Radiation background

A good powder photograph has sharp intense lines superimposed on a background of minimum intensity. However, the diffraction lines themselves vary in intensity, because of the structure of the crystal itself, and an appreciable background intensity may exist, due to a number of causes. The two effects together may cause the weakest diffraction line to be almost invisible in relation to the background. 

Diffraction of the continuous spectrum. Each crystal in a powder specimen forms a weak Laue pattern, because of the continuous radiation component of the incident beam. This is true whether or not that particular crystal has the correct orientation to reflect the characteristic component into the Debye ring. Many crystals in the specimen are therefore contributing only to the background of the photograph and not to the diffraction ring, and the totality of the Laue patterns from all the crystals is a continuous distribution of background radiation. If the incident radiation has been so chosen that very little fluorescent radiation is 

Diffraction and scattering from other than the specimen material. 

Our universe started with an event called the Big Bang. At the moment of its creation the universe was an infinitesimally small point, called a singularity, that contained all of the matter and radiation. The radiation in the singularity 

Very recently there have been two balloon-borne instruments, Maxima and Boomerang, that have measured smaller pieces of the sky but at higher spatial resolution. These experiments have concentrated on how the spatial variations of the background match to predictions made from various models of the universe. They are consistent with a universe that is flat rather than curved and imply that most of the matter in the universe must be in a currently unobservable form, which has the designation dark matter. 

The bottom of Fig. 4 shows another profound measurement by COBE. The far infrared absolute spectrophotometer (FIRAS) instrument on COBE measured the spectrum of the cosmic background radiation and found it to be an essentially perfect fit to a blackbody spectrum. The solid line in the figure is a theoretical perfect blackbody spectrum and the squares are the measurements. Any cosmological theory must be able to predict this remarkably perfect fit. These two measurements are among the most important observations ever made in infrared astronomy. 

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Both emission and absorption processes rely on the background radiation, which is present throughout the universe and which has a wavelength distribution characteristic of a black body and a temperature of about 2.7 K. This radiation is a consequence of the big bang with which the universe supposedly started its life. 

Most of the data on radiation health effects have come from medical monitoring of Japanese atomic bomb survivors. For survivors who received radiation exposures up to 0.10 Sv, the iacidence of cancer is no greater than ia the geaeral populatioa of Japanese citizens. For the approximately 1000 survivors who received the highest radiation doses, ie, >2 Sv, there have been 162 cases of cancer. About 70 cases would have been expected ia that populatioa from aatural causes. Of the approximately 76,000 survivors, as of 1995 there have beea a total of about 6,000 cases of cancer, only about 340 more cases than would be expected ia a group of 76,000 Japanese citizens who received only background radiation exposure (59). 

The choice of a detector for a specific appHcation should be made in order to minimize the cooling requirements and the magnitude of the background radiation noise therefore, in detector selection the cutoff wavelength should be only slightly greater than that required by the appHcation. If the... 

High power pulsed lasers are used to produce plasmas and thus to sample and excite the surfaces of soHds. Improvements in minimum detectable limits and decreases in background radiation and in interelement interference effects result from the use of two lasers (99) (see Surface and interface analysis). 

Background Radiation. If the radiation from a radioactive source is measured, the spectmm also includes contributions from the radiations from the surrounding environment. This includes radiations from the radioactivity in the materials in and around the detector, including the stmcture of the building or nearby earth. There is also cosmic radiation that comes from space and interacts with the earth and atmosphere to produce radiations that may enter the detector, and thus is observed. 

The production of characteristic X rays is determined by the cross sections discussed above, but the observed X-ray spectra include both these characteristic peaks and a continuous background radiation. A detailed investigation of the origin of... 

Figure 2 Experimental and calculated background radiation production cross sections for (a) 360 g/cm plastic foil and (b) 200 fig/cm Al foil. ...

Figura 3 Calculated detection limits for trace eiements in 1 mg/cm specimens of carbon, aiuminum, and caidum (100 xC of 3-MeV protons). The dashed curves represent the detection limits if the background radiation is due oniy to secondary eiectron bremsstrahiung.

Different analytical techniques are used for detection of the elemental composition of the solid samples. The simplest is direct detection of emission from the plasma of the ablated material formed above a sample surface. This technique is generally referred to as LIBS or LIPS (laser induced breakdown/plasma spectroscopy). Strong continuous background radiation from the hot plasma plume does not enable detection of atomic and ionic lines of specific elements during the first few hundred nanoseconds of plasma evolution. One can achieve a reasonable signal-to-noise ra-... 

R. W. Wilson, The cosmic microwave background radiation, pp. 113-33 in I.es Prix Nobel 1978. Almqvist Wiksell International, Stockholm 1979. A. A. Penzias, The origin of the elements, pp. 93-106 in Les Prix Nobel 1978 (also in Science 105, 549-54 (1979)). 

A. A. Penzias and R. W- Wilson (Holm-del) discovery of cosmic microwave background radiation. 

Multichannel instruments are capable of measuring the intensities of the emission lines of up to 60 elements simultaneously. To overcome the effects of possible non-specific background radiation, one or more additional wavelengths may be measured and background correction (see Section 21.12) can be achieved. 

In summary, pulse-height analysis (PHA) prior to a Mossbauer measurement is an essential step in tuning a Mossbauer spectrometer. PHA allows the adjustment of the y-detection system to the Mossbauer photons and the reduction of noise by rejecting nonresonant background radiation. 

There is always some level of ambient radioactivity in an environment. This radiation is known as background radiation. Measure and record the background activity by recording the counts registered by the gamma ray detector in 30-s intervals for 5 min. Record the data in Data Table 1. 

Place the sample near the detector. The precise location of the sample is not critical, provided that the detector is registering a good signal. However, the sample should not be moved once it has been placed in a satisfactory location. Record the number of counts registered by the detector in 30-s intervals in Data Table 2 until the count rate becomes indistinguishable from the background radiation count rate recorded in step 2. 

Convert the background radiation readings in Data Table 1 to a count rate in cpm by multiplying them by 2. 

Convert the readings in Data Table 2 to a count rate in cpm by multiplying them by 2. Subtract the background radiation rate obtained in step 2 from each count rate to obtain the corrected count rate. 

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