NUCLEAR TECHNOLOGIES AND METHODS 143 rate is corrected by the background, the lower [6]. Commission Recommendation of 20 Dec 2001 on the limit of detection of radon concentration [15] is protection of the public against exposure in drinking 1/2 3 water supplies. Off. J. European Commission L. 344/45. LLD=1.645*(rb/tb) /S=0.011 Bq/dm at tb=30 min counting time. ec.europa.eu/energy/nuclear/radioprotection/doc/ legislation/01928_en.pdf. The monitor based on the transfer membrane [7]. Proposed Radon in Drinking Water Rule. www.small- of radon from water shows a high sensitivity equal biz-enviroweb.org/pub_video/epadocs/hdocs/h10.pdf. 3 to 87 cpm/(Bq/dm ), which allows to measure [8]. Machaj B., Bartak J.: Nukleonika, 49, 1, 29-31 (2004). 3 radon concentration in water 1 Bq/dm with a [9]. Radon Detection in Drinking Water. http://las.perk- relative error of approximately 4% at 2 level in inelmer.com/applicationssummary/applications/ a 30 min period. Response time to step variation Radon+Detection.htm. of radon concentration in water from 0 to 90% of [10]. Kim G., Burnett W.C., Dulaiova H., Swarzenski P.W., the step is 45 min. Moore W.S.: Environ. Sci. Technol., 35, 4680-4683 (2001). References [11]. Surbec H.: www.nucfilm.com/ustron.pdf. [12]. Bassignani A., Colomb G., Degli Esposti L., Fresca [1]. Radon entry via potable water. In: Radon and its de- Fantoni R., Giacomelli G., Maltoni G., Mandrioli G., cay products in indoor air: Eds. W.W. Nazaroff, A.V. Mascoli M., Materazzi F., Patrizii L., Sirri G., Ugolott Nero. J. Willey and Sons, New York 1998, pp.131-157. D.: http://www.lngs.infn.it/lngs/report99/lngs_exp20_ [2]. Burnett B., Nelson T., Corbet R., Robinson L., Weaver 99.pdf. J., McKisson J.E., Lane-Smith D.: www.nucfilm.com/ [13]. Portable radon concentration gauge MR-1. http:// burnett_98.pdf. www.ichtj.waw.pl/. [3]. Zalewski M., Karpińska M., Mnich Z., Kapaa J., Za- [14]. Lucas H.F.: Rev. Sci. Instrum., 28, 680-683 (1957). lewski P.: J. Environ. Radioact., 53, 2, 167-173 (2001). [15]. Technical Bulletin. Determining Lower Limit of [4]. Pachocki K., Gorzkowski. B., Majle T., RóŜycki R., Detection (LLD) and Minimum Detectable Activity Penski J., Poręba I.: Annuals of the National Insti- (MDA) for Radiation Measurements. http://www. tute of Hygiene, 47, 285-293 (1966). rss.usda.gov/publications/mdatb.htm. [5]. Przylibski T.A., Mamont Cieśla K., Kusyk M., Dorda J., Kozłowska B.: J. Environ. Radioact., 75, 2, 193-209 (2004).

THE LIQUID SCINTILLATION COUNTER LG-1CS Jakub Bartak, Bronisław Machaj, Jan P. Pieńkos

Liquid scintillation counters (LSC) are widely used The width of the single channel analyser is pro- for the measurement of nuclides emitting alpha gramed by the user. (e.g. Pu-239, Rn-222) or beta radiation (e.g. radio- Automatic gain control of the measuring chan- ctive carbon C-14, tritium H-3) in medicine, radio- nel is based on reference light pulse from light logical protection, hydrology or scientific research. emitting diode LED placed in light guide SW con- Some of the LSC are sophisticated, very heavy and trolled by a pulse generator GI. During the gain expensive instruments for fine measurements of control, generator pulses are fed to the LED and very low activities as in determination of the age reference light pulses are generated. The LED of very old objects by measuring C-14 activity [1]. pulse amplitude is converted by an analog-to-digit In the applications of LSC technique for measure- ADC converter and is read out by the microproc- ment of radiopharmaceuticals, determination of essor circuit. With the start of gain control, the radioactive contamination where the activities mea- photomultiplier tube high voltage is increased and sured are much higher, less sophisticated, cheaper the light pulse amplitude is measured, until the and smaller counters can be adopted [2]. Small di- pulse amplitude reaches nominal value. After the mension model LG-1CS of LSC was developed in the Department of Radioisotope Instruments and 1 Methods, Institute of Nuclear Chemistry and Tech- nology (INCT) that is designed to measure beta 2 and alpha activities. 3 General view of the counter is shown in Fig.1. 4 Block diagram illustrating the principle of opera- 5 tion is shown in Fig.2. Measured sample in a vial 6 together with a liquid scintillator SC is placed in the front of photo-cathode of a photomultiplier tube PMT. Output pulses from the PMT after am- plification and shaping in pulse amplifiers A1, A2 8 7 Fig. 1. General view of the liquid scintillation counter are passed through a single channel analyser W1 LG-1CS: 1 – knob for rotation of chamber vial with scintilla- and counted by a programmable pulse counter tion cocktail in lead shielding, 2 – locking device for the under the control of microprocessor system. Pulse cover of rotary chamber, 3 – chamber for vial with scintilla- count rate (multiplied by a calibration factor) is a tion cocktail, 4 – vial holder, 5 – vial with liquid scintillator, measure of the activity of the measured sample. 6 – cover for rotary chamber, 7 – keyboard, 8 – display. 144 NUCLEONIC CONTROL SYSTEMS AND ACCELERATORS nominal pulse amplitude had been reached, the gain control procedure was finished. The counter is equipped with a serial port RS232 and a universal port USB enabling transmission of measured data to an external personal computer PC.

Fig.4. Pu-239 and background differential spectra measured with Ultima Gold scintillator [3]. Window width – 50 mV, window step voltage – 50 mV.

Fig.2. Block diagram of the liquid scintillation counter: Differential spectrum of the count rate of the SC – liquid scintillator in !28x60 mm vial; SW – light guide; scintillator background, Fig.3, practically coincide LED – light emitting diode of reference light pulse; PMT with the C-14 spectrum in the range 50-950 mV. – photomultiplier tube; GI – pulse generator approxi- Analyzing the minimum detectable activity (MDA) mately – 1000 pps; A1,A2 – pulse amplifiers; W1 – window [4] for different ranges of pulse amplitude of regis- width of single channel analyser; ADC – analog-to-digit tered pulses, it was found that the lowest activity converter; ZNN – low voltage power supply; ZWN – high can be measured in the measuring range 150-1600 voltage power supply; DAC – digit-to-analog converter; RS mV. The MDA of C-14 for such a range is 47 dpm, – serial port RS232; USB – universal series bus; uP – micro- processor. for 10 min counting time and at 50% detection ef- ficiency of the scintillator. To decrease the background due to natural ra- The counter is able to measure pulses with the diation, the scintillator is shielded with lead 15 mm amplitude from 20 mV to 4 V. This corresponds to thick. The vial with a liquid scintillator is placed in the energy of beta radiation 156/950*20=3.3 keV a rotary chamber protecting the photomultiplier up to 650 keV at a pulse amplitude equal to 4 V. tube against daylight. Additionally, the light tight- Beta nuclides with higher radiation energy can be ness is increased by a cover (Fig.1) that is closed measured in a second energy range that is 3 times after the scintillator vial is entered. higher, i.e. in the range 10 keV up to 1950 keV. Differential spectrum of liquid scintillator Ul- Differential spectrum of Pu-239, Fig.4, lies tima Gold [3] was measured first with the internal above the background spectrum of the liquid scin- single channel scanning analyser. 20 cm3 of the tillator, thus the background counts can be cut off. cocktail in a standard glass vial !28x60 mm was Analysis carried out shows that the best pulse mea- entered for measurement. In a similar way, differ- suring range is 700-1600 mV. The MDA for Pu-239 ential spectra were measured when the liquid is 8.6 dpm for 10 min counting time and at 50% scitillator contained C-14 beta nuclide (Emax=156 cocktail detection efficiency. keV) and then when the scintillator contained The Pu-239 pulse amplitude of 1000 mV cor- Pu-239 alpha nuclide (E=5.1 MeV). Results of responds to 5.1 MeV. Thus, the energy range (20 the measurements are shown in Figs.3 and 4. mV-4 V) is equal to 0.1-20 MeV. Other basic parameters are the following: • diameter of PMT photocathode – !38 mm, • instability of measuring channel <2%, • counting time – 1 min up to 120 h, • lead shielding – 15 mm, • data transmission – RS232 and USB ports. References [1]. Quantulus Liquid Scintillation Spectrometer. http://las. perkinelmer.com/catalog/productinfopage.htm?prod- uctid=1220-003. [2]. Thriathler LSC. http://www.wolflabs.co.uk/Triathler_ lsc%201.htm. [3]. Liquid Scintillation Cocktails & Vials. http://www.perkin- elmer.com/cocktails. [4]. Technical Report. Determining Lower Limit of Detec- tion (LLD) and Minimum Detectable Activity (MDA) Fig.3. C-14 and background differential spectra measured for Radiation Measurements. www.rss.usda.gov/publi- with Ultima Gold scintillator [3]. Window width – 50 mV, cations/mdatb.htm. window step voltage – 50 mV.