Cobalt monitor

The nuclear reaction Co (n,y) Co is the most frequently used monitoring reaction for thermal neutrons over longer exposure periods. The only stable cobalt isotope Co reacts with thermal neutrons to Co as well as directly to Co since ° Co decays almost completely (99.74%) to Co with a halflife of 10.2 min it can be ignored in the determination of neutron fluence data. Thus, in practice, °Co can be regarded as the only reaction product. Both y transitions of Co at 1.17 and 1.33 MeV show transition probabilities of 100% each. 

In most cases, as a fluence monitor small disks or wires of an Al-Co alloy with 0.1 to 1% Co are used by such a dilution of the monitor substance, self-shielding and flux depression effects are avoided. In general, since interfering radionuclides produced in the diluting element are small, chemical separations before activity measurement of Co are normally not needed. Frequently, the cobalt impurities in stainless steels, which are in the range of 100 to 1000 ppm, can be directly used as a fluence monitor in such cases, however, the accurate cobalt concentration in the material has to be additionally determined by chemical analysis. 

The thermal neutron cross section of Co is 3.73 10 cm, the newly formed °Co shows a corresponding value of 2.0- lO cm. These comparatively high values result in a bumup of both isotopes in the course of the irradiation, so that the number of Co atoms present at the end of irradiation (even when corrected 

The danger of errors in the experimental determination of the thermal neutron fluence caused by other nuclear reactions is small. Co is also produced in stainless steels by (n,p) reaction from Ni and by (n,a) reaction from Cu, with both reactions being induced by fast neutrons. Since the cross sections of both reactions are very small (5 10 cm and 4.2 10 cm respectively), no interference is to be expected in light water reactors when monitor materials of the usual purity are used. Due to the 5.28-year halflife of Co, details of the irradiation history of this monitor material can only be neglected at gross irradiation periods of up to one year for longer periods, interruptions in the operation history of the plant, such as shutdowns, load reductions etc., have to be corrected for. 

Due to its favorable properties, the nuclear reaction Fe (n,p) Mn is one of the standard reactions for the determination of fast neutron fluences. Since iron is the main constituent of most of the materials to be investigated, this determination can be carried out without the use of a specific fluence monitor. The halflife of Mn of 312 days permits the use of this nuclear reaction for irradiation periods of up to about two years, provided that corrections for details of the irradiation history are made. Tlie favorable energy of the Mn y ray of 0.835 MeV (transition probability 100%) frequently allows a direct activity measurement by use of a high-resolution Ge detector only in the investigation of materials with low iron contents such as Zircaloy, a chemical isolation of the monitor nuclide prior to the activity measurement is leconunended. This can be carried out, for example, by precipita- 

---

The thermal neutron flux to which a Zircaloy-2 specimen was exposed while in-pile was determined by measm ing and comparing the amount of the induced activities, Zr -Nb -, in the specimen and in a control sample of Zircalo,y-2. The latter was irradiated together with a cobalt monitor in a separate experiment in which the specimen and cobalt did not contact solution. Tor some experiments which contained steel specimens, similar measurements were made utilizing the activity. The fission power density in the uranium solution adjacent to a specimen was calculated from the resulting value for the thermal flux together with the value for the fission cross. section of uranium, assuming 200 Mev per fission. 

Recently, comparisons have been made between values for the flux indicated by cobalt monitors within the test system and by Zircaloy-2 within the same system. The flux values determined from the cob alt were about 25 less than those from Zircaloy-2. The cobalt values may be more valid, hut the consistency between Zircaloy-2 results has been good, and the Zircaloy-2 values are employed throughout in presenting the results [50]. 

Because the chemiluminescence intensity can be used to monitor the concentration of peroxyl radicals, factors that influence the rate of autooxidation can easily be measured. Included are the rate and activation energy of initiation, rates of chain transfer in cooxidations, the activities of catalysts such as cobalt salts, and the activities of inhibitors (128). 

Analysis of zinc solutions at the purification stage before electrolysis is critical and several metals present in low concentrations are monitored carefully. Methods vary from plant to plant but are highly specific and usually capable of detecting 0.1 ppm or less. Colorimetric process-control methods are used for cobalt, antimony, and germanium, turbidimetric methods for cadmium and copper. Alternatively, cadmium, cobalt, and copper are determined polarographicaHy, arsenic and antimony by a modified Gutzeit test, and nickel with a dimethylglyoxime spot test. 

Because process mixtures are complex, specialized detectors may substitute for separation efficiency. One specialized detector is the array amperometric detector, which allows selective detection of electrochemically active compounds.23 Electrochemical array detectors are discussed in greater detail in Chapter 5. Many pharmaceutical compounds are chiral, so a detector capable of determining optical purity would be extremely useful in monitoring synthetic reactions. A double-beam circular dichroism detector using a laser as the source was used for the selective detection of chiral cobalt compounds.24 The double-beam, single-source construction reduces the limitations of flicker noise. Chemiluminescence of an ozonized mixture was used as the principle for a sulfur-selective detector used to analyze pesticides, proteins, and blood thiols from rat plasma.25 Chemiluminescence using bis (2,4, 6-trichlorophenyl) oxalate was used for the selective detection of catalytically reduced nitrated polycyclic aromatic hydrocarbons from diesel exhaust.26... 

The flow-cell design was introduced by Stieg and Nieman [166] in 1978 for analytical uses of CL. Burguera and Townshend [167] used the CL emission produced by the oxidation of alkylamines by benzoyl peroxide to determine aliphatic secondary and tertiary amines in chloroform or acetone. They tested various coiled flow cells for monitoring the CL emission produced by the cobalt-catalyzed oxidation of luminol by hydrogen peroxide and the fluorescein-sensitized oxidation of sulfide by sodium hypochlorite [168], Rule and Seitz [169] reported one of the first applications of flow injection analysis (FTA) in the CL detection of peroxide with luminol in the presence of a copper ion catalyst. They... 

Although the rate of hydrolysis of nitroammine cobalt(III) complexes can be easily measured spectrally, it proved important to monitor for loss of NHj or NOj" groups in the early stages by using selective electrodes. The dominant loss of NH3 or NOf in the first stage depended on the complex. 

The second-order rate constants k for the base hydrolysis of a number of cobalt(lll) complexes were measured with a simple flow apparatus using conductivity as a monitoring device. Equal concentrations (Ag) of reactants were used. Show that a plot of R,/R — R, vs time is linear, having slope s, and that... 

Bencko V, Wagner V, Wagnerova M, et al. 1986. Human exposure to nickel and cobalt Biological monitoring and immunobiological response. Environ Res 40 399-410. 

In other imaging systems, ammonia or other amines are thermally released from the cobalt(III) complex upon reduction. This imagewise release can be monitored either as a pH change, causing a color-forming reaction, or by incorporating into the system a component such as o-phthalal-dehyde, which reacts with ammonia to form a black dye. 

Control Room. The cobalt control room was designed to enable the operator to observe directly the carrier loading area and the entrance to the cobalt cell and to view remotely the interior of the cell by use of television cameras (Figure 11). From the control room the operator can control the conveyor functions and the source elevator in addition to monitoring air flow, water activity, and pool water level. 

To calibrate the cobalt source, three systems are most often used ferrous sulfate, ferrous sulfate-cupric sulfate, and ceric sulfate. Dosimeters of these solutions are prepared by filling 5-ml. chemical-resistant glass ampoules with approximately 5 ml. of solution and flame-sealing the ampoules. The ampoules are then arranged in phantoms of Masonite or similar materials (Figure 13) to simulate the food items. These phantoms are placed in containers similar to those used for food products, and arranged in the conveyor carrier in which they are transported into the irradiation cell. Because of the upper dose limit of the ferrous sulfate and ferrous sulfate-cupric sulfate dosimeters (40,000 and 800,000 rads, respectively), these systems can be used only to establish the dose rate in the facility and not to monitor the total dose during food irradiation. The ceric dosimeter which... 

The major objective in validating a radiation sterilization process, regardless of whether the mode of radiation is cobalt-60, cesium-137, or electron beam, is to determine the D value of the indicator micro-organism used to monitor the process. With radiation sterilization, the D value is defined as the dose of radiation in Mrads or kilograys necessary to produce a 90% reduction in the number of indicator microbial cells. The D value depends on such factors as temperature, moisture, organism species, oxygen tension, and the chemical environment and/or phys-... 

A chromophore such as the quinone, ruthenium complex, C(,o. or viologen is covalently introduced at the terminal of the heme-propionate side chain(s) (94-97). For example, Hamachi et al. (98) appended Ru2+(bpy)3 (bpy = 2,2 -bipyridine) at one of the terminals of the heme-propionate (Fig. 26) and monitored the photoinduced electron transfer from the photoexcited ruthenium complex to the heme-iron in the protein. The reduction of the heme-iron was monitored by the formation of oxyferrous species under aerobic conditions, while the Ru(III) complex was reductively quenched by EDTA as a sacrificial reagent. In addition, when [Co(NH3)5Cl]2+ was added to the system instead of EDTA, the photoexcited ruthenium complex was oxidatively quenched by the cobalt complex, and then one electron is abstracted from the heme-iron(III) to reduce the ruthenium complex (99). As a result, the oxoferryl species was detected due to the deprotonation of the hydroxyiron(III)-porphyrin cation radical species. An extension of this work was the assembly of the Ru2+(bpy)3 complex with a catenane moiety including the cyclic bis(viologen)(100). In the supramolecular system, vectorial electron transfer was achieved with a long-lived charge separation species (f > 2 ms). 

---