Gamma Detectors, Planar Design-
We have two Canberra model GL2020RS low energy Germanium (LeGe) planar detectors coupled with low background cryostats in low background shields. These have 2000mm2 active area crystals with a detection range from 10 kiloelectron volts(keV) to 1200 keV. Our detectors have been used regularly to detect 210Pb or 234Th in sediments and 7Be from rainwater. With their large surface area and low background, these detectors are useful to anyone who is trying to detect environmental levels of a gamma emitting radioisotope in either a liquid or solid matrix. Samples are generally placed in 8 ounce polystyrene jars, set on top of the Ge crystal, and gamma events are counted. Since the detectors will accumulate a full spectrum of identifiable gamma peaks within each counting period, multiple radionuclides can be quantified during a single count. These instruments are coupled with Windows-based spectroscopy software by Aptec, for data processing.
Gamma Detectors, Well Design-
The facility operates 2 Canberra closed-end coaxial large volume well detectors. Both detectors have a 4p counting geometry, a detection range past 3000keV and are coupled with low background cryostats and shields. In order to reduce background, detectors GCW4023S3 and GCW4030S use a large plastic scintillator detector and coincidence timing circuit to create an anti-coincidence veto gate suitable for both germanium detectors. Either APTEC PCMCA or Canberra Genie 2000/LYNX are used as processing MCA software. These well detectors are the most sensitive gamma counter designs available, thus providing high quality data and minimizing counting time.
The facility has been selected as a beta testing site for Canberra Industries. In 2013 the facility will acquire and test a next generation Canberra SAGe well detector with a larger crystal which provides greater efficiency and superior energy resolution compared with our other 28 mm wells.
Beta Detectors, Gas Flow Proportional Counters-
We have 6 Riso GM-25-5, five sample low-level beta detectors. These accept any samples that can be mounted on a 25mm filter holder. These detectors have very low backgrounds, 0.17 counts per minute, and can have counting efficiencies as high as 55%. Beta particle detection is not readily nuclide specific so there can be some sample purification necessary before counting. These detectors count five samples at a time using Windows-based software designed by Riso in Denmark. They are ideal for all beta detection except for the weakest beta emitters (3H and 14C), which cannot penetrate the mylar window into the counting chamber.
Alpha Detectors, Surface Barrier-
Alpha analysis is conducted via CANBERRA’s Alpha Analyst with an eight Passivated Implanted Planar Silicon (PIPS) detectors. Operating through Genie 2000, Apex-Alpha™ software controls detector set-up and calibration, quality assurance, sample analysis and reporting. Alpha decay occurs in the elements of higher atomic number and the alpha particles high energy and short travel requires some chemical purification and electroplating, precipitating or evaporation onto a metal disc for counting. These have traditionally been used for plutonium, long-lived thorium isotopes and other actinides. The facility can do these purification and plating steps if needed.
Delayed Coincidence Counting System-
The short-lived 223Ra and 224Ra isotopes can be counted using a delayed coincidence alpha scintillation techniques. Samples are collected by passing 1092s liters of seawater through MnO2-impregnated fibers which are partially dried and placed in a scintillation cell attached to a photo multiplier tube. Inert gas is recirculated through the system and the gaseous Radon daughters rapidly reach equilibrium with the parent Ra on the fiber. The detector discriminates between the alpha decay of the short-lived daughters of 223Ra and 224Ra via delayed coincidence counting techniques. Two of these systems are available as part of the facility.
Sediments by Gamma Spectrometry-
One of the largest uses of the facility is for the analyses of gamma emitting radionuclides in marine sediments. Dried and ground sediment samples are placed in calibrated counting jars on the planar and well gamma counters for 12-48 hours. From measurement of isotopes such as 210Pb, 226Ra, 234Th, 137Cs and 7Be, one can often calculate the rates of mixing (bioturbation and physical mixing) and sedimentation in coastal and deep ocean sediments, on time scales of days to 100 years.
Water, Particles, Sediment Traps for 234Th-
Thorium-234 is a particle-reactive naturally occurring decay product of uranium-238. Detection of 234Th can be accomplished via beta or gamma methods depending upon sample size and application. Measurement of the difference in 234Th and 238U activities in seawater provides an in-situ tracer of particle export and cycling in the upper ocean on time scales of days-weeks. This approach is becoming a "core" measurement in many biogeochemical studies. Additional alpha spectrometry is often performed to evaluate chemical yields from extraction procedures as well as for specific actinide analysis in marine and ground water samples.
Groundwater and Coastal Waters for Radium Isotopes-
The radium isotopes 226Ra (t1/2 1620 years), 228Ra (t1/2 5.75 years), 224Ra (t1/2 3.66 days), and 223Ra (t1/2 11.4 days), are naturally produced in sediments and are highly soluble and enriched in groundwater and coastal surface waters. As such, measurement of the flux of Ra out of sediments can be used to quantify groundwater discharge, and measurement of surface water Ra gradients can be used to quantify flow and horizontal mixing processes in the coastal ocean. Current methods now allow for the rapid detection of 223Ra and 224Ra using a customized delayed coincidence counter. 226Ra and 228Ra are efficiently detected using a gamma "well" detector.