Counting of the Sample

After irradiation is completed, the sample is counted using an appropriate system. The qualitative and quantitative determination of an isotope is based on the analysis of the energy spectrum of the radiations emitted by the radioisotope of interest. Sometimes it may be necessary to use information about the half-life of the isotope(s). In such a case, counting may have to be repeated several times at specified time intervals. 

The counting system depends on the radiation detected. Modem activation analysis systems depend on the detection of gamma rays and X-rays and very 

A basic counting system for activation analysis consists of a detector [Ge or Si(Li)], electronics (i.e., preamplifier, amplifier), and a multichannel analyzer (MCA). Modern MCAs do much more than record the data. They are minicomputers or are connected to computers that store and analyze the recorded data. Examples are the ADCAM architecture offered by EG G ORTEC and the Genie-ESP VAX-based Data Acquistion and Analysis System offered by Canberra. 

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The extraction of two typical agricultural products from environmental matrices were chosen as examples for the operation of this system. Diuron, a phenylmethylurea, was freshly spiked onto Tama soil. This soil was characterized and shown to have 3.1% organic material and 14 % clay fraction. In addition, a phenyl metabolite of NUSTAR, a systemic fungicide, on wheat previously unextractable by SFE was extracted. The wheat sample was not classified for its chemical composition. Both samples were treated with radiolabeled compounds (E. I. du Pont de Nemours and Company, Du Pont Agricultural Products, Wilmington, DE) and extraction results are from liquid scintillation counting of the sample extract. Chromatographic evaluation of the Diuron from soil extracts has previously been published (2). 

The channels ratio method makes use of existing counts within the sample vial. This method is suitable when large numbers of counts are present, but it becomes very time consuming with samples containing few counts, because a long time is required to accumulate sufficient counts for statistical accuracy. Most modern scintillation counters therefore employ an automatic external standardization system of quench analysis to avoid the time required for the internal channels ratio method. This method utilizes a specially selected external y radiation source carried in a lead-shielded chamber that is buried in the instrument. Before the regular counting of the sample, the external standard is... 

In the infancy of the LSC technology the final sample volume exceeded 40 ml. The volume of the vial has since subsequently been standardized to 20 ml for over two decades. Most of the instruments are also built accordingly to measure and collect the counts of the sample in such vials, and until recent the volumes of the liquid scintillator have been usually between 8 and 10 ml. Ever increasing number of LSC samples has been raising the annual costs and physical volume of the scintillators and vials. Since the research funds at the same time have become in short supply, the economical factors had to be taken into account. The first cost reduction by search for inexpensive scintillators generated a wide variety of cocktails and many research groups have created their own. Another cost reduction in the LSC was the introduction of the plastic vial in 1963. 

The rapidity of the publication of the results was impressive After the decisive experiment on December 17 (counting of the samples continued until December 21), the revised manuscript was finished on December 22 (just before Christmas). Otto Hahn called up his friend Paul Rosbaud, a freelance of Die Naturwissenschalfen practically in charge of natural science. He came over, got the article, brought it to the printing shop, and replaced another article in press by the one of Hahn and Strassmann. The galley proofs were finished by December 27 and the article was out by January 6, 1939. 

The dialyzers are shaken in a constant temperature shaking machine at 3TC. At the desired intervals two dialyzers are removed and 0.5 ml counting samples are taken from front and back compartments thus, each point on the dialysis curve is established by means of individual dialyzers in duplicate. No dialyzer is returned to the shaker and sampled again after a further interval since errors are introduced if several samples are taken from the same dialyzer because of the change in the ratio of solution volume to membrane area. The samples are counted with a gamma scintillation spectrometer. Although we base our calculations on the counts of the samples taken from the front compartments (in which the concentration of labeled chromium(III) is increasing), counts of the samples from the backs are obtained as a check. 

Column A contains a simple count of the sample number starting from 1. 

In the Sg experiments [108], a 950 pg cm " Cm target was bombarded with 3 X 10 Ne ions s at 121 MeV to produce Sg with a half-life of s [109] (Todays knowledge [103, 104] shows the presence of two states one with a half-life of about 9 s and one with about 15 s). Totally, 3,900 identical separations were conducted with a collection and cycle time of 45 s and a total beam dose of 5.48 X 10 Ne ions. The transport efficiency of the He(KCl) jet was 45%. On the average, counting of the samples started 38 s after the end of collection. The overall chemical yield was 80%. 

For phytoplankton cultures it may be necessary to dilute the sample in order to avoid coincidence corrections. From a preliminary count of the sample, and with reference to Table XVII, it may be determined if the count exceeds an acceptable level of coincidence. Generally, counts which have a coincidence correction of greater than 5% should be diluted. Dilution to a known volume should be made with membrane-filtered (0.22 p) sea water. 

Find the radioactive count of the sample, using precisely the same assembly as that used for phytoplankton samples. Evaluate the counting rate in counts per minute, having counted a total of at least 10,000—preferably 50,000 — counts. Correct this rate for any coincidence correction and for the natural background rate if this is significant. 

Instructions for starting the sampling of interferogram and precise counting of the sampling points are controlled by the output signals (g) and from the two laser interferometers and the counter shown in Figures 5.5 and 5.6. 

A method of computing activation cross sections by determination of the absolute disintegration rate of a foil irradiated in a known thermal flux is described. The method can also be used to determine an unknown thermal flux in which a foil has been irradiated, provided that an activation cross section for the foil material is assumed. The method used could be applied to many materials which become activated by thermal neutrons and whose resulting beta activity has a sufficiently long half-life so that counting of the sample is not a problem. The techniques involved in absolute beta counting are emphasized. 

The excess heat of solution of sample A of finely divided sodium chloride is 18 cal/g, and that of sample B is 12 cal/g. The area is estimated by making a microscopic count of the number of particles in a known weight of sample, and it is found that sample A contains 22 times more particles per gram than does sample B. Are the specific surface energies the same for the two samples If not, calculate their ratio. 

In this problem you will collect and analyze data in a simulation of the sampling process. Obtain a pack of M M s or other similar candy. Obtain a sample of five candies, and count the number that are red. Report the result of your analysis as % red. Return the candies to the bag, mix thoroughly, and repeat the analysis for a total of 20 determinations. Calculate the mean and standard deviation for your data. Remove all candies, and determine the true % red for the population. Sampling in this exercise should follow binomial statistics. Calculate the expected mean value and expected standard deviation, and compare to your experimental results. 

In research environments where the configuration and activity level of a sample can be made to conform to the desires of the experimenter, it is now possible to measure the energies of many y-rays to 0.01 keV and their emission rates to an uncertainty of about 0.5%. As the measurement conditions vary from the optimum, the uncertainty of the measured value increases. In most cases where the counting rate is high enough to allow collection of sufficient counts in the spectmm, the y-ray energies can stih be deterrnined to about 0.5 keV. If the configuration of the sample is not one for which the detector efficiency has been direcdy measured, however, the uncertainty in the y-ray emission rate may increase to 5 or 10%. 

By number count the great majority of particles in the outside air are likely to be less than 1 pm. By weight, these small particles will account for a very small proportion of the sample. A filter with a high efficiency measured by weight of particles trapped may be almost transparent to the small ones. Very high counts can be found in rural areas from pollen or agricultural activities. 

To determine the ftq, value of Hg a solid sample is used, in which some of the iodine is present as radioactive 1-131. The count rate of the sample is 5.0 X 1011 counts per minute per mole of L An excess amount of Hg2I2(s) is placed in some water, and the solid is allowed to come to equilibrium with its respective ions. A 150.0-mL sample of the saturated solution is withdrawn and the radioactivity measured at 33 counts per minute. From this information, calculate the ft, value for Hg2l2. 

Fig. 8-10. Contour maps showing spectrograph sensitivities for the iron Ka line for various positions of the sample, (a) At surface of sample holder, (b) 0.16 inch below surface of sample holder, (c) 0.32 inch below surface of sample holder. The sensitivity changes with the x-ray optical system, with the goniometer setting, and with the distance of the sample below the surface of the sample holder. The contour interval is 20 counts per second. (Authors unpublished results.)...

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... 

How can we determine the amount of substance present if we can t count the atoms directly We can find the amount if we know the mass of the sample and the molar mass, M, the mass per mole of particles ... 

Molar mass is important when we need to know the number of atoms in a sample. It would be impossible to count out 6 X ID23 atoms of an element, but it is very easy to measure out a mass equal to the molar mass of the element in grams. Each of the samples shown in Fig. E.2 was obtained in this way each sample contains the same number of atoms of the element (6.022 X 1023), but the masses vary because the masses of the atoms are different (Fig. E.4). The same rule applies to compounds. Flence, if we measure out 58.44 g of sodium chloride, we obtain a sample that contains 1.000 mol NaCl formula units (Fig. E.5). 

In the technique developed by Willard Libby in Chicago in the late 1940s, the proportion of carbon-14 in a sample is determined by monitoring the (1 radiation from C02 obtained by burning the sample. This procedure is illustrated in Example 17.4. In the modern version of the technique, which requires only a few milligrams of sample, the carbon atoms are converted into C ions by bombardment of the sample with cesium atoms. The C ions are then accelerated with electric fields, and the carbon isotopes are separated and counted with a mass spectrometer (Fig. 17.19). 

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