The vapor and liquid densities of the alkali metals rubidium, cesium, sodium, and potassium were measured from room temperature up to near the critical point by a method which utilizes radioactive isotopes of the alkali metals. Critical temperature and critical density were then estimated by several methods of correlation including those of (1) Rowlinson, (2) Cailletet and Mathias, and (3) Kordes, and by a generalized correlation of reduced density versus reduced temperature. Critical temperatures of the alkali metals estimated by these correlating methods were as follows: 2573°±350°K for sodium, 2223°±600°K for potassium, 2093°±25°K for rubidium, and 2057°±40°K for cesium. Corresponding critical densities were: 0.206±0.041 g/cc for sodium, 0.194±0.37 g/cc for potassium, 0.346±0.009 g/cc for rubidium, and 0.428±0.012 g/cc for cesium. The critical temperature and critical density of lithium were estimated using the liquid-density data for lithium reported by Tepper and the generalized correlation of reduced density with reduced temperature. The estimated critical temperature of lithium was 3223°±600°K and the estimated critical density 0.120±0.033 g/cc. The technique used in these measurements involved charging a metal capsule (molybdenum or molybdenum—tungsten alloy) with a calculated amount of alkali metal, sealing the capsule, and irradiating it in a thermal neutron flux to obtain a radioactive isotope of the metal being tested. The irradiated capsule was then placed in an insulated inductively heated high-temperature cell made of molybdenum—tungsten alloy. Gamma radiation emanating from the vapor and liquid regions in the capsule passed through the thin capsule wall (30 mil) and telescoping holes in the high-temperature cell to a sodium iodide crystal outside the furnace. The measured radiation was proportional to the density of the vapor or liquid. While this method of measurement was demonstrated on alkali metals, the technique is general and can be applied in many cases to measure vapor and liquid densities at high temperatures (up to 220°K) for metals, alloys, fused salts, and other materials.

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Equipment and techniques have been developed for measurement of densities of metals at high temperatures (up to 2200°K). The equipment and techniques were demonstrated using alkali metals but are generally applicable not only to most metals but also to many other substances. Determination of vapor and liquid densities of alkali metals at high temperatures and pressures was accomplished by measuring the radiation emanating from the vapor and liquid phases of a radioactive alkali metal contained in a high pressure cell fabricated from a molybdenum-30 wt.% tungsten alloy. The procedure was used to obtain vapor and liquid phase densities of the alkali metals at temperatures up to 2200°K and pressures up to 500 atm, which approach critical conditions. The radiation counting method involves sealing the metal whose density is to be measured in a containment capsule, irradiating the capsule and metal to produce a gamma emitting isotope of the metal, charging the capsule into the high temperature cell, and counting the activity of the vapor phase and the liquid phase at temperatures ranging from room temperature up to the critical point. A calibration of density vs irradiation level is performed at low temperatures where accurate liquid density data are available.