Passive Shielding

One of the most fundamental techniques for background rejection is the use of passive high-Z materials (usually lead) to shield the central detector elements from 

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Ambient radioactivity originating from the surroundings is suppressed as much as possible by multiple passive shielding layers. 

The detector was set up with Nal(Tl) anticoincidence, heavy passive shielding of lead and old iron to absorb external gammas, and borated paraffin to absorb cosmic... 

Figure 13.22 The initial system - standard low background germanium detector in an anticoincidence shield and a heavy multilayered passive shield (reproduced by permission of R.L. Brodzinski)...

Use 100 mm of lead in the passive shield, with, if possible, a layer of aged lead on the inside, especially if lower energies are of interest. 

The qualitative thermodynamic explanation of the shielding effect produced by the bound neutral water-soluble polymers was summarized by Andrade et al. [2] who studied the interaction of blood with polyethylene oxide (PEO) attached to the surfaces of solids. According to their concept, one possible component of the passivity may be the low interfacial free energy (ysl) of water-soluble polymers and their gels. As estimated by Matsunaga and Ikada [3], it is 3.7 and 3.1 mJ/m2 for cellulose and polyvinylalcohol whereas 52.6 and 41.9 mJ/m2 for polyethylene and Nylon 11, respectively. Ikada et al. [4] also found that adsorption of serum albumin increases dramatically with the increase of interfacial free energy of the polymer contacting the protein solution. 

Shielded polyplexes with improved blood circulating properties are interesting tools for systemic cancer therapy (see Sect. 4.2). Nanoparticles can take advantage of the enhanced permeability and retention (EPR effect) [89] for passive tumor targeting. The EPR effect is based on the leakiness of tumor vasculature, due to neovascularization in growing tumors, combined with an inadequate lymphatic drainage. Nanoparticles with an elongated plasma circulation time can extravasate and passively accumulate at the tumor site. 

Passive samplers are used for specific applications such as for indoor air environments or as passive dosimeters. In this approach, the air containing the organic diffuses to and adsorbs on a solid sorbent without active pumping. The organics are subsequently thermally desorbed or extracted from the sorbent using a solvent (e.g., see Shields and Weschler, 1987). 

Shields, H. C and C. J. Weschler, Analysis of Ambient Concentrations of Organic Vapors with a Passive Sampler, JAPCA, 37, 1039-1045 (1987). 

Lead s durability (its chemical inertness) and malleability make it useful in the construction industry. The inertness of lead under normal conditions can be traced to the passivation of its surface by oxides, chlorides, and sulfates. Passivated lead containers can be used for transporting hot concentrated sulfuric acid but not nitric acid, because lead nitrate is soluble. Another important property of lead is its high density, which makes it useful as a radiation shield because its numerous electrons absorb high-energy radiation. The main use of lead today is for the electrodes of rechargeable storage batteries (see Box 12.1). 

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