Neutron howitzers

Neutron Howitzer Irradiation. Whole-coin irradiation, however, has the advantage that the internal portions of the coin are irradiated and activated, thus permitting nondestructive analysis of the entire coin. One method we have devised is based on such irradiation but in a neutron beam that is about 100,000,000th as intense as that in a reactor. Only the silver in the coin is activated sufficiently for detection. None of the 255-day half-life isotope is detected, and the analysis is based on silver isotopes with half-lives of 24 sec (110Ag) and 2.4 min (108Ag). This silver activity dissipates in 10-15 min, and the coin is unharmed. We have analyzed over 4000 coins by this method which is described in Refs. 2, 3, 4. 

Since we do not weigh these streaks, we assumed that the Ag + Cu + Zn + As + Sb + Au = 100%. By comparing the radioactivity levels in the streaks with those in the standard alloy streaks (each corrected for decay to a common reference time) we can then calculate the percent ratios Cu/Ag, Au/Ag, Zn/Ag, Sb/Ag, and As/Ag. The combination of these ratio data and the above equation allows us to calculate the individual percent values. Various chemical analyses reported in the literature for silver coins and art objects indicate that other elements such as lead and tin (which we do not detect) are usually present at less than 2%. There are notable exceptions however. Some types of coins from certain periods contain up to 10-15% Pb + Bi. As a result, we have always performed direct neutron Howitzer silver analysis on at least a few coins of each general type that are analyzed by streak analysis. The silver data for the Howitzer analysis are invariably lower than those for the streak analysis, but this is to be expected for two reasons the... 

Figure 4. Silver fineness as determined by neutron Howitzer analysis for about 400 Sasanian drachms. Alternating open and closed circles are used to assist visually in associating data points with ruling monarchs listed at top of graph.

Combined neutron Howitzer and streak analyses of 125 one- and two-peso coins not only reveal the real gold content of these issues but help to establish the silver standard, which was probably aimed at 900 thousands fine (90% silver), a creditable alloy and better than the 800 thousands (80%) of the federal subsidiary coinage. It was therefore possible to estimate fairly closely the theoretical composition and value of the Zapatista silver coinage. 

For the thermal neutron activations, a lOmg Californium-252 ( Cf) source was utilized in a special assembly constructed from a Neutron Howitzer (Trademark of Reactor Experiments, Inc). The assembly is a 6-cu ft cylindrical Lucite tank filled with distilled water and containing three access ports (Fig 3). The large spherical container on the right is the shielded storage cask for the Cf neutron source when not in use... 

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