Decay under vacuum

Isotopes are also used to determine properties of the environment. Just as carbon-14 is used to date organic materials, geologists can determine the age of very old substances such as rocks by measuring the abundance in rocks of radioisotopes with longer half-lives. Uranium-238 (t1/2 = 4.5 Ga, 1 Ga = 10y years) and potassium-40 (t,/2 = 1.26 Ga) are used to date very old rocks. For example, potassium-40 decays by electron capture to form argon-40. The rock is placed under vacuum and crushed, and a mass spectrometer is used to measure the amount of argon gas that escapes. This technique was used to determine the age of rocks collected on the surface of the Moon they were found to be 3.5-4.0 billion years old, about the same age as the Earth. 

Wu et al. (5) recently interpreted the thermal decomposition mechanism of tetramethylammonium-exchanged Y zeolite. The order of occurrence of the gaseous decay products is (mole %) (CH N (50), CH4 (11), (CH3)20 (10), CO (9), CH3OH (6), H2 (4), C4H8 (4), C2H4 (trace), for the decomposition carried out at 275°C under vacuum. At this temperature, a displacement reaction of water nucleophile on the tetramethyl cation, forming methanol and trimethylamine, is proposed ... 

Figure 10.14 The decay of fluorine contamination with continued reactor use, involving multiple intermittent chamber evacuations and O2 and Ar plasma-treatments of new substrates other than this intermittent use and times when chamber-cleaning procedures were employed, the reactor is continually under vacuum, indicating that this contamination does gradually pump out.

The starch was irradiated under vacuum in borosilicate glass tubes, transferred to quartz tubes, and sealed under vacuum for scanning. Initial scanning was performed immediately after preparation and decay rate studies were made on these samples, allowing them to age at room temperature. 

Radical formation in the polyimides on y-radiolysis under vacuum has been investigated by ESR spectroscopy. At 77 K the radical spectra of the irradiated polyimides are composed principally of two partially overlapping singlets (8,9), one of which decays when the temperature is raised to 200 K. This component has been assigned to anion radicals formed at 77 K by trapping of thermalized electrons in the polymer matrix. The other singlet has been assigned to neutral radicals with the free electron delocalized over several units of the polymer chain. These radicals are thus similar to the radicals formed on UV photolysis. Other minor radical components are also sometimes evident in the spectra, for example in that of Ultem that has been irradiated at 77 K (9). 

Although it is well known that when polyvinylchloride (PVC) is irradiated with UV light, discoloration, dehydrochlorination, decomposition and crosslinking occur, few ESR studies on PVC irradiated with UV light have been reported. Nishijima et al. (86) observed an ESR spectrum after UV irradiation of a polyvinylchloride film with light of 185 nm at —196° C under vacuum. The sample was heated at various temperatures for 10 min after UV irradiation at —196° C. The changes in a spectral shape, radical concentration, A Hmsl and A H are shown in Figs. 9 and 10. It is apparent that a broad component decays out at about —100° C and a narrow component at about 80° C. The former could be attributed to some kinds of alkyl radicals and the latter to... 

Pressure decay systems normally operate to a specified pressure (usually within the pack) which is then monitored for pressure drop. Leaks down to 10 3 cm3/s can be detected. There is also a pressure increase method where leakage from a pack under vacuum can be detected as a positive pressure change (assuming an excellent vacuum chamber seal is achieved). 

Figure 2. Electron spin resonance spectra of decay in vacuum at room temperature (9.0 Mrads). (a) Primary radical R under vacuum, (b) radical transformation, (c) intermediate spectrum, (d) polyenyl radical.

Fig. 27. Solid-echo decays at 23°C of mori silk fibroin films and fiber without drying. The immersion times of the silk films in 80% methanol and temperature of the silk films under vacuum before NMR measurement are indicated.

Radiation modification of PC and PSF as plastic teeth was successfully attained by specific irradiation techniques, that is, the irradiation at elevated temperature of the Tg in inert gas atmosphere or under vacuum. The hardness and wear resistance of PC and PSF improved with a small dose of 3-5 kGy, whereas the decay of mechanical properties and coloring were scarce. By applying EB irradiation using the EB accelerator, the gradient of hardness can be formed in PC teeth. Namely, the surface area of teeth could be hardened selectively by the lower voltage of the EB accelerator. 

Fig. 21. Decay of extractable monomer [M] as a function of time. HDDA, photopolymerized in an ESR tube for 90 s at 20 "C and stored under vacuum at room temperature. Initiator 0.25 wt.-° HMPP. Intensity 0.2 mW cm". (From Ref. with permission)...

From the radioactive decay constants and measurement of the amount of argon in a rock sample, the length of time since formation of the rock can be estimated. Essentially, the dating method requires fusion of a rock sample under high vacuum to release the argon gas that has collected through radioactive decay of potassium. The amount of argon is determined mass spectrometrically,... 

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