Twenty Years of Microtron Laboratory Activities at CTU in Prague

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.


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 constructed in Czechoslovakia.

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 Laboratory of Nuc1ear Reactions (nowadays Flerov LNR) of the Joint 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 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 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 frequency 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 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 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.retDl, 9-bending magnet D2, l0-electron beam line, I lfirst quadrupole doublet, l2-quadrupole doublets ofindividual beam lines, l3steering vertical magnets D3, l4a,b,c-vacuum valve, ion pump, turbo molecular pump, l5drive for angular displace- mentoftheextractionchannelfororbitselection(steppingmotor,absoluredigitalencoder),16-driveforadjustingthelengthof the extraction channel (stepping motor, absolute digital encoder), l7-vacuum bushings of magnet coils currenileads, lbca- bling vacuum bushings, lgvacuum 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 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 secondary electron emission from thin wires placed at the periphery ofthe electron beam.The main advantage ofthis system consists 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 stabilizatíon 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.

Microtron beam applications
An internal beam was obtained from the new microtron in 1990, and an external beam in 1991.Most of the applications 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 lndustry 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 calibrated at the state metrological institute.The arrangement enabled radiation beams with a high quality index (Fig. 7) to be obtained, and homogeneous 10 x IOcm 2 photon and eIectron 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 arrangement 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 bremsstrahlung 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 W targets 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.
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 in the warer phantom (from ref.The same on-line arrangement was also used to study the radiation changes in different types of optical fibres [14] (pure fibre, scimillation fibres, shiftered fibres) in high intensity bremsstrahlung fields, dose to the e-gamma converter with dose rates up to 10 Gy/s.(Fig. ll).
Besides the applications cited above, in some individual cases tests were made of the radiation effects in other materials (polystyrene and polyethylene radiation modifications) and radiation hardness of electronic elements and circuits.
For an experimental study of radiation changes in solid-state samples during irradiation by high integral electron fluxes (covering the range from 10 9 to 10 16 electron/cm 2 ) at difTerent energies up to 22 MeV, a special facility for absolute e1eetron flux measurement was installed [15].It eonsists of a Faraday cup, constructed specially for this purpose in the laboratory, fixed on a telescopic optical bench (Fig. 12), and conneeted by a triaxial eable with a Keithley e1eetrometer in the control room.The required form of the energy spectrum of the eleetron fields is produeed by combining several seattering foils inserted in the eleetron f1ight path [16].The facility enables irradiation of samples under well-defined electron fields with precise integral flux measurements.
The Faraday cup will also be used for measuring integral electron fluxes when preparing polarized Li 6 D targets in the framework of collaboration between JINR, Charles University and the Czech Technical University in Prague.
A possible mierotron application tested in the laboratory in the past was for producing neutrons from gamma-n or from gamma-fission processes.A MnS0 4 bath was used to determine the total neutron yield from a lead or uranium convertor, potentially surrounded by a layer of heavy water.The experimentally determined neutron yields were in the order of 10'IS-1 in the 47t solid angle.Mter moderation, the thermal neutron tlux near the convertor was about 10 12 m-2 s-1 A further line of applications was the experimental production of radionuclides for labelling some pharmaceutical medical products.This direetion had been seriously considered since the start of the first microtron.The main field of interest was the production of lnI.Soon after the start of the first microtron, a glass apparatus was assembled in cooperation with the Physical Institute of the Czechoslovak Academy of Sciences, comaining a target permanently cooled by liquid nitrogen.Together with the Institute of Mineral Raw Materials in Kutná Hora, several experiments were performed in the late of 1980s, using natural xenon with the aim to assess the attainable yield of 123 1 [17].Limited financial resources of both the Prague faculty and the Kutná Hora institute prevented the implememation of experiments with gas enriched in 124Xe contem.
In the first half of the 1990s, supported by the Grant Agency of the Czech Republic, the laboratory was in a position to construct a target supplemented by a stainless steel filling and recyding apparatus, using cryogenic pumping [18].Mter the first veriflcation experiments with natural xenon gas, tests were carried out with xenon enriched to 11 % [19].The aim of these experiments was to check which the parameters were important for commercial production, such as the 123[ yield, optimum electron energy, optimum irradiation time and post irradiation die out period, effective washing out procedure of the irradiation product, its radiochemical purity, production reproducibility, and so on (Fig. 13).
Calculation of 123 1 production yield, determination of optimum length of irradiation to get maximum concentration of this radioisotope after the end of irradiation, and the length of the die out period, enabled the laboratory to define economic irradiation conditions, taking into account the necessary radiation protection of personneJ during the experiments.
Today the apparatus is stili used for pumping and filling defined quantities of the enriched gas to other apparatus.The know-how gained in constructing and exploiting this pilot apparatus was used to advantage by the microtron labo- 0.0001 +l-IJ~"""'Uo+J-.l..l.fJ...u..t.u..l1"""'''''''' ratory in the design and construction of a production apparatus ordered by the Institute of Mineral Raw Materials in Kutna Hora (Fig. 14).
Meanwhi!e, new interest in radioisotope production appeared from the NucIear Physics Institute ofthe Czech Academy of Sciences at Řež, which asked the microtron laboratory to construet severa!plants for 123} production and for production of Rubidium-Krypton generators at the cycIotron, by irradiation medium pressure gas targets.The experience gained while constructing apparatus for producing 123 1 by irradiation of high-pressure Xe targets by microtron bremsstrahlung, served as a basis for constructing Kr 1 [20] and Xe 1 apparatus (Fíg.15), the former for routine production of 81Rb•BlmKr and the latter for 123} at the cyclotron in Řež [21].Today, this production helps to meet the increasing demand from nuclear medicine in the Czech Republie.
5 Involvement of the microtron laboratory in the education process The microtron laboratory has played a very important role in the teaching process, especially in the education of students in fields such as experimentaJ nuclear physics, neutron physies, neutronography, activation analysis, dosimetry, changes of material properties induced by radiation, solid state physics, nucIear chemistry and principles of acceleration technology.About 30 diploma projects in these fields have been performed by students of the faculty in the mierotron laboratory.The laboratory is involved in courses for students from abroad organized by the department of dosimetry, and provides a practicaI opportunity to participate in special microtron applications in radiation dosimetry and activation analysis.
The microtron laboratory is often visited by students of secondary schools and universities and by members of the pubIie.

Conclusion
The microtron laboratory now faces major moderniza_ tion of the microtron instaliarion, during li,t i.n tn.frigt frequency magnetron generator fiom the'1960s will be re_ placed by a modern one.As a consequence, the microtron will be out of action for some time.An eri of the microtron historv in the Czech Republic is thus coming ro an end.This r..-.6 to us an appropriate moment to summarize n,rrenty years of history of microtron laboratory activities at CTU in'prague.

Fig
Fig. I: View on microtron MT 22 from the side of the beam extraction system 50