Radiochemical detectors

Unlike the majority of other HPLC detectors, radiochemical detection is based on time, and detection of low abundance peaks can be improved by adjusting the flow rate. Slower flow rates allow for more counting time and therefore increased accuracy and sensitivity. Even so, radiochemical detection often suffers from reduced sensitivity due to high background. 

The standard technique to check the radiochemical purity of a labeled compound is thin layer chromatography/instant thin layer chromatography (TLC/ITLC) combined with a radioactivity counter. This method is quick and easy to perform. Different isomeric forms of a radiopharmaceutical may not be determined. A more sophisticated method for quality control is high performance liquid chromatography (HPLC) combined with a radioactivity detector. Radiochemical purity as well as isomeric forms and small modifications due to instability, radiolysis, etc., may be determined. To achieve an accurate determination of non-chelated radiometal, a mM solution of DTPA (pH 4-5) has to be added to the HPLC sample to ensure a complexation of the free metal. If DTPA is not added, unchelated metal might bind and stick to the HPLC column and thus indicate a too high radiochemical purity (Smith-Jones etal. 1998). 

The following instrumental analysis textbooks may be consulted for further information on the detectors and signal analyzers used in radiochemical methods of analysis. 

Alpha counting is done with an internal proportional counter or a scintiUation counter. Beta counting is carried out with an internal or external proportional gas-flow chamber or an end-window Geiger-MueUer tube. The operating principles and descriptions of various counting instmments are available, as are techniques for determining various radioelements in aqueous solution (20,44). A laboratory manual of radiochemical procedures has been compiled for analysis of specific radionucHdes in drinking water (45). Detector efficiency should be deterrnined with commercially available sources of known activity. 

The NAA measurements on the paper samples were made at the Breazeale Nuclear Reactor Facility at the Pennsylvania State University with a TRIGA Mark III reactor at a flux of about 1013 n/cm2-sec. Samples were irradiated from 2 to 20 min and counted for 2000 sec, after a 90 min decay time for Ba and a 60 hr decay for Sb, Analyses were performed instrumentally, without radiochemical separation, using a 35cm3 coaxial Ge-Li detector and a 4096-channel pulse height analyzer. With these procedures, detection limits for Ba and Sb were 0.02ug and 0.001 ug, respectively. These sensitivities are comparable to those obtained by GA s radiochemical separation procedure, and are made possible by the use of the higher neutron output from the more powerful reactor and in combination with the higher resolution solid state detector... 

The detection of the migrating sample boundary in CE can be accomplished by UV, fluorescent, electrochemical, radiochemical, conductivity, and mass spectrometry (MS) means. The use of high-sensitivity detection systems is always a key issue in CE applications. The sensitivity of HPCE detectors may be at least 2 to 3 orders of magnitude better than that of HPLC detectors. Since the detection cell volume is very small, the concentration sensitivity... 

The difficulties include the inconvenience of handling radioactivity and the necessity for obtaining an accurate radiochemical analysis of two phases containing several elements (which often involves complicated spectra). Highly sensitive instrumentation is required for the analysis e.g. a Li-Si surface barrier detector for a particles, a 2 r gas counter for (3-radiation and a Li-Ge detector for 7-radiation. Great care is required during source preparation, which is best done by electrodeposition. 

Neutrino detectors are placed at great depths, at the bottom of mines and tunnels, in order to reduce interference induced by cosmic rays (Fig. 5.3). Two methods of detection have been used to date. The first is radiochemical. It involves the production by transmutation of a radioactive isotope that is easily detectable even in minute quantities. More precisely, the idea is that a certain element is transformed into another by a neutrino impact, should it occur. Inside the target nucleus, the elementary reaction is... 

Radioactivity of uranium can be measured by alpha counters. The metal is digested in nitric acid. Alpha activity is measured by a counting instrument, such as an alpha scintillation counter or gas-flow proportional counter. Uranium may be separated from the other radioactive substances by radiochemical methods. The metal or its compound(s) is first dissolved. Uranium is coprecipitated with ferric hydroxide. Precipitate is dissolved in an acid and the solution passed through an anion exchange column. Uranium is eluted with dilute hydrochloric acid. The solution is evaporated to near dryness. Uranium is converted to its nitrate and alpha activity is counted. Alternatively, uranium is separated and electrodeposited onto a stainless steel disk and alpha particles counted by alpha pulse height analysis using a silicon surface barrier detector, a semiconductor particle-type detector. 

Another consideration when choosing a detector is whether it is important to preserve the separated analytes, either for use or for further analysis. Some methods, such as evaporative laser scattering detection and mass spectrometry, destroy the sample during the measurement. Other methods, such as fluorescence or radiochemical detection, may require chemical labeling of the analytes ... 

FIGURE 7.12 Schematic for radiochemical detection of gamma-emitting analytes. The analytes within each peak are measured by a photomultiplier tube placed in close proximity to or incorporated into the aluminum shell of the detector. 

This work was supported by the Intramural Research Program of the National Institutes of Health, National Institute of Biomedical Imaging and Bioengineering. The authors would like to thank Terry M. Phillips for his guidance and for his valuable assistance, especially with the sections on electrochemical and radiochemical detectors. 

Radiochemical separations are necessary for many elements when only a Nal detector is available. Even with a Ge(Li) detector, a radiochemical separation increases the sensitivity and accuracy and permits the determination of some elements whose radioactivities are masked by stronger activities in the multi-element spectrum of a coal sample. For example, mercury, selenium, gallium, and zinc in most coals are below the limit of detection instrumentally even with the resolution of a Ge(Li) crystal (7), but can be determined after radiochemical separations as is described later. 

In the radiochemical procedure the irradiated coal sample and mercuric oxide carrier are digested with sulfuric acid, followed by nitric acid. Water and potassium bisulfate are added to drive off any nitric acid remaining. The mercury is separated by a standard dithizone extraction, and the extract is counted for the 0.077 MeV photopeak of 197Hg with the Nal detector. 

The Davis or Cl detector was the detector used to define the solar neutrino problem, and another type of radiochemical detector, the SAGE/GALLEX detectors, was used to further define the problem. These detectors, GALLEX in Italy and SAGE in Russia, are based on the reaction... 

The direct observational evidence for the occurrence of neutrino oscillations came from observations with the Cerenkov detectors. The SNO detector found one-third the expected number of electron neutrinos coming from the sun in agreement with previous work with the radiochemical detectors. The Super Kamiokande detector, which is primarily sensitive to electron neutrinos, but has some sensitivity to other neutrino types found about one-half the neutrino flux predicted by the standard... 

Outline how you would construct a radiochemical neutrino detector based upon 115In. 

Fig. 3.61. Flow cells for the Berthold radiochemical detector A, for homogeneous scintillation ...

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