History of Czech microtrons

The first microtron MT 22 was built in the second half of seventies of the last century [1] in close collaboration with the Flerov Laboratory of Nuclear Reactions of the Joint Institute for Nuclear Research in Dubna (Russia). The cyclic accelerator itself works on the principle invented by the V.I. Veksler (1944) and on the technical solution invented by S.P. Kapitza and V.N. Melekhin (1959). With exception of the main electromagnet coils and power supply, the acceleration resonant cavities, some parts of the ferrite insulator and high vacuum pumps, transferred to Prague from Dubna, all other systems were designed in the Microtron laboratory and manufactured by Czech industry, mostly in ČKD Prague. Two iron yokes were made, one for Prague, another piece for Dubna. The 3 GHz, 1,8 MW peak power, magnetron high frequency source was taken and adapted from a military radar installation. The original system for extraction of electrons at variable energies was designed on the Prague microtron. The maximum energy was set to 22 MeV, suitable for routine activation analysis, especially of samples from geological mineral ore prospecting. For analysis of gold ore samples (reaction gamma-gamma prime) an extra 10 MeV extraction channel was introduced in the acceleration chamber. To minimize the costs of shielding against penetrating gamma radiation, a bomb shelter from the Second World War was chosen and adapted for the microtron laboratory. The microtron, situated at the end of a long corridor, required additional concrete shielding with double, heavy shielded entrance doors, only in one direction.

This first microtron started the operation in 1981. A full-automated pneu-post for sample transportation and a multiple detector system was designed and made by the collaborating Institute of Mineral Raw Materials in Kutná Hora. Almost one hundred thousands samples of gold containing ores, coming from mineral ore prospecting in Czechoslovakia, were analysed during eight years of operation. These radiochemical analyses occupied nearly completely the microtron capacity. The rest was used for other radiochemical applications and for improvement of the microtron.

A second microtron of the same type was built in Czechoslovakia in Kutná Hora (Institute of Mineral Raw Materials - 1990) with substantial support from the side of the microtron laboratory, mainly for, besides other applications, commercial production of 123I for medical purposes. This microtron has been recently disassembled.

After ten years of successful operation, the microtron MT 22 at the CTU was replaced (1989-1991) by a new, so called chamberless type MT25 [2], proposed commonly in the framework of Prague-Dubna collaboration, covered by a Czech patent certificate [3]. Its advantage consists in the elimination of a distinct and very complicated vacuum acceleration chamber, the vacuum iron yoke of the main electromagnet replacing the acceleration chamber. This solution reduces to a minimum the number of vacuum gaskets, which moreover become easily controllable and accessible for replacement. Two iron yokes, designed in the Prague microtron laboratory, were made by ČKD Prague, one of them having been sent to Dubna. The construction of the chamberless microtrons in Prague and Dubna became realizable by the possibility to get from the USSR industry hollow copper leads for inner water cooling, encased in a vacuum tight copper envelope, mutually isolated by Al2O3. A pair of coils was made in Dubna for Prague. Although the coils, situated inside the vacuum tight iron yoke, increase significantly the pumped surfaces, the experience proved, that operational vacuum can be reached. At present time, several nearly identical chamberless microtron are in use, one of them in Prague, another in Dubna. They differ mainly in the beam extraction and beam transport systems.

The MT25 microtron has been reconstructed and modernized since 2004 (after its transition to the NPI ASCR in 2003). This is reconstruction whole RF line, power supplies and measuring and control systems.

[1] Šimáně, Č., Vognar, M.: První československý mikrotron, JE 27/1981/421-425, JE 28/1982/14-16

[2] Vognar, M.: Výstavba bezkomorového mikrotronu MT25 na FJFI ČVUT a perspektivy jeho využití, 9.celostátní seminář “Jaderné metody v hornictví, geologii, geofyzice a geochemii” Mariánská u Jáchymova, 1991

[3] Vognar, M., Belov, A., Šimáně, Č., Pokrovskij, V.: Electromagnet for an electron accelerator, ČSFR Patent 239494