Acta Polytechnica Vol. 43 No. 2/2003 Twenty Years of Microtron Laboratory Activities at CTU in Prague M. Vognar, Č. Šimáně, D. Chválil A concise review is presented ojtke activities at tke Prague microtron laboratory, starting witk tke construction ojtke first micmtron in tke Czechoslovak Republic, covering R&D connected with tke design and building ofeleetron accelerators ofthis type, applications ofelectron and bremsstrahlung beams andfields in applied radiation dosimetry, in the study ofradiation-induced changes ofoptical and other physical properties ofinorganic and organic substances (e.g., scintillation crystals suck as PbW04, opticalfitrres, semiconductors),};or activation analysis ojsamples, especially from geological mineral ore prospecting (gold ores and otkers), Jor radioisotope production (J 3/Jor medical diagnostic purposes), et cetera. Participation oJtke microtron laboratory in tke education oJstudents oJtkeJaculty in variousJzelds ojapplied dosimetry and other microtron applications is also discussed. Keywords: microtron design and construction, electron beams, bremsstraklung,radioisotope production, radiation damage. 1 Introduction From time to time it is useful to recapitulate the history of efforts made in a specific direction of technical development, and to evaluate the achievements. In our case we will look back at the history of building and applying microtrons in the Microtron Laboratory at the Faculty of Nuc1ear Sciences and Physical Engineering of the Czech Technical University in Prague. Practically with our own hands we budt the only circular accelerators, apart from betatrons, to have been con­ structed in Czechoslovakia. 2 Microtron MT 22 The first microtron MT 22 (Fíg. I) was built in the second half of the 1970s [1] in c10se collaboration with the Labora­ tory of Nuc1ear Reactions (nowadays Flerov LNR) of theJoint Institute for Nuc1ear Research in Dubna (former USSR). The accelerator itself was of the same type as the microtron at the LNR, working on the principle invented by Veksler and improved by S. P. Kapitza. With the exception of the main electromagnet coils and power supply, the acceleration reso­ nant cavities, some parts of the ferrite insulator and high vacuum pumps, transferred to Prague from Dubna, all other Fig. I: View on microtron MT 22 from the side of the beam extraction system 50 systems were designed in the Microtron laboratory and man­ ufactured by Czech industry, mostly by ČKD Prague. 1wo iron yokes were made, one for Prague, and the other for Dubna. The 3 CeV, 1.8 MW peak power, magnetron high fre­ quency source was taken from a military radar installation and adapted. A new aspect of the Prague microtron design was the original system for extraction of electrons at variable energies (Fíg. 2). The maximum energy was set to 22 MeV, suitable for routine activation analysis, especially of samples from geological mineral ore prospecting. For analysis or gold ore Fig. 2: Pian view of microtron MT 22 and the extraction system. 1- main electromagnet, 2- main electromagnet coils, 3- vacuum acceleration chamber, 4- high frequency line, 5- cavity resonator, 6- current probe, 7- beam extraction channel, 8- thin Al exit windows, 9- quadrupole doublet, 10- electron beam lines, 11- bending magnet. Acta Polyrechnica Vol. 43 No. 2/2002 samples (reaction gamma-gamma prime) an exrra l0 MeV exrracrion channel was introduced in the acceleration cham_ ber. To minimize rhe costs of shielding against penetrating gamma radiation, a second world war bomb shelier was chol sen and adapted for the microtron laboratory. The microtron, situated ar rhe end of a long corrido4 required addition_ al concrere shielding with double, heavy shielded enrrance doors, only in one direction. This first microtron came into operarion in 19g0. A ful_ ly-automarized pneu-post for sample transportation and a multiple derecror sysrem was designed and made by the collaborating Institute of Mineral Raw Materials in Kutnd Hora. Almosr one hundred thousand samples of gold_bear_ ing.ores, coming from mineral or. p.orp..iing in Czechoslo_ vakia, were analysed during eight years of opiration. These radiochemical analyses almost totally occupied the microtron capacity. The rest was used for other radiochemical applica- tions and for improving of the microtron as such. A second microtron of the same rype was built in Czecho- slovakia in Kutn6 Hora, with substantial supporr from the microtron laboratory mainly for commercial production of t23l for medical purposes. Due to the organizational and other changes at the Institute of Mineral Raw Materials in recenr years, this microtrcn was disassembled. 3 Chamberless microtron MT 25 After ten years of successful operation, the microtron MT 22 at CTU was replaced berween 1989 and l99l by the neq so called chamberless type MT 2b [2], jointly proposed in the fi'amework of Prague-Dubna collaboration, covered by a Czech patenr cerrificate [3]. It has the advantage of elimi- nating the need for a distinct and very complicated vacuum acceleration chambe4 the vacuum iron yoke of the main electromagnet replacing the acceleration chamber (nS. 3). This solution reduces to a minimum the number of vacuum Fig. 3: Insight in the open chamberless microtron MT 25 gaskets, which moreover become easily controllable and ac- cessible for replacement. Two iron yokes, designed at the Prague microrron laboratory were made by iKD prague, one ofwhich was sent to Dubna. The construction of the chamber- less microtrons in Prague and Dubna was possible due to the availability from Soviet industry of hollow copper leads for inner water cooling, encased in a vacuum tight copper enve- lope, mutually isolated by AIrOr. A pair of coils was made in Dubna for Prague. Although the coils, situated inside the vacuum tight iron yoke, significantly increase the pumped surfaces, experience proved that an operational vacuum can be achieved. At the present time, several nearly identical chamberless microtrons (ng.a) are in exploitation, one of them in Prague, anorher in Dubna. They differ mainly in the beam extraction and beam transport systems. External step motors are used in the Prague extraction system for rwo separate movements of the telescopic iron ex- traction channel, remotely controlled by absolute electrome- chanical turn encoders [4], also developed in the microtron laboratory. The electron beam is guided by a beam rransport Fig' 4: Microtron MT 25 facility of the Faculty of Nuclear Sciences and Physical Engineering in Prague. l- HF power input (3 GHz, peak power 2 MW, pulse length 2.5 microseconds, pulse repetition rate 400 s-I), 2- wave guide, 3- cavity resonator, 4- electromagnet with vacuum tight magnetic yoke, 5- main coils, 6- electromagnet current supply, 7- beam extraction channel, 8- bending rnug.ret Dl, 9- bending magnet D2, l0- electron beam line, I l- first quadrupole doublet, l2- quadrupole doublets ofindividual beam lines, l3- steering vertical magnets D3, l4a,b,c- vacuum valve, ion pump, turbo molecular pump, l5- drive for angular displace- mentoftheextractionchannelfororbitselection(steppingmotor,absoluredigitalencoder),16-driveforadjustingthelengthof the extraction channel (stepping motor, absolute digital encoder), l7- vacuum bushings of magnet coils currenileads, lb- ca- bling vacuum bushings, lg- vacuum gauges, 20- beam mean current and beam position induition pick up, 21- vacuum exit window (Al foil 0.1 mm), 22- electron beam diaphragm for gamma fields, 23- electron beam diaphragm Al 30 mm, aperture 2 mm for electron fields, 24- metrological workstation. 5l Acta Polytechnica Vol. 43 No. 2/2003 system to one of three selectable workplaces. Two of them are provided with an induction pick up system for continuous mean electron current measurement and for beam position control dose before the beam exit [5]. A system for automatic stabilization of the beam position, dose behind its exit to the air through the thin Al foil, has been installed, using second­ ary electron emission from thin wires placed at the periphery ofthe electron beam. The main advantage ofthis system con­ sists in the fact that the wires absorb a negligible portion of the electron energy and therefore need no supplementary cooling (Fíg. 5). The same principle has been proposed and already experimentally tested, for beam position control at Fig. 5: Front view on the wire pick up system for automatic stabili­ zatíon of the exit electron beam critical points of the electron transport system, such as the entry oriftce of the extraction channel, the entries to the deflecting dipole magnets and magnetic quadrupole lenses. To prevent deterioration of the beam quality by scattering on the pick up wires, they will be made retraetable from the beam path. 4 Microtron beam applications An internal beam was obtained from the new microtron in 1990, and an external beam in 1991. Most of the applica­ tions were oriented to radiatíon dosimetry. The idea was to establish in the Czech Republic a secondary standardization laboratory, using standard high-energy electron and gamma fields, for calibrating dosimeters from oncology departments. Supported by the Grant Agency of the Czech Ministry for ln­ dustry and Commerce, an experimental arrangement (Fíg. 6) was installed [7], consisting of an optica! bencll with a water phantorn and an optically centred collimator system with sets of interchangeable bremsstrahlung fiJters and scattering foils. The measuring part included a set ofionisation chambers cal­ ibrated at the state metrological institute. The arrangement enabled radiation beams with a high quality index (Fig. 7) to be obtained, and homogeneous 10 x IOcm2 photon and eIec­ tron fields precise to 65 % to be generated, complying with the ICRP IAEA standards (Fíg. 8) [8]. Lack of funding and the requirement to dedicate the microtron exclusively for dosimetric metrology, which was an unacceptable condition for the faculty, forced the laboratory to abandon this project and to work on other physical and pedagogical applications. One option was to use the instalIed experimental ar­ rangement to study radiation induced effects in a range of .,;: -' / ~.... ..' Fig. 6: Scheme ofthe installation in the beam path for dosimetric applications and for irradiation in well defined electron or bremsstrah­ lung fields (from ref. 7.) 1- electron beam line, 2- quadrupole doublet, 3- beam mean current and beam position induction pick-up, 4- diaphragm 5- first indexed turret with two Wtargets 1.5 and 3 mm and one Sn foil 0.2 mm, second indexed turret with combined Al-Cu scattering foils, 7- light source, 8-primary conical stainless collimators, g-secondary rectangular W-steel collimator, 10- third turret with scattering foils, 11- water phantom on mobile support, 12- two-dimensiona! scanning system THERADOS, 13- laser, support table with water storage tank under the optical bench. 52 Acta Polytechnica Vol. 43 No. 2/2002 materials, which also required well defined radiation fields with well known radiation doses and dose rates. Atrention was prirnarily paid ro oprical changes induced in scintillation crys_ tals, such as PbWOr, BGO, yAB used in big detecror sysiems, e.9., the AILAS elecrromagnetic calorimeter ar CERN. For this purpose the installation was supplemented with additional parts specially developed and instailed for optical spectrometry in the wavelengrh range from 300 to g00 b) 1@ ly,dq plq of 0E 9hd6 ncB €. - lO McV u I t0 cor dcpart. ft6urd i. w{tcr dtmrgn- f 3 5 V ar- r:r:3t-rrtttrl LmJ Itudos ,lq of dx gbols fcld €., - 19 Mrv fl ! l0 c.n dcelh. nrlssaad io warca phsrcn. in the warer phantom (from ref. 8) nanomerers, to measure the light transmission coeflicient of the crystals tgl. To mainrain a consranr remperature during the experiments, a special thermostat with peltier elementi was built with forced air circulation in the irradiation volume lEg gl. Today, the arrangemenr is mainly used for resring rhe irradiation effecrs in various types of scintillation crystahlt0l, illl, [2], [3], (ng. l0), ordered by the manufacrurer in cooperation with the Technical University in Liberec. For o c| o€ I k an 3 to ?5 7, 2, @ o ! I E! o I 6 Dcptb irr BrOlco Fig' 7: Depth absorbed dose measurements in the water phantom (from ref 8.). a) c^entral axis depth absorbed dose distribution for max-imumphotonenergies l0Mevand lgMev(SSD-lm,fieldsize l0x l0cm2;,b;..lr,i;:;;;;iu*isd.pthabsorbeddosecurves for three elecrron beams energies: l-9,g MeV; 2-14,6 Mev; 3-1g,7 Mev (field size l0 " i0 ;r) - ta t( ts. II ta IO Er E -rt a2 E t E - 3 O - f;t O r- [3t2lItEtattr fml lio-1331 -tr-1251 -EtCi -OCUE -Cl.ElE -ClEIS -CUII( -DSFCU ..!;;---,.::--.-:;;-;.--;;.----;:;.~.,------;M;_ --"'"' n~~1- -SI'lIflK ~ ~ 1 .. "-~.::.--~.--;.----;:;.,---;:..;--;.:,--;.=­ _,c/o.optIO