Helium isotope separation

Different combinations of stable xenon isotopes have been sealed into each of the fuel elements in fission reactors as tags so that should one of the elements later develop a leak, it could be identified by analyzing the xenon isotope pattern in the reactor s cover gas (4). Historically, the sensitive helium mass spectrometer devices for leak detection were developed as a cmcial part of building the gas-diffusion plant for uranium isotope separation at Oak Ridge, Tennessee (129), and heHum leak detection equipment is stiU an essential tool ia auclear technology (see Diffusion separation methods). 

While the considerations presented above apply to most simple liquids (we exclude polymeric liquids, glasses, liquid crystals, etc.), liquid helium needs a separate discussion. Two helium isotopes are known, He and He. Upon liquefaction they form liquids with very unusual properties. They remain liquid under their own... 

Aije, J., Valli, K. Helium-jet ion guide fm an on-line isotope separator. Nucl. Instrum. Methods 179, 533-539 (1981)... 

Hauser, A. W. Schrier, J. Schwerdtfeger, P. Helium tunneling through nitrogen-functionalized graphene pores Pressure- and temperature-driven approaches to isotope separation. J Phys Chem C 2012, 116, 10819-10827. 

Helium, the second element in the periodic table, has atomic number 2. This means its nucleus contains two protons and has a 2+ charge. The neutral atom, then, contains two electrons. There are two stable isotopes, helium-4 and helium-3, but the helium found in nature is almost pure helium-4. Helium is found in certain natural gas fields and is separated as a by-product. Sources of helium are rare and most of the world supply is produced in the United States, mainly in Texas and Kansas. 

As a nuclear reaction, the s process is relatively well understood, but the problem lies in identifying an astrophysical site for it and determining the relevant physical parameters, such as neutron flux, mean time separating two neutron captures, and temperature. It has been shown that the most propitious temperatures are those of helium fusion. Added to the fact that the surfaces of certain red giants are rich in s isotopes, such as radioactive technetium and barium, this observation confirms the idea that the s process may be related to helium fusion regions in stars. 

Helium-4 Normal-Superfluid Transition Liquid helium has some unique and interesting properties, including a transition into a phase described as a superfluid. Unlike most materials where the isotopic nature of the atoms has little influence on the phase behavior, 4He and 3He have a very different phase behavior at low temperatures, and so we will consider them separately Figure 13.11 shows the phase diagram for 4He at low temperatures. The normal liquid phase of 4He is called liquid I. Line ab is the vapor pressure line along which (gas + liquid I) equilibrium is maintained, and the (liquid + gas) phase transition is first order. Point a is the critical point of 4He at T= 5.20 K and p — 0.229 MPa. At this point, the (liquid + gas) transition has become continuous. Line be represents the transition between normal liquid (liquid I) and a superfluid phase referred to as liquid II. Along this line the transition... 

In a study of the long-lived protactinium isotopes produced from thorium bombarded by high energy deuterons or helium ions (175), pieces of thorium metal of 25 mil thickness were used to increase the total yield of the protactinium. On the other hand, when the time for chemical separations had to be shortened in order to study the short-lived protactinium Isotopes, thinner pieces of thorium, 5 mils or less in thickness, were used to ensure rapid dissolution. In some cases, thorium nitrate powders wrapped in aluminum were used as the target in order to reduce further the time for dissolution of the target. 

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