Liquid scintillation counting (LSC) is an analytical technique, which measures activity of Radionuclides from the rate of light photons emitted by a radioactive sample dissolved in a liquid scintillator. It is a powerful technique especially for detection and quantification of low energy Radionuclides as it enjoys advantage of 100% source detector geometry.
In this technique the radioactive material (such as 3H or 14C) is dissolved in liquid scintillation system consisting of a solvent (such as toluene) and a scintillating solute (such as PPO) contained in a glass or plastic vial. The radiations emitted by the radioactive sample transfer energy to solvent molecules, which get excited and transfer their energy to solute molecules, which on de-excitation emit photons. Some times a secondary solute is used to match the wavelength of light emitted to that of PMT characteristics. The number of photons emitted is proportional to energy of radiation and is detected using photo multiplier tubes and associated electronic circuits.
The Liquid Scintillation Counting System Type LS620 presented here is designed using advanced technology. It is a computer controlled manually operated system for detecting even small amounts of beta or alpha radioactivity. It has wide range applications in Biology, Medicine and Environmental studies. The system has been fabricated for the quantitative estimation of beta emitters like, 3H, 14C and 32P. Usage of excellent quality high efficiency matched pair of photo multiplier tubes, precision fast and slow preamplifiers, USB based spectroscopy quality MCA, rugged manually operated sample chamber with lead shield allow the system to offer high efficiency even for low energy beta emitters such as tritium and also a very low background with ability to count radioactivity in bulk quantity of samples one by one.
The instrument is highly reliable and incorporates several built in features such as coincidence correction to achieve low background. The USB bus interface of the MCA provides an excellent connectivity with most of the new PCs and lap top computers. The LSC application software provided, integrates with the hardware providing online display of spectrum build up and measure of the activity of the sample.
* Note : Manufactured by NUCLEONIX SYSTEMS based on Technology
from Bhabha Atomic Research Center, Mumbai.
System block diagram of manual PC based liquid scintillation counting system
System block diagram of manual PC based liquid scintillation counting system is sown above. It consists of a sample chamber that encloses a matches pair of photo multiplier tubes. The photo multiplier tubes are mounted axially and view the scintillating solution contained in a glass or plastic vial. The vial consists of the radioactive sample dissolved in the liquid scintillation solution. The radiations emitted by the radioactive sample transfer energy to solvent molecules, which get excited and transfer their energy to solute molecules, which on de-excitation emit photons. Some times a secondary solute is used to match the wavelength of light emitted to that of PMT characteristics. The light is emitted in duration of few nano seconds, as organic scintillators are fast scinitllators. The number of photons emitted is proportional to energy of radiation and is detected by the two photo multiplier tubes. The response of the Photomultiplier tube to the photons incident on it is a current that flows for a time equal to charge collection time. The time integral over the duration of the current is Q or the total amount of charge generated in that specific interaction.
Specifications
This system essentially consists of the following units. l Sample chamber with matched pair of PMTs with built-in slow fast pre-amplifiers l MINIM bin with power supply - MB403 l High voltage units - HV502 - 2 no’s (0-200V @ 1mA) l Signal processing unit - SP560 l 8K MCA module - MC100U l Personal computer system with processing software l A set of unquenched standards (optional)
MINIBIN AND POWER SUPPLY MB 403 Mini bin : Accommodates SIX / EIGHT single bit modules or combination of multiple widths with Amphenol connectors. Minibin is primarily designed with the objective of conserving bench space and to achieve significant saving in cost of the Minibin based systems. Bussed wiring is provided to the power connectors to distribute +/- 12V and +/- 24V. A control panel with ON/OFF switch, low voltage test sockets is provided on the right extreme side of the bin.
Minibin Dimensions : 11.75"width X 11.00 depth (upto connectors) X 8.75" height. Power supply : This is either two and half bit module or a compact box type enclosure fitted at the back of this bin, which generates highly regulated D.C voltages. Input : (230V + 10%) A.C, 50Hz. D.C Output : +12V @ 1A, -12V @ 1A, +24V @ 0.5A, -24V @ 0.5A 48 watts maximum. Regulation : Better than +/- 0.1% Noise & Ripple : Less than 3 mv Stability : +/- 0.5% after a 24 hr warmup at constant line, load & ambient temp.
HIGH VOLTAGE UNIT (HV 502) : a. Output voltage variable continuously from 0V to 2000 volts
b. Output current (max) 1mA
c. Load & Line regulations : Better than 0.005% of full scale
d. Indefinite over load & short circuit protections and self recovery
e. Output ripple less than 20mv.
f. Dimensions : Single / Two bit module
Signal Proessing Unit SP560 :
This is a two bit module. This receives input signal from the two PMTs. Both slow pre-amp output & fast pre-amp outputs are fed to this. This module has co-incidence circuits, summing amplifier & shaping circuits.
Slow pre-amp signals after summing amplifier & shaping are fed to MCA for storing PHA spectrum, under coincidence condition. i.e. when both the PMTs give signal output.
Detector :
Cocktail consisting of radioactivity dissolved in liquid scintillator contained in glass or plastic vial viewed by matched pair of photo multiplier tube. PMT 9829QA or its equivalent.
Housing :
Manually operated light sample chamber shielded with lead.
Internal LSC standards : l Determination of counting efficiency by internal standardization is the original analytical technique, to which other efficiency determination methods are referred. l Certified internal LSC standards are calibrated in DPM per unit weight rather that DPM per unit volume. l The advantage of calibrating in DPM per gram is that the calibration remains constant with changes in temperature.
Conventional size standards (20mL vial)
Background standard
3H standard, ~2.5 x 105 DPM
14C standard, ~1 x 105 DPM
Slow preamplifier(s) : One for each PMT are built-in. These two outputs are summed & fed to MCA.
Fast pre-amplifier(s) : One for each PMT are built-in to check for time co-incidence output to achieve low Background.
Specifications of PMT 9829B QA : Two matched PMT power of E.T. make 9829B
Photocathode : Blalkali
Quantum efficiency at peak : 30%
Luminous sensitivity : 85 µA/lm
Dynodes : 12LFBeCu
Anode sensitivity
Nominal anode : 500 A/lm
Max. rated anode : 2000 A/lm
Dark current at 20oC
DC at nominal A/lm : 3 nA
DC at max. rated A/lm : 12 nA
Temperature coefficient time
Single electron rise time : 2ns
Single electron fwhm : 3ns
Weight : 130g
Specifications of USB-MCA module : This card is wiredup into a single bit NIM module.
MCA resolution : 256, 512, 1K, 2K, 4K and 8K channels
Spectrum memory : 128 K bytes single port SRAM
Max counts / channel : 31 bit (2 Giga counts)
Pulse processing time : 7 ms including ADC conversion time of 5 ms
Pile up rejection : Active high TTL input from spectroscopy amplifier
DNL : Better than +/- 1%
INL : Better than +/- 0.05% F.S
MCA Input : Single channel, 0 to +10 volts
Timer : 32 bit, integrated in FPGA
Preset Time : LIVE or CLOCK, 1 sec to 136yrs
LLD, ULD, Baseline : Digitally set through software
Power requirement : 5V, ~ 500 mA through USB cable directly
(No external power supply required)
Output : Live display of acquisition of beta spectrum
Efficiency : (a) For tritium (3H) (in toluene based scintillating solution) > 40% with background less than 60 cpm with BG standard vial, without BG standard vial 40 cpm.
(b) For carbon (14C) (in toluene based scintillating solution) > 90% with background less than 100 cpm with BG standard vial, without BG standard vial 100 cpm.
Operating environment : Air conditioned (20-25oC), dust free environment.
Power : System operates on 230V, 50Hz, AC supply
Plot No.162 A & B, Phase-II, IDA, Cherlapally, Hyderabad-500051 India Phone:+91-40-32918055, 32914548 Fax : 040 - 27262146
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