large cross-section neutron beams in neutron tomography, imaging and radiography.
VWNM's can be used in 18 MeV Cyclone-18 of Yerevan’s oncological center for outlet beam profile direct measurements in medical treatment. Another area of usage can be diagnostics of neutron beam planned to be generated at Cyclone-18 for use in a broad class of studies and experiments (engineering of materials, biological, chemical, and physical systems investigations, astrophysics, nuclear physics, and material science).
Preliminary experiments and tests is planned to perform on the spontaneous fission neutron sources accessible in Yerevan Physics Institute. Proposed Project is based on the results of three years work on ISTC Project A-347. Different types of Vibrating wire monitors were installed and successfully tested in accelerators, as well as for hard X-ray beam monitoring [5, 6]. Vibrating Wire Monitor with large aperture was further installed at the Fermilab High Intensity Neutrino Source (USA) to measure the proton beam halo [7]. For the invention, construction and successful test of the "Vibrating Wire Scanner" diagnostic system Faraday Cup award was assigned to Dr. S.G.Harutyunyan at 2008 [8].
The main tasks of the Project are following:
- Development of composite wire model for estimations of VWNM technical characteristics;
- Development and manufacturing of composite wires with Gd layers;
- Development of two types vibrating wire resonators sensitive to neutron fluxes;
- Development of vibrating wire resonators and electronics for resonant target VWNM;
- VWNM and RT-VWNM tests using radioactive elements and neutron beams of research reactor;
- Marketing of VWNM and RT-VWNM.
In scientific aspect to achieve the purposes of the Project important theoretical investigations in area of neutron interaction with matter, numerical simulations based on finite element analysis and experimental work will be done.
At present there are agreements with Prof. T. Reetz (HTM Reetz GmbH, Germany) Mr J. Bergoz (Bergoz Instrumentation, France) and Prof. M. Chung (UNIST, Korea) on collaboration for application of developed sensors based on vibrating wire for profiling of neutron beams.
During the Project realization Vibrating wire neutron monitor different modifications will be developed, with definitely commercial interest. VWNM and RT-VWNM monitors can be widely used for all types of neutron beam diagnostics in neutron sources including centers of neutron therapy, as well as at Yerevan’s oncological center.
Fourteen specialists from YerPhI will take part in present Project, which have a great experience of theoretical and experimental investigation in neutron science and technology, accelerator physics and technology, beam instrumentation, as well as in area of automation of physical experiments, chemistry and structural mechanics
The project is supposed to carry out during 36 months.
When carrying out the Project worldwide experience of vibrating wire based sensors development, as well as the experience and achievements of Project participants will be used.
Proposed Project meets the ISTC status and is stimulated by the necessity to create alternative jobs for scientists and engineers, who previously were engaged in defense programs of the USSR.
