Gamma-rays emission

Neutron Activation Analysis Few samples of interest are naturally radioactive. For many elements, however, radioactivity may be induced by irradiating the sample with neutrons in a process called neutron activation analysis (NAA). The radioactive element formed by neutron activation decays to a stable isotope by emitting gamma rays and, if necessary, other nuclear particles. The rate of gamma-ray emission is proportional to the analyte s initial concentration in the sample. For example, when a sample containing nonradioactive 13AI is placed in a nuclear reactor and irradiated with neutrons, the following nuclear reaction results. 

When irradiation is complete, the sample is removed from the nuclear reactor, allowed to cool while any short-lived interferences that might be present decay to the background, and the rate of gamma-ray emission is measured. 

As noted earlier, gamma-ray emission is measured following a cooling period in which short-lived interferents are allowed to decay away. The initial activity therefore, is determined by extrapolating a curve of activity versus time back to f = 0 (Figure 13.15). Alternatively, if the samples and standards are irradiated simultaneously, and the activities are measured at the same time, then these activities may be used in place of Aq) and (Ao)s in the preceding equations. 

Short-lived gamma-ray Emission from impurities... 

Plot of gamma-ray emission as a function of time showing how the analyte s initial activity is determined experimentally. 

The concentration of Mn in steel can be determined by a neutron activation analysis using the method of external standards. A 1.000-g sample of an unknown steel sample and a 0.950-g sample of a standard steel known to contain 0.463% w/w Mn, are irradiated with neutrons in a nuclear reactor for 10 h. After a 40-min cooling period, the activities for gamma-ray emission were found to be 2542 cpm (counts per minute) for the unknown and 1984 cpm for the standard. What is the %w/w Mn in the unknown steel sample ... 

Radioactive transformations are accomplished by several different mechanisms, most importantly alpha ptirticle, beta particle, tuid gamma ray emissions, Each of tliese mechanisms are sponuuieous nuclear transformations. The result of these transformations is tlie formation of different tuid more stable elements. 

Several different mechanisms, most importtuitly alplia particle, beta particle, and gamma ray emissions accomplish radioactive transformations. Each of these mechanisms is a spontaneous nuclear transformation. Tlie result of tliese transformations is the formation of different stable elements. Tlie kind of transformation that will ttike place for any given radioactive element is a function of the type of nuclear inslabilitv as well as the mass/energy relationship. The nuclear instability is dependent on the ratio of neutrons to... 

Particle-induced gamma-ray emission (PIGE) in which nuclei are excited and gamma-rays are generated by the ion beam. 

PIGE proton-induced gamma-ray emission (or PIGME)... 

At present, most PET scanners can acquire in both a two-dimensional as well as a three-dimensional mode, whereas SPECT cameras measure in a three-dimensional mode. The physical property of the dual-positron gamma-rays emission lends itself to mathematical reconstruction algorithms to produce three-dimensional images in which the calculations are much closer to exact theoretical ones than those of SPECT. This is, in part, due to the two-photon as opposed to single-photon approach. PET can now achieve resolutions, for example in animal-dedicated scanners, in the order of 1 or 2 mm. The resolution is inherently limited theoretically only by the mean free path or distance in which the positron travels before it annihilates with an electron, e.g. those in biological water 2-8 mm. SPECT, although achieving millimeter resolution with the appropriate instrumentation, cannot quite achieve these levels. 

Gamma-ray anisotropy or nuclear orientation thermometry OOl-l Spatial distribution of gamma-ray emission Spatial distribution related to Boltzmann factor for nuclear spin states Useful standard forT < IK... 

Carbon - The aerosol collected on the Tissuequartz filters was analyzed for total carbon by the nondestructive technique of proton-induced gamma-ray emissions (17) and graphitic carbon (soot) by calibrated optical reflectance at Washington University, St. Louis, Missouri. The estimated error in measurement of total carbon per filter was 5 yg or 20%, whichever was greater. The estimated error for soot was lyg or 20%, whichever was greater. 

PIGE particle-induced gamma-ray emission spectroscopy... 

If the trapped gas is radioactive with a high gamma-ray emission, such as krypton-79, which has a half-life of 34 hrs, the clathrate may then... 

Beta-Minus Decay 370 Positron Decay 3 72 Electron Capture Decay 3 72 Gamma Ray Emission 373... 

Figure 5.3. Left. The gamma-ray emission from XX annihilation in a rich, Coma-like, nearby galaxy cluster is shown Mx = 70 — 500 GeV (from top down). The integral flux is compared to the sensitivity of ongoing and planned gamma-ray experiments, as labelled. Right. The diffuse synchrotron emission spectrum of secondary electrons produced in XX annihilation is shown to fit the Coma radio-halo spectrum the green area represent the prediction of a model in which the x annihilates predominantly into fermions, while the blue area represent the gauge-boson dominated x annihilation (from Colafrancesco Mele 2001).

A similar mechanism of gamma-ray emission can be also provided by the annihilation of Dark Matter particles (see Colaffancesco at this Meeting) which produce both direct gamma-ray emission and secondarily produced gamma-rays through 7r° — 7 I 7, as above. 

There is not yet, however, a definite detection of diffuse gamma-ray emission from galaxy clusters. While there is a preliminary evidence of gamma-ray emission from a dozen bright, radio-active clusters which host powerful radio galaxies and Blazars and are associated to unidentified EGRET sources (Co-lafrancesco 2002), many of the quiet, X-ray selected clusters only have upper limits for their emission at E > 100 MeV. 

---