Nal detectors

The Compton profile measurements on Cu and Cu 953AI0047 were performed at ID 15b of the ESRF. Figure 1 shows the setup of the scanning-type Compton spectrometer used. It consists of a Si (311) monochromator (M), a Ge (440) analyzer (A) and a Nal detector (D). The signal of an additional Ge solid state detector (SSD) was used for normalization. ES, CS and DS denote the entrance slit, the collimator slit and the detector slit, respectively. For each sample 10 different directions were measured with approximately 1.5-2 x 103 7 total counts per direction. The incident energy was 57.68 keV for the Cu and 55.95 keV for the Cuo.953Alo.047 measurement. 

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. 

The only interference is from 82Br which is, of course, more serious if counting is done with a Nal detector. Bromide is removed by adding bromide carrier to the nitric acid solution of the products at 40°C and precipitating with silver nitrate. Mercuric bromide is soluble in warm dilute nitric acid and is quantitatively retained in solution. 

The irradiated sample and the polyethylene container in which it was irradiated, along with mercury carrier, are wet-ashed with a mixture of nitric, sulfuric, and perchloric acids under good reflux conditions. The mercury is isolated from the digest by precipitation as mercuric sulfide and is purified by electrodeposition as elemental mercury on gold foil, which is then counted for 197Hg and 24-hr 197mHg x- and y-ray activity with a thin Nal detector. 

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 y-ray spectroscopic information was obtained using an array of five Ge detectors with pentagonal Nal anti-Compton shields located at 63° to the beam and three additional Ge detectors at 24°. Two-fold or higher coincident events from these detectors were used to trigger the 72 Nal detectors of the Spin Spectrometer (SS) at ORNL. [JAA83] An average Compton suppression factor of 3.5 for the Co spectrum was obtained. The Ge detectors were placed at 20.8 cm from the target. 

The event tapes were processed in order to (1) linearize and match the gains of the Nal detectors, (2) separate the neutron from the y pulses in the SS, and (3) construct the total pulse height, H, and the coincidence fold k. 

Figure 16.9. Annual modulation of the total counting rate (background plus possible dark matter signal) in seven years of data with the DAMA-Nal detector. A constant counting rate has been subtracted to give the residuals. The significance of the modulation is 6cr and its period is 1 year. The interpretation of the yearly modulation as due to a WIMP signal is controversial. (Figure from Bernabei et al.(2003).)...

Fig. 1. (Top) y-Ray energy spectrum of the reaction 19F(p,ay)160 measured by a 3-inch Nal detector. Proton energy is 2.7 MeV, sample material is fluorapatite. (Middle) y-Ray energy spectrum for the same reaction acquired by a high purity germanium detector. The sample is meteoritic material. Low energy lines from several other nuclear reactions can be identified. (Bottom) Low-energy y-ray spectrum from 19F(p,p y)19F inelastic scattering recorded with a thin Ge(Li) detector. Reproduced with permission from Grambole and Noll [59],...

To determine the Cu/0 ratio, the outputs from the two linear amplifiers (one for each 12.7cm X 12.7cm Nal detector) were combined in a sum invert amplifier, the output of which was split and connected to two separate singlechannel analyzers. The energy window of one analyzer was set from 0.45—0.60 MeV to count the Cu, and the other was set from 4.85—... 

Relative efficiency compared to 3" x 3" Nal detector as defined in IEEE 325. 

Many configurations of Nal detectors are commercially available, ranging from very thin crystals for X-ray measurements to large crystals with multiple phototubes. Crystals built with a well to allow nearly spherical (4jt) geometry counting of weak samples are also a standard configuration. 

The HELINUC system is mounted on a helicopter (equipped with a Nal detector) and linked to ground-based equipment (with a germanium detector). 

Nal bench-top gamma spectrometric system comprising of Nal detector (8% resolution), lead shielding to house Marinelli beakers and polyethylene bottles (0.5 or 1 litre), amplifier, high voltage devices etc. PC aided system with printer, plotter and software for spectra and data evaluation. 

Set of Nal detectors having different sizes of crystals. Quick low resolution measurements. 

Small size CdTe, Csl or Nal detectors. Gamma-detection in restricted areas (such as inside fuel assemblies). 

Determination of Mo Impurity. Determination of Mo should be performed with a sample of the fresh eluate (37 MBq) by y-spectrometry. A lead absorber 6-mm thick is placed between the sample and the Nal-detector. The fraction of y-radiation measured at 740 keV ( Mo) should not exceed the reading obtained with a reference... 

A 1-mCi point isotropic gamma source is located 0.10 m away from a 60° spherical shell of a Nal detector, as shown in the figure below. Assuming that all the pulses at the output of the photomultiplier tube are counted, what is the counting rate of the scaler The gamma energy is 1.25 MeV. 

SS, 3 X 2nun). A Nal detector continuously measures the passing activity through the sample loop. A GC run is remotely started at the moment that the sample loop is filled with [ NJ-NHs. The GC is equipped with a Haysep P column (SS, mesh 80-100, O.D. 1/8). The GC run is started at 90°C, and after 1 minute, the temperature is increased with 10°C/min to 130°C. The products are analysed by a thermal conductivity detector (TCD). A Nal detector, directed at the TCD, monitors labelled products. Behind the TCD detector, a heated electrical 3-way valve selects a small part from the [ NJ-NHg peak. Depending on the experiment, a pulse time of 2-10 s is used to inject the labelled gaseous ammonia into the reactant stream. The required specific activity of the radioactive ammonia pulse is 0.1 MBq/ml minimum to meet the statistical requirements of the positron emission profiling experiments. 

For many years, a 3 x 3 Nal detector was the standard for routine measurements of y-active materials. The detector consisted of a cylindrical piece of thallium-activated Nal 7.6 cm (i.e., 3 in.) in diameter and 7.6 cm in height. The hygroscopic crystal was enclosed in an aluminum can (with a glass window on one end) to protect it from moisture. These crystals could be manufactured sufficiently reproducibly that efficiencies could be taken from tabulations. Another useful geometry was similar except for a coaxial well where small samples could be counted at nearly 100% detection efficiency. 

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