Measuring effective radium concentration with less than 5Â g of rock or soil

► We show how to measure radon source, effective radium concentration, with less than 5 g of sample. ► Using data and Monte-Carlo we show that the method and the estimated uncertainties are reliable. ► The detection limit is moderate (2-5 Bq kg−1) but sufficient in numerous applications. ► Dispersion of effective radium concentration at small spatial scales (mm to cm) can be studied. ► Temperature sensitivity can be studied; large values of about 3% °C−1 are reported for pitchblende.