Counting cyclotron

In anticipation of the development to operational status of the ion or direct counting systems, it would be helpful if we could compare these values with projected counting errors for the two types of direct counting systems being developed. Table 4 lists projections for the Rochester Van de Graaff facility [49] and the University of California Lawrence Berkeley cyclotron system employing an external ion source [31,50]. Table 4 also lists the sample sizes and approximate measurement periods for both systems. This data illustrates the potential extention in dating... 

S. D.-H. Shi, C. L. Hendrickson, and A. G. Marshall. Counting Individual Sulfur Atoms in a Protein by Ultrahighresolution Fourier Transform Ion Cyclotron Resonance Mass Spectrometry Experimental Resolution of Isotopic Fine Structure in Proteins. Proc. Natl. Acad. Sci. U.S.A., 95(1998) 11532-11537. 

The range of applicability of equation 11.122 depends on the limits of detection of in the sample. The current maximum age attained by direct radioactivity counting is about 4 X 10" a. To measure residual radioactivity, the total carbon in the sample is usually converted to CO2 and counted in the gas phase, either as purified CO2 or after further conversion to C2H2 or CH4. To enhance the amount of counted carbon, with the same detection limit (about 0.1 dpm/g), counters attain volumes of several liters and operate at several bars. More recent methods of direct detection (selective laser excitation Van de Graaif or cyclotron acceleration) has practically doubled the range of determinable ages (Muller, 1979). 

A different method became available with modern meson factories, where the characteristic X-radiation from exotic atoms can be studied under optimized conditions and with reasonable count rates. Such experiments require the use of high-intensity external beam lines together with a particle concentrator like the cyclotron trap and a high-resolution low-energy crystal spectrometer. 

In practice, one equidistantly discretizes (digitizes) the continuous (analog) time variable t as t = tn = nAt = nr (n = 0,1,2,..., N — 1), where the nonnegative integer n counts the time. The quantity r is the time increment (the time lag) or the sampling time, which is also called the dwell time r = T/N in ion cyclotron resonance mass spectroscopy (ICRMS) [44], Hereafter, we shall write ... 

Shi, S.D.H., Hendrickson, C.L. and Marshall, A.G. (1998) Counting individual sulfur atoms in a protein by ultrahigh-resolution Fourier transform ion cyclotron resonance mass spectrometry experimental resolution of isotopic fine structure in proteins. Proc. Natl. 

Experiments performed to date with cyclotrons have used positive ions obtained from carbon dioxide and a gas ion source. This is an advantage in the sense that it permits standard pretreatment practices developed over the last 30 years at decay counting laboratories to be routinely employed up to the point of measurement (29). On the other hand, beam currents using gas ion sources are characteristically significantly less than those from the solid samples currently used with electrostatic type accelerators. In addition, memory effects, which make comparisons of a standard to an unknown difficult, have been reported. The first cyclotrons used for radioisotope measurements had previously been used extensively for nuclear physics experiments and the production of high energy ions. Because of these experiments, some cyclotron systems have apparently been contaminated. For the 88-in. Berkeley cyclotron, the construction of an external ion source was designed to attempt to overcome this problem. Unfortunately, the efficiency of the beam transport system in the external ion source introduced other problems (30). 

A (estimated to contain 0.25 micrograms of 94 ) was placed near the screened window of the ionization chamber embedded in paraffin near the beryllium target of the 37-inch cyclotron. The neutrons produced by the irradiation of the beryllium target with 8 MeV deuterons give a fission rate of 1 count per minute per microampere. When the ionization chamber is surrounded by a cadmium shield, the fission rate drops to essentially zero. 

The bottom pole piece of the Harvard cyclotron had been laid on April 14 by the first week in June Robert Wilson s cyclotron group saw signs of a beam. The Wisconsin long-tank Van de Graaif came on line at 4 million volts on May 15 and the 2 MV short-tank Van de Graaif on June 10. In July the first physics experiment completed at Los Alamos counted the number of secondary neutrons Pu239 emitted when it fissioned. In this experiment, says the Los Alamos technical history, the neutron number was measured from an almost invisible speck of plutonium and found to be somewhat greater even than for The experiment thus established... 

The spheres for the first work described below were made of UsOs powder tamped into thin Cu shells which in turn were covered with cadmium to remove thermal neutrons. They were imbedded in a large graphite block 81 by 81 by 107 cm which was bombarded with neutrons from the reaction LF(, w)Be produced by 8-Mev protons ia the Princeton cyclotron. The resonance absorption was measured by sampling the material in the sphere, removing fission products and UX by the method described by Anderson, and counting the induced 24-min beta activity. This method suffered from certain inaccuracies. A second improved method was therefore used later. Both methods and the results obtained with them are described below. 

After bombardment, the hot central-sphere block was removed and the uranium oxide from one hemisphere was taken out and broken lip in a mortar, dissolved in HNO3, and the ether purification made, as described by Anderson, after which a thick-dish sample was prepared for counting, as in previous work. The cyclotron beam was monitored from run-to-run with (a), two Pbl2 samples placed at 10 cm in the block (at room temperature), (b), with a Pbl2 sample placed on the cyclotron outside the block, and (c), with a thin... 

To apply CPAA one needs access to a cyclotron, a radiochemical laboratory, and y-spectrometry and jS" "-counting equipment. The time needed for one analysis is determined by the half-life of the radionuclide induced and whether a radiochemical separation is necessary or not. The inherent complexity and costs are the major drawbacks of CPAA. 

An important property of the MOT is the ability to catch atoms whose optical frequencies are shifted from the laser frequency by only a few natural linewidths. This property has been applied for ultrasensitive isotope trace analysis. Chen et al. (1999) developed the technique in order to detect a counted number of atoms of the radioactive isotopes Kr and Kr, with abundances 10 and 10 relative to the stable isotope Kr. The technique was called atom trap trace analysis (ATTA). At present, only the technique of accelerator mass spectrometry (AMS) has a detection sensitivity comparable to that of ATTA. Unlike the AMS technique based on a high-power cyclotron, the ATTA technique is much simpler and does not require a special operational environment. In the experiments by Chen et al. (1999), krypton gas was injected into a DC discharge volume, where the atoms were excited to a metastable level. 2D transverse laser cooling was used to collimate the atomic beam, and the Zee-man slowing technique was used to load the atoms into the MOT. With the specific laser frequency chosen for trapping the Kr or Kr isotope, only the chosen isotope could be trapped by the MOT. The experiment was able to detect a single trapped atom of an isotope, which remained in the MOT for about a second. 

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