Recoil rooms

The early work of Sutin and Dodson (85) on neutron-irradiated ferrocene exemplifies the results and problems of recoil chemistry. After dissolving their samples in hexane and extracting with aqueous solutions they isolated, after further purification, radioactive FeCp2 and a species which emerged as ionic iron(III). Adsorbed on the walls of the glass vessels remained another species soluble in acetone which accounted for up to 50-60% of the radioactive iron. This species has not yet been identified. The FeCp2 activity accounted for some 10-12% of the Fe, which increased on standing several weeks at room temperature or 2-3 days at 110° C, as is shown in Table III. 

The example is typical for many applications of Mossbauer spectroscopy in catalysis a catalyst undergoes a certain treatment, then its Mossbauer spectrum is measured in situ at room temperature. Flowever, if the catalyst contains highly dispersed particles, the measurement of spectra at cryogenic temperatures becomes advantageous as the recoil-free fraction of surface atoms increases substantially at temperatures below 300 K. Secondly, spectra of small particles that behave superparamagne-... 

The high atomic mass yields small recoil energies and thus a large recoU-free fraction even at room temperature (/300 0.95 for Ta metal [175]) this allows measurements of the resonance effect to be made over a wide temperature range up to about 2,300 K. 

The paper summerizes the experimental data on the equilibrium factor, F, the free fraction, fp, the attachment rate to the room air aerosol, X, the recoil factor,, and the plateout rates of the free, qf, and the attached, q3, radon daughters, determined in eight rooms of different houses. In each room several measurements were carried out at different times, with different aerosol sources (cigarette smoke, stove heating etc.) and under low (v<0.3 It1) and moderate (0.3[Pg.288]

The concentration of radon decay products and therefore the factors F and fp are influenced by the basic processes of the attachment, recoil and deposition (plateout) and by room specific parameters of radon emanation and ventilation (Fig. 1). ... 

This paper will summerize our experimental data on the equilibrium factor (F), the free fraction (f ), the attachment rate to the room air aerosol (X), the recoil factor r and the plateout... 

Fig. 14 shows the comparison of the photographs from Chandra and Avedisian (1991) with simulated images of this study for a subcooled 1.5 mm n-heptane droplet impact onto a stainless-steel surface of 200 °C. The impact velocity is 93 cm/s, which gives a Weber number of 43 and a Reynolds number of 2300. The initial temperature of the droplet is room temperature (20 °C). In Fig. 14, it can be seen that the evolution of droplet shapes are well simulated by the computation. In the first 2.5 ms of the impact (frames 1-2), the droplet spreads out right after the impact, and a disk-like shape liquid film is formed on the surface. After the droplet reaches the maximum diameter at about 2.1ms, the liquid film starts to retreat back to its center (frame 2 and 3) due to the surface-tension force induced from the periphery of the droplet. Beyond 6.0 ms, the droplet continues to recoil and forms an upward flow in the center of the... 

Thus making samples not too thick helps in getting sharper spectra and facilitates the quantitative interpretation. Finally, particularly in the Mossbauer spectra of small catalyst particles, one should be aware of the temperature dependence of the absorption area through the recoil-free fraction. If the spectrum contains contributions from surface and bulk phases, the intensity of the former will be greatly underestimated if the spectrum is measured at room temperature. The only way to obtain reliable concentrations of surface and bulk phases is to determine their spectral contributions as a function of temperature and make an extrapolation to zero Kelvin [13]. 

Surface phases have low Debye temperatures. As a result, the recoil-free fraction may be low at room temperature (see Fig. 5.2). Thus, measuring at cryogenic temperatures will increase the Mossbauer intensity of such samples considerably. But there can also be other circumstances which call for low temperature experiments. 

Let us now discuss some recent work by Sano and myself on completely characterizing barium stannate, a material first proposed by Plotnikova, Mitrofanov, and Shpinel (21), as a source for tin Mossbauer spectroscopy. It is easily prepared, is a stoichiometric compound and has all the properties one desires in a Mossbauer matrix. The recoil-free fraction at room temperature is about 0.55 with about a 10% error. The line width extrapolated to zero absorber thickness is about 6% larger than natural—i.e., the line width observed is ca. 0.318 mm./sec. at zero ab-... 

FIGURE 7. Laser induced fluorescence (LIF) spectrum of the CN(X2e+) fragment produced in the ArF laser photolysis of BrCN. Note that the recoil fragment energy is such that the recoil velocities of nascent CN produced with Br(2P3/ ) or Br(2p-]/2) fragments are 3.3 x 10 and 2.0 x 10 cm/s, respectively. The collision frequency will be up to a factor of six higher than it is at room temperature. 

In order to avoid recoil effects all samples must be solid but, since measurements are made at 4.2 K, samples which are fluid at room temperature can be measured without difficulty. The amounts required are relatively small, typically 20-30 mg for 127I and 2-20 mg for 129I these amounts refer to the mass of iodine contained in the sample. Samples are usually pressed into discs or mulled with grease, and can often be recovered unharmed. In the case of 129I this is essential, as the expensive isotope must be recycled. 

Direct searches for superheavy elements in the U+ U reaction were undertaken at the unilac by several groups. All these efforts remained without positive evidence. The data are summarized in Figure 13. The curve labeled chem [106] was obtained with off-line chemical separations [107] and an assay for a-and spontaneous fission activities here, the 10 picobam level was reached for half-lives between several days and years. Attempts to detect short-lived nuclides were less sensitive. The curve labeled gas holds for an on-line search [108] for components volatile at room temperature. wheel [106] refers to fission track detection in the unseparated product mixture deposited on a rotating catcher, rec [109] to implantation of recoil atoms in a surface barrier detector, and JET to on-line transport from target to detector with a gas jet [91,110],... 

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