Scintillation activators

As it happens, the band gap of sodium iodide is large and photons emitted by de-excitation of electrons directly from the conduction band would be far outside of the visible range. This makes detection of the light difficult. Not only that, the bulk of the material absorbs the emitted photons before they reach the photomultiplier. Both problems are solved by using an activator. In the case of Nal, this would be thallium and for Csl it is thallium or sodium. The shorthand descriptions for these activated scintillators are Nal(Tl), CsI(Tl) and CsI(Na). 

Not all of the energy absorbed from the gamma-ray will be re-emitted as scintillation photons. Nal(Tl), the most efficient in terms of light output, will only release about 12 % of the total. The rest of the energy is retained as lattice vibrations or heat. The size of the actual output signal from the detector will also depend upon how well the response of the light detector matches the scintillation-light spectrum. 

Transitions from some of the excited states to the ground state may be forbidden. An electron finding itself in one of these states must first be promoted by thermal excitation to a level which is not forbidden before it can de-excite. The lifetime of these states can be much longer that the normal excited states. The slowly decaying 

Scintillation detectors have an advantage over HPGe semiconductor detectors in that they can be operated at room temperature, and indeed the light output from Nal(Tl) is at a maximum at room temperature. The light output is reasonably constant over the normal range of room temperature but decreases beyond that. Other materials behave in a similar manner but the temperature of maximum response differs from material to material. 

Even though the response of the detector crystal may be reasonably constant within the normal operating temperature range, the stability of the electronic system may not be so forgiving. 

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Cerium is one of the most widely used activators, which improve the working characteristics of many scintillators. Determination of the valence state of cerium in single crystals of alkaline and rare-earth borates allows to establish the nature of activator centers for purposeful influence on the scintillation efficiency of the matrix. 

A scintillation counter registers emitted radiation caused by the disintegration of nuclides. If each atom of nuclide emits one count, what is the activity of a sample that registers 3.00 X 104 disintegrations in five minutes ... 

In the phosphor-photoelectric detector used as just described, the x-ray quanta strike the phosphor at a rate so great that the quanta of visible light are never resolved they are integrated into a beam of visible light the intensity of which is measured by the multiplier phototube. In the scintillation counters usual in analytical chemistry, on the other hand, individual x-ray quanta can be absorbed by a single crystal highly transparent to light (for example, an alkali halide crystal with thallium as activator), and the resultant visible scintillations can produce an output pulse of electrons from the multiplier phototube. The pulses can be counted as were the pulses-from the proportional counter. 

The concrete block walls of the cell housing the generator tube and associated components are 1.7 meters thick. The facility also includes a Kaman Nuclear dual-axis rotator assembly for simultaneous transfer and irradiation of reference and unknown sample, and a dual Na iodide (Nal) scintillation detector system designed for simultaneous counting of activated samples. Automatic transfer of samples between load station to the rotator assembly in front of the target, and back to the count station, is accomplished pneumatically by means of two 1.2cm (i.d.) polyethylene tubes which loop down at both ends of the system and pass underneath the concrete shielding thru a pipe duct. Total one-way traverse distance for the samples is approx 9 meters. In performing quantitative analysis for a particular element by neutron activation, the usual approach is to compare the count rates of an unknown sample with that of a reference standard of known compn irradiated under identical conditions... 

The sampling of solution for activity measurement is carried out by filtration with 0.22 pm Millex filter (Millipore Co.) which is encapsuled and attached to a syringe for handy operation. The randomly selected filtrates are further passed through Amicon Centriflo membrane filter (CF-25) of 2 nm pore size. The activities measured for the filtrates from the two different pore sizes are observed to be identical within experimental error. Activities are measured by a liquid scintillation counter. For each sample solution, triplicate samplings and activity measurements are undertaken and the average of three values is used for calculation. Absorption spectra of experimental solutions are measured using a Beckman UV 5260 spectrophotometer for the analysis of oxidation states of dissolved Pu ions. 

Fluorescent materials are very important in medical research. Dyes such as fluorescein (21) can be attached to protein molecules, and the protein can be traced in a biological system by exciting the fluorescein and looking for its emissions. The use of a fluorescent material allows the detection of much smaller concentrations than would otherwise be possible. Because fluorescent materials can be activated by radioactivity, they are also used in scintillation counters to measure radiation (see Box 17.2). 

Uniformly labeled C-8-D with a specific activity of 2.99 juc/mg was administered orally to pregnant females at 2 /xg/kg/day from 6-15 days of gestation. Three females were sacrificed on alternate days during days 6-20 of pregnancy. Triplicate samples of fetus, placenta, blood, brain, abdominal fat, and sartorius muscle were procured from each female. The samples were dissolved in 1 ml of Soluene (Packard Instruments) to which 15 ml of Aquasol were added. Each sample vial was counted for 30 min in a Nuclear Chicago Mark I liquid scintillation counter. 

D-Xylulose 5-phosphate (ii-threo-2-pentulose 5-phosphate, XP) stands as an important metabolite of the pentose phosphate pathway, which plays a key fimction in the cell and provides intermediates for biosynthetic pathways. The starting compound of the pathway is glucose 6-phosphate, but XP can also be formed by direct phosphorylation of D-xylulose with li-xylulokinase. Tritsch et al. [114] developed a radiometric test system for the measurement of D-xylulose kinase (XK) activity in crude cell extracts. Aliquots were spotted onto silica plates and developed in n-propyl alcohol-ethyl acetate-water (6 1 3 (v/v) to separate o-xylose/o-xylulose from XP. Silica was scraped off and determined by liquid scintillation. The conversion rate of [ " C]o-xylose into [ " C]o-xylulose 5-phosphate was calculated. Some of the works devoted to the separation of components necessary while analyzing enzyme activity are presented in Table 9.8. 

The sources most commonly used so far consisted of sintered disks containing about 100 mg ZnO enriched with 90% Zn. The disks were irradiated with 12 MeV deuterons or 30 MeV He particles, to yield the 78 h activity of Ga, and then annealed by heating in oxygen to 700-1,000 K for about 12 h and cooling down slowly (about 50 K h ) to room temperature. A Nal scintillation counter, 2-3 mm thick, is suitable for the detection of the 93 keV y-rays. Because of the relatively high transition energy, both source and absorber are generally kept at liquid helium temperature. 

Biggin CD, Cook GT, MacKenzie AB, Pates JM (2002) Time-efficient method for the determination of Pb-210, Bi-210, and Po-210 activities in seawater using hquid scintillation spectrometry. Anal Chem 74 671-677... 

Figure 9.3 The whole-body counter of University Medical Centre, Utrecht, The Netherlands. The counter has a mobile shadow shield with two Nal(Tl) scintillation detectors (4x6 in) placed at opposite sites of the subject. The lead shielding is 100 cm long with a diameter of 90 cm. The scanner moves on rails over a distance of 240 cm with an adjustable speed. Extreme variations in geometry yield practically the same value for 59Fe activity. The equipment can be used for measurements in man and small animals.

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