TECHNICAL DESCRIPTION OF LIULIN-4 TYPE LET SPECTROMETERS (LETS) Tsvetan Dachev 1 , Plamen Dimitrov 1 , Borislav Tomov 1 , Yury Matviichuk 1 1 Solar-Terrestrial Influences Laboratory (STIL), Bulgarian academy of Sciences, 1113 Sofia, Bulgaria Introduction The Linear Energy Transfer (LET) spectrometer (LETS) Liulin-4 type main purpose is to measure the specter (in 256 channels) of the deposited energy from primary and secondary particles at the aircraft altitudes, at Low Earth Orbits (LEO), outside of the Earth magnetosphere on the route and on the surface of the planets of Solar system. It sensitivity was proved against neutrons and gamma radiation, which allows monitoring of the natural background radiation also. The LETS is a miniature spectrometer-dosimeter containing: one semiconductor detector, one charge-sensitive preamplifier, 1 or more microcontrollers and a flash memory. Different modifications of LETS use displays and/or Global Positioning System (GPS) receivers. Pulse analysis technique is used for the obtaining of the deposited energy specter, which further is converted to the deposited dose and flux in the silicon detector. The unit is managed by the microcontrollers through specially developed firmware. Plug-in links provide the transmission of the stored on the flash memory data toward the standard Personal Computer (PC). PC is used for the full management of the LETS through standard serial or parallel communication port. On the PC is stored the full data sets and it is used for data visualization and preliminary analysis. Spectrometer development history Liulin-4 type spectrometer is a successor of the Bulgarian-Russian dosimeter-radiometer LIULIN [1], which was installed in the working compartment of the MIR space station in 1988. LIULIN measurements were carried out under a wide variety of solar and geomagnetic activity conditions. They provide an excellent opportunity to study effects on the dose-rates and fluxes in the near Earth radiation environment over long time periods, as well as rapid changes, induced by solar proton events and geomagnetic disturbances in the 1989-1994 time frames [2-7]. Main characteristics of Liulin-4 LETS was proved on the Liulin-3M instrument, which was designed together with scientists from Radiation physics office GSFC, NASA [8,9] for continuous monitoring of the radiation environment of the BION-11/12 satellites and on aircrafts. The LIULIN-3M instrument was calibrated by proton fluxes with different energies at the Indiana University Cyclotron Facility in June 1997 and was used for space radiation measurements during commercial aircraft flights [7a,b]. Liulin-4 type Mobile Dosimetry Unit was developed in a cooperation with Dr. R. Beaujean under CONTRACT of 27.09.1996 between STIL-BAS and the Rektorat der Christian-Albrechts-Universitaet zu Kiel on the topic "Dosimetrie im Weltraun", conducted under DARA proposal 50WB9418 from 28.03.1996. Modifications of the LETS for purposes of scientific projects First use of Liulin-4 type LETS was in the Mobile Radiation Exposure Control System - Liulin-E094 (Figure 1). It contains 4 active individual dosimeters of Liulin-4 type and worked successfully between May and August 2001 on the board of US Laboratory module of the International Space Station (ISS). The system was a part of the experiment Dosimetric Mapping E094, which was placed in the US Laboratory Module in the composition of the Human Research Facility (HRF). The German (University of Kiel) build two DOSimetric Fig.1. External view of the Liulin-E094 system. One of the Mobile dosimetry Units is seen alone at the right hand side of the system Other 3 units are inside of the Control and Interface Unit (CIU)