Microtron MT25

The Prague microtron MT25 is a cyclic electron accelerator with Kapitza's resonator. The electrons are accelerated by an RF electric field of constant amplitude and frequency in a constant uniform magnetic field. In the vacuum chamber electrons follow circular paths with a common tangent point. The accelerating cavity, which is supplied with the RF field, is located at this point.



Schematic layout of the MT25 microtron. 1 – magnetron, 2 – phase shifter, 3 – circulator, 4 – water load, 5 – accelerating cavity,
6 – main magnet (vacuum chamber), 7 – electron trajectories, 8 – adjustable beam extractor, 9 – first deflector

 

Beam Parameters of Microtron MT25:

Electron beam

Energy Ee: Maximum 25 MeV, energy is variable in steps of 1 MeV from 12 up to 25 MeV or in steps of 0,5 MeV from 6 up to 15 MeV. The energy can be adjusted continuously in the range of step interval.
Output beam current: Pulsed, pulse length is 3,5 µs, standard repetition rate is 423 Hz, mean maximum current is about 30 µA in dependence on electron energy.
Energy dispersion: tens of keV
Beam distribution: Electron beam line with magnetic lenses and deflectors to workstations A, B a C.
Beam diagnostician: The mean electron current is measured by induction pick-up system before exit window of electron beam line, the beam position is indicated by a wire sensors.
Beam line output: The thin Al window is placed on exit of electron beam.
Beam output: The collimated beam spot dimension behind exit window is adjustable from 2 to 20 mm, the possibility of standard homogenous fields 10 × 10 cm2 in 1 m distance from exit window (homogeneity ±5%).
Beam power: Ns = Is · Ee , for maximal energy and electron current it is about 750 W.
Dose rate: About 100 Gy/(min·µA) in standard homogenous electron fields 10 × 10 cm2 at 1 m distance from exit window, in case of collimated beam it is several hunderds Gy/(min·µA).

Photon beam - Bremsstrahlung

Physical quality: Bremsstrahlung is an electromagnetic radiation produced by the deceleration of electrons passed through tungsten (W) target placed behind exit window.
Energy spectrum: Continuous, upper boundary of bremsstrahlung energy is equal electron energy Ee.
Photon output direction: Rotary symmetrical towards electron motion before photon emission, photon beam is axial symmetric with forward intensity peak. The conical iron collimators are used for the homogenous photon fields (homogeneity better ± 5%).
Dose rate: About 10 Gy/(min·µA) in axis of collimating beam in 1 m distance from tungsten target, about 0,5 Gy/(min·µA) in homogenous field 10 × 10 cm2 in 1 m distance. (E = 25 MeV)

Neutrons

Origin: Photonuclear reaction (γ, n) or uranium photo fission.
Yields: 2,5·1011 neutrons/s (Pb), 5·1011 neutrons/s (U), isotropic emission to all the solid angle. Proportion of bremsstrahlung dose to neutrons dose is about 2-10 %.
Energy spectrum: in case of Pb it is up to 2 MeV, in case of U it is about 30% less





Monitoring center of the MT25 microtron