Crystal Wrapping

It shoidd be noted that with tin low-noi.se PIN or APD readout we hav( developed, it is possil)le to actually measure the resj)onse of t in crystal to low energy photons, su(4i as the 511 keV and 1275 keV 7 lin/ s of Na sco Se ction 5.3.3. This is iiot po.s.sibh with the 500 keV of nois/ )( r crystal in th/ CLEO II calorimet/ r [8]. The ( ( ualization of light output for each crystal can thus be done using tin actual readout syst( m in situ. 

---

In a study conducted by the Armour Research Foundation (Ref 41) a-Pb azide crystals wrapped in a thin A1 foil were subjected to fast and thermal neutrons in the heavy water pile at Argonne National Laboratory. With a thermal flux rate of about 10l4n/cm2/sec the crystals were irradiated for 8, 17 and 170 hours. The crystals decompd to a brown powder which was identified as Pb carbonate by X-ray techniques and infrared absorption spectra. From a mass spectrographic analysis of the isotopes of carbon and oxygen in the decompn products, it was determined that the mechanism of carbonate formation is a reaction with the atmosphere by broken surface bands produced by the neutrons. Subsequently, Raney (Ref 60) reported... 

However, conductive elastomers have only ca <10 of the conductivity of soHd metals. Also, the contact resistance of elastomers changes with time when they are compressed. Therefore, elastomers are not used where significant currents must be carried or when low or stable resistance is required. Typical apphcations, which require a high density of contacts and easy disassembly for servicing, include connection between Hquid crystal display panels (see Liquid crystals) and between printed circuit boards in watches. Another type of elastomeric contact has a nonconducting silicone mbber core around which is wrapped metalized contacts that are separated from each other by insulating areas (25). A newer material has closely spaced strings of small spherical metal particles in contact, or fine soHd wires, which are oriented in the elastomer so that electrical conduction occurs only in the Z direction (26). 

Chemists were greatly surprised when soccer-ball-shaped carbon molecules were first identified in 1985, particularly because they might be even more abundant than graphite and diamond The C60 molecule (10) is named buckminsterfullerene after the American architect R. Buckminster Fuller, whose geodesic domes it resembles. Within 2 years, scientists had succeeded in making crystals of buckminsterfullerene the solid samples are called fullerite (Fig. 14.32). The discovery of this molecule and others with similar structures, such as C70, opened up the prospect of a whole new field of chemistry. For instance, the interior of a C60 molecule is big enough to hold an atom of another element, and chemists are now busily preparing a whole new periodic table of these shrink-wrapped atoms. 

The structure of SWCNTs is characterized by the concept of chirality, which essentially describes the way the graphene layer is wrapped and is represented by a pair of indices (n, m). The integers n and m denote the number of unit vectors (a a2) along the two directions in the hexagonal crystal lattice of graphene that result in the chiral vector C (Fig. 1.1) ... 

It is difficult to prepare stretched oriented fibres and such fibres may differ in their polymer chain conformation compared with the unstretched materials. Furthermore the quality of the single crystal X-ray data is poor and difficult to interpret. In contrast power X-ray data of relatively high quality may be obtained from polycrystalline polymer films. Lightfoot, Mehta and Bruce (1992) have obtained the first crystal structure of a polymer-salt complex, PEOjiNaClQ from powder X-ray data. Fig. 5.11(h). The structure is similar to the corresponding PEOjiNal structure, the PEO chains are wrapped around the Na ions with each Na ... 

This problem first emerged from the necessity to reconcile topological and structural data of nucleosomes and chromatin. As soon as a minichromosome could be reconstituted from pure DNA and histones, the total reduction of the DNA linking number (Lk) was found to be equal to the number of nucleosomes, which was also true for the native Hl-bearing SV40 minichromosome [12]. On the other hand, the first low-resolution crystal of the core particle showed that DNA was wrapped with 1 3/4 turns of a left-handed superhelix. Assuming linker DNAs... 

In conclusion, nucleosome cleavage periodicity data, together with direct measurements of a defined nucleosome crystal structure, suggest that wrapped DNA may be overtwisted to different extents, but in no case do these overtwistings appear sufficient to explain the paradox ( 0.5 bp/turn overtwisting required, against 0.2 bp/turn observed at the most). 

---