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Diffusive Radiation in Random Media

#A-655


Diffusive Mechanism of Radiation in Random Media: Application to Particle Detection and Astrophysics

Tech Area / Field

  • INS-DET/Detection Devices/Instrumentation
  • PHY-PFA/Particles, Fields and Accelerator Physics/Physics
  • SAT-AST/Astronomy/Space, Aircraft and Surface Transportation

Status
8 Project completed

Registration date
18.12.2000

Completion date
08.11.2006

Senior Project Manager
Glazova M B

Leading Institute
Yerevan Physics Institute, Armenia, Yerevan

Supporting institutes

  • Byurakan Astrophysical Observatory, Armenia, Byurakan

Collaborators

  • Thomas Jefferson National Accelerator Facility, USA, VA, Newport News\nUniversity of Southern California, USA, CA, Los-Angeles

Project summary

The investigation of the radiation that originates at the passage of relativistic charged particles through a dielectric random inhomogeneous medium is the aim of this project.

It is well known that charged particle moving through a random medium emits electromagnetic waves. One can explain the rise of this radiation as follows: the charged particle forms an electromagnetic field (pseudophoton) in the medium that is converted into radiation by scattering on inhomogeneities of permittivity. So, the main problem here consists in correct consideration of the pseudophoton scattering in random media.

In earlier works on this problem the multiple scattering of the pseudophoton has not been taken into account. We showed that in the range of wavelengths l<< l << L, lin (where l is the mean free path of photon in medium, lin is the inelastic mean free path of the photon , L is the characteristic size of the system) the role played by the multiple scattering is essential. It leads to a diffusion of pseudophoton and this diffusion gives the principal contribution. The conversion of pseudophoton into radiated photon is more efficient in the appropriate region for the multiple scattering than in the single scattering region due to the fact that one takes into account the correlation effect of the random dielectric instead of considering the local single scattering only.

Note that the contribution of diffusion to the radiation intensity is L2/l2 times larger than that of single scattering which reproduces all the results of conventional transition radiation theory. The ratio L2/l2 has the sense of average number of pseudophoton scattering events in the medium. The diffusive mechanism of radiation (DR) in random media is realized under assumption that the photon is weakly absorbed and the condition lin>>l is satisfied. Taking into account the weak absorption of pseudophotons one obtains that the average number of scattering events is equal to lin/l.

Note that the contribution of diffusion to the radiation intensity is L2/l2 times larger than that of single scattering which reproduces all the results of conventional transition radiation theory. The ratio L2/l2 has the sense of average number of pseudophoton scattering events in the medium. The diffusive mechanism of radiation (DR) in random media is realized under assumption that the photon is weakly absorbed and the condition lin>>l is satisfied. Taking into account the weak absorption of pseudophotons one obtains that the average number of scattering events is equal to lin/l.

In case of one-dimensional inhomogeneity the diffusive contribution leads to a strong energy dependence of radiation intensity and owing to that one can assume that such random systems are of promise for registration (detection) of relativistic charged particles. One dimensional random inhomogeneity can be created by placing parallel plates in a homogeneous mediun or in vacuum randomly. Plates can be made of, for example, polymers.

The experimental study of diffusive radiation mechanism in the frame of this project is suggested.

At the first stage it is planned to investigate the visual and infrared range radiation emitted from 1 – 10 MeV electrons passing through a random stack of plates. It is planned to use LAE-5 and LAE-10 accelerators having the following characteristics:

1. In LAE-5 the electrons are accelerated to 1- 5 MeV , the repetition rate of pulses ranges from 100 to 300 Hz, the pulse duration is 4 msec and an average maximum current 500 mcA.
2.
3. In LAE-10 the electrons are accelerated to 6 -10 MeV , the repetition rate of pulses ranges from 100 to 300 Hz, the pulse duration 1-2 msec and average maximum current 150 mcA.

We plan to construct a special vacuum chamber that will comprise the random system. The angular and frequency dependence of DR will be studied in the ranges of visible light and infrared. The construction of device for registration of emitted photons is planned. The whole construction will be optimizied preliminary by Monte Carlo simulations.

At the second stage of experimental research we should like to study the DR mechanism in the X-ray wavelength range. This aim in view we plan to investigate the propagation of X-rays in a random stack of plates. The diffusive scattering of X-rays would mean that an appropriate DR mechanism in this system is also possible.

DR in the X-ray region will allow to use it for detecting of relativistic charged particles. In this part of the project a closed collaboration with Dr.Lebedev from the Jefferson National Laboratory in USA which has old traditions in detection physics, is intended.

Moreover, we plan to investigate the manifestations of DR in random nanostructures. These systems are of great interest because of their possible use as sources of soft X-rays. Such sources have advantages if one compare them with the usually used for these goals synchrotron radiation because one can use Mev electrons instead of the Gev ones . In this part of the project we will collaborate with the Prof. Verhoeven from the Institute for Atomic and Molecular Physics in the Netherlands. His group actively is engaged in analogous problems.

We plan theoretically to investigate the fluctuations of radiation intensity under the diffusional mechanism of radiation. It is well known that they play an important role in the problem of wave propagation in random media.

And, at last, we assume to investigate the possibility of DR showing in astrophysics. There are all the necessary conditions for forming of X-ray DR in many astrophysical objects. The X-ray observations of some types of astrophysical objects carried out by astronomical satelites show that a non-thermal component of X-ray radiation exists in their spectra . There is no any common point of view concerning their origin. We hope to interpret the X-ray spectra of some types of astrophysical objects by the DR. On theoretical and astrophysical problems we will collaborate with Prof.Nieuwenhuizen from the Amsterdam University.

So the characteristic features of DR allow to distinguish it as a novel radiation mechanism which has fundamental as well as applied value.A group of high quality specialists is created for performing the project. The most part of them earlier was engaged in the defense projects of former USSR.


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