The Natural Rock Sample is a 35 oz (1 kg) sample of granite gravel, enclosed in a container that may be sealed using a pair of ball valves and end caps.

Granite generally contains trace amounts of both uranium and thorium that has been in the rock since it was formed, millions if not billions of years ago. Therefore all the progeny of both the uranium and thorium decay chains are fully supported and in full equilibrium down to ²²⁶Ra and ²²⁴Ra.

When the Natural Rock Sample is kept under conditions of stable absolute humidity, it constitutes a rock-steady emitter of both radon and thoron. It is possible to take advantage of that property to check the performance of a RAD8 or RAD7.

The device, as it stands, can provide a means for monitoring long-term changes in both radon and thoron sensitivity. After calibration at Durridge, and with the optional Thoron Calibration Check Kit (sold separately), the device can be used for low-precision checking of the radon sensitivity and the thoron sensitivity of a RAD8 or RAD7. The radon sensitivity can be checked up to once per month, and the thoron sensitivity can be checked any time. A copy of the Durridge-issued Natural Rock Sample Calibration Form can be previewed here (PDF).

Natural Rock Sample Physical Specifications

Natural Rock Sample Thoron Calibration

The Natural Rock Sample System comes already calibrated for radon. Thoron calibration is also offered as an option. Thoron calibration may be performed when the Natural Rock Sample System is purchased, or at any time thereafter.

Thoron Calibration Check Kit

Check the thoron calibration of the RAD7 using the components in the Thoron Calibration Kit. The Thoron Calibration Check Kit consists of a Pump, Power Adaptors, Laboratory Drying Unit, Needle Valve, Custom Flow Meter, Tubing Set, and Carrying Case. (See photo and components diagram.) The Thoron Calibration Check Kit requires the Natural Rock Sample, which is not included.