Radioactive samples

Radioactive samples can be analyzed by emission spectroscopy, although all the precautions usually associated with radioactive samples must be followed. Fortunately, sample sizes for spectrographic analysis are small, so total radioactivity in the laboratory should be small. 

Of particular concern is the safe removal of the vaporized products of excitation. An enclosed arc-spark stand is mandatory, as is an efficient exhaust system. The exhaust system should be constructed so air can be removed but the particulate matter retained in a filter or some other collection device. 

Sample preparation, prior to spectrographic analysis, should be done under approved safety conditions in a room separate from the spectrographic laboratory. Laboratories and personnel should be monitored on a regular basis to ensure that radioactivity remains below a safe level at all times. With 

One of the more interesting applications of gas analysis was that developed by Evens and FasseE to determine oxygen in niobium. An argon atmosphere is used with dc arc excitation and concentrations of oxygen in the range of 0.004-0.60 wt % can be determined by their method. 

Seeley and Skogerboe have described a combined sampling-analysis technique for the determination of trace elements in particulate matter in the atmosphere. Porous cup graphite spectroscopic electrodes are used as filters to collect the particulates and then form the sample electrode for emission spectroscopic determination of element concentrations. 

---

Radioactive samples requite other, special techniques. Some are discussed in Reference 22 (see Radioactive tracers). 

Assay of Radioactive Compounds. The radioactive samples were counted on steel planchets in a Nuclear Chicago Model D-47 low-background gas-flow counting chamber with an absolute counting efficiency (estimated by comparison with a standard) of about 20%. 

In this context, k is called the decay constant. The law tells us that the activity of a radioactive sample is proportional to the number of atoms in the sample. As we saw in Section 13.4, a first-order rate law implies an exponential decay. It follows that the number N of nuclei remaining after a time t is given by... 

Predict the amount of a radioactive sample that will remain after a given time period, given the decay constant or half-life of the sample (Example 17.3). 

A radioactive sample contains 3.25 X 1018 atoms of a nuclide that decays at a rate of 3.4 X 1013 disintegrations per 15 min. (a) What percentage of the nuclide will have decayed after 150 d (b) How many atoms of the nuclide will remain in the sample (c) What is the half-life of the nuclide ... 

Most Mossbauer experiments are currently performed with commercially available radioactive sources. For some applications, however, a so-called source experiment may be useful, in which the sample is labeled with the radioactive parent-isotope of the Mossbauer nucleus such as Co. The y-radiation of the radioactive sample is then analyzed by moving a single-line absorber for Doppler modulation in front of the detector. 

The first nuclear microbeam with a spatial resolution of 1 pm was built by Watt et al. (1981), and the first sub-micron instrument was built by Grime et al. (1987). Khodja et al. (2001) have published a description of the nuclear microprobe at the Pierre Sue Laboratory in France, which is a national facility dedicated to microbeam analysis. Its unique facility is that it is capable of analysing radioactive samples by means of a dedicated beamline. Figure 4.1 shows a schematic diagram of the apparatus. 

EXAMPLE 22.6. Calculate the time required for a radioactive sample to lose one-third of the atoms of its parent isotope. The half-life is 33 min. 

A locally resolved detection of radioactive samples after chromatographic separation can be performed by imaging techniques which work either indirectly with Eu3+- or P-doped sensor plates and laser activated emission or directly by a micro channel array detector which works like an open Geiger-Muller counter. 

In many instances non-linear functions can be linearised and in this way a non-linear, iterative fitting procedure can be reduced to an explicit linear fit. A typical example is the exponential decay of the intensity of the emission of a radioactive sample. We use the data already used for Figure 4-4, produced by the function Data Decay. m. 

Actinide metal samples are characterized by chemical and structure analysis. Multielement analysis by spark source mass spectrometry (SSMS) or inductively coupled argon plasma (ICAP) emission spectroscopy have lowered the detection limit for metallic impurities by 10 within the last two decades. The analysis of O, N, H by vacuum fusion requires large sample, but does not distinguish between bulk and surface of the material. Advanced techniques for surface analysis are being adapted for investigation of radioactive samples (Fig. 11) ... 

The adaptation for radioactive samples described in Fig. If has been performed by J. Naegele basically for the investigation of surface and bulk properties of actinide sohds such as discussed in Chap. E. In that chapter, a more detailed description for the characteristics of the system is given... 

Half-life The time in which half the atoms in a radioactive sample undergo decay. 

The leach rates seen here can be compared to those from a similar test on simulated waste glass of the same composition. The leach rates for cesium and strontium from the fully radioactive Rlass were the same as the leach rates of the simulated glass. Although both tests were done on bulk glass samples, they differed in configuration. The fully radioactive samples were disks cm (height) x 3.5 cm (diameter)], and the... 

Form of Reported Data. The mass of material leached can be determined either by any standard gravimetric method or, in the case of radioactive samples, by measurement of the radioactivity dissolved in the leaching medium. Since different ions are often leached at different rates, it is important to specify the ion when quoting leach rate results. To avoid confusion on this matter, it has been suggested( ) that long-term leaching results be reported as (fraction of A leached) (cm /g) day) , where A is the specific ion analyzed for. 

Carter, J. A., Sites, J. R., Analysis of Radioactive Samples by Spark Source... 

What is meant by the half-life of a radioactive sample ... 

Thus all probability ends up in the state N = 0, which is therefore called an absorbing state. All other states (N 1) are depleted in the course of time they are called transient states. They can only occur because the decay products disappear into an infinitely large universe. For finite physical systems transient states are excluded, see V.5. If our radioactive sample were enclosed in an impermeable container there would be a non-zero probability for the emitted particles to be reabsorbed. Such a... 

A radioactive sample shows the following counts for one-minute intervals 2642 2650 2649 2641 2641 2637 2651 2636. Find the average deviation, the standard deviation, and the 90% confidence interval for a single value and for the mean. 

Alpha particles from radioactive samples, or He2+ ons accc cralc(l n a cyclotron, may be used to bring about other nuclear reactions. For example, they may bombard a beryllium metal target ... 

Radioactive decay with emission of particles is a random process. It is impossible to predict with certainty when a radioactive event will occur. Therefore, a series of measurements made on a radioactive sample will result in a series of different count rates, but they will be centered around an average or mean value of counts per minute. Table 1.1 contains such a series of count rates obtained with a scintillation counter on a single radioactive sample. A similar table could be prepared for other biochemical measurements, including the rate of an enzyme-catalyzed reaction or the protein concentration of a solution as determined by the Bradford method. The arithmetic average or mean of the numbers is calculated by totaling all the experimental values observed for a sample (the counting rates, the velocity of the reaction, or protein concentration) and dividing the total by the number of times the measurement was made. The mean is defined by Equation 1.1. 

The mean counting rate for the data in Table 1.1 is 1222. If the same radioactive sample were again counted for a series of ten observations, that series of counts would most likely be different from those listed in the table, and a different mean would be obtained. If we were able to make an infinite number of counts on the radioactive sample, then a true mean could be calculated. The true mean would be the actual amount of radioactivity in the sample. Although it would be desirable, it is not possible experimentally to measure the true mean. Therefore, it is necessary to use the average of the... 

The Observed Counts and Sample Deviation from a Typical Radioactive Sample... 

The number of disintegrations emitted by a radioactive sample depends on the purity of the sample (number of radioactive atoms present) and the decay constant, A. Therefore, radioactive decay is also expressed in terms of specific activity, the disintegration rate per unit mass of radioactive atoms. Typical units for specific activity are mCi/mmole and /r,Ci//rmole. 

Color quenching is a problem if chemical substances that absorb photons from the secondary fluors are present in the scintillation mixture. Since the secondary fluors emit light in the visible region between 410 and 420 nm, colored substances may absorb the emitted light before it is detected by the photocells. Radioactive samples may be treated to remove colored impurities before mixing with the scintillation solvent. 

Point quenching occurs if the radioactive sample is not completely dissolved in the solvent. The emitted j8 particles may be absorbed before they have a chance to interact with solvent molecules. The addition of solubilizing agents such as Cab-O-Sil or Thixin decreases point quenching by converting the liquid scintillator to a gel. 

---