Gamma-ray counters

Gamma-ray counters are usually made from a cylinder of activated sodium iodide, which is enclosed in an aluminum shell, with one side of the cylinder lying in close proximity to a photomultiplier tube. The flow cell is composed of a U tube or coil (Figure 7.12) placed into the core of the cylinder. Radioactivity present in the column effluent is blocked from escaping by the dense sodium iodide, and the energy is converted into light, which is measured by the photomultiplier tube. 

NA—neutron activation sample irr. for 20 min. (JR-3, 2Xl013N/cm2/sec), 3 day cooling, diss. in hot HN03, add 2 mg. each Ag+, Au2+, add HC1, heat, add H20, heat, cool, filter. Solution extracted with EtAc and counted for 198Au (.411 Mev). AgCl ppt. counted 110mAu (.656 Mev). Gamma-ray counter, Nal (Tl) detector, 3 inch X3 inch. 

Gamma-ray counters Gamma ray detector Gamma-rays... 

A special benefit of counting radiations in coincidence is that the process permits the absolute measurement of the activity of the radionuclide. For the beta-particle counting efficiency sp and the gamma-ray counting efficiency y (in count/disintegration), the activity A is related to the three count rates. The count rate Rp of beta particles in the beta-particle counter, the count rate Ry of gamma rays in the gamma-ray counter, and the coincidence count rate Rpy (in count/s) yield A (in Bq) by ... 

Radiometric ore sorting has been used successfully for some uranium ores because uranium minerals emit gamma rays which may be detected by a scintillation counter (2). In this appHcation, the distribution of uranium is such that a large fraction of the ore containing less than some specified cut-off grade can be discarded with tittle loss of uranium values. Radioactivity can also be induced in certain minerals, eg, boron and beryllium ores, by bombarding with neutrons or gamma rays. 

The gamma rays are detected today with sodium iodide crystals scintillation counters. The counters, 6 to 12 in. long (15 to 30 cm) are shock mounted and housed in the drill collars. Several types of measurements can be made total gamma rays, direction-focused gamma rays, spectral gamma rays. 

In MWD, the recording speed is the rate of penetration which rarely exceeds 120 to 150 ft/hr or 2 to 2.5 ft/min, two orders of magnitude less than the logging speed. Counters can be made shorter and time constant longer (up to 30 s or more). This results in a better accuracy and a better bed definition. Figure 4-269 shows an example of comparison between an MWD gamma ray log and the wireline log ran later. 

Spectral Gamma Ray Log. This log makes use of a very efficient tool that records the individual response to the different radioactive minerals. These minerals include potassium-40 and the elements in the uranium family as well as those in the thorium family. The GR spectrum emitted by each element is made up of easily identifiable lines. As the result of the Compton effect, the counter records a continuous spectrum. The presence of potassium, uranium and thorium can be quantitatively evaluated only with the help of a computer that calculates in real time the amounts present. The counter consists of a crystal optically coupled to a photomultiplier. The radiation level is measured in several energy windows. 

By counting the gamma of low energy reaching the first counter a Pe curve sensitive to the nature of the formation can be recorded. A special counter protection fairly transparent to low-energy gamma ray (beryllium) is used. Table 4-129 shows the value of Pe for various lithologies. 

Gas-filled detectors are used, for the most part, to measure alpha and beta particles, neutrons, and gamma rays. The detectors operate in the ionization, proportional, and G-M regions with an arrangement most sensitive to the type of radiation being measured. Neutron detectors utilize ionization chambers or proportional counters of appropriate design. Compensated ion chambers, BF3 counters, fission counters, and proton recoil counters are examples of neutron detectors. 

Proportional counters are extremely sensitive, and the voltages are large enough so that all of the electrons are collected within a few tenths of a microsecond. Each pulse corresponds to one gamma ray or neutron interaction. The amount of charge in each pulse is proportional to the number of original electrons produced. The proportionality factor in this case is the gas amplification factor. The number of electrons produced is proportional to the energy of the incident particle. 

Proportional counters can also count neutrons by introducing boron into the chamber. The most common means of introducing boron is by combining it with tri-fluoride gas to form Boron Tri-Fluoride (BF3). When a neutron interacts with a boron atom, an alpha particle is emitted. The BF3 counter can be made sensitive to neutrons and not to gamma rays. 

A 3.5 ml portion in a 4 ml polyethylene vial was irradiated for 5 min. Another portion, 3.0 ml in a 3.5 ml silica vial, was irradiated for 3 d. After the short irradiation, 3 ml of the irradiated solution were transferred into an activity-free vial and submitted to y-ray spectrometry with a Ge(Ii) detector coupled to a 4000-channel analyser. After the long irradiation, the sample was allowed to cool for 3 d, then the surface of the silica ampoule was cleaned with dilute nitric acid and the sealed ampoule was placed in the counter (the background activity of the ampoule was negligible). Gamma-ray energy and the areas under peaks were calculated by computer. To determine the half-fives of the nuclides produced, the counting was repeated at appropriate intervals. 

The hematite with adsorbed Co-57 or Sb-119 along with the solution was subjected to emission Mossbauer measurement at 24 1°C with the experimental setup shown in Figure 2. The absorber, Fe-57-enriched potassium ferrocyanide (0.5 mg Fe-57/cm2) or barium stannate (0.9 mg Sn-119/cm2), was driven by a Hanger 700-series Mossbauer spectrometer connected to a Tracor-Northern TN-7200 multi-channel analyzer. The Mosssbauer gamma-rays of Co-57 and Sb-119 were detected respectively with a Kr(+3% carbon dioxide)-filled proportional counter and with a 2 mm-thick Nal(Tl) scintillation counter through 65 pm-thick Pd critical absorber for Sn K X-rays. The integral errors in the relative velocity were estimated to be of the order of 0.05 mm/s by repeated calibration measurements using standard absorbers. 

The internal standard ratio method for quench correction is tedious and time-consuming and it destroys the sample, so it is not an ideal method. Scintillation counters are equipped with a standard radiation source inside the instrument but outside the scintillation solution. The radiation source, usually a gamma emitter, is mechanically moved into a position next to the vial containing the sample, and the combined system of standard and sample is counted. Gamma rays from the standard excite solvent molecules in the sample, and the scintillation process occurs as previously described. However, the instrument is adjusted to register only scintillations due to y particle collisions with solvent molecules. This method for quench correction, called the external standard method, is fast and precise. 

Gamma Rays (Nuclear-decay y-Rays, 0.5-m.e.v. Photons from Annihilation of Positrons or X-rays). The development of large sodium iodide crystal 7-ray spectrometers (13) has made possible high detection efficiencies (close to 100% for some 7-ray energies). Also, whole-body counters utilizing large cylindrical liquid scintillators provide a detection efficiency of 15% for the 7-rays emitted from potassium-40 in the human body (23). 

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