Measure Thoron with Lucas Cell

Our Lucas cell and portable radon detector can also be used for quantitative analysis of radon-220, also called thoron.

Thoron is discriminated from radon-222 by means of the difference in half-life of these two isotopes and their solid daughter products. We look at the count rate over the first few minutes after filling the Lucas cell.

Thoron's half-life is 55 seconds. When the thoron nucleus emits an alpha particle it changes into polonium-216, a solid alpha emitter with a half-life of 0.158 seconds, which doubles the count rate. This shows up in our alpha counter as a count rate falling at a half-life of 55 seconds.

Radon-222, on the other hand, shows a slowly increasing count rate in the first few minutes. The 3.8-day of Radon-222 gives a steady count rate. This steady count is augmented by its solid alpha emitting daughter, polonium-218, which grows into equilibrium with its half-life of 3.05 minutes. Radon-222 and its daughter together therefore give a slowly increasing count rate.

Click here to see graphs for these two isotopes of radon.

Our instruments can also measure thoron in an intrinsically safe manner, as long as you can get the gas sample back to the radon monitor in a very few minutes. The half-life of thoron is 55 seconds. Click here for details.
  • We measure: radon - radium - thoron - radon daughters - alpha radiation.
  • The Lucas cell is recognized as the most sensitive and reliable method for these elements.
  • Intrinsically safe functions.
  • Sensitive to geochemical trace levels necessary for radon in lake water and for radon-thoron isotope ratios.
  • immune to beta and gamma radiation.
  • one monitor works with a number of (less expensive) detectors.
  • Our instruments are used around the world in exploration for uranium, oil & gas, groundwater and hydrothermal, and in environmental protection, health physics, earthquake prediction, and evaluation of hydrocarbon and NAPL contamination etc.
  • Same instruments used for radon and radium in soil, sediment, plant parts, rocks, water, soil gas, air, snow, food, and for radon and thoron daughters in air.
  • Winter and summer, from the Sahara Desert to the Canadian Shield, our instruments have faced up to severe field conditions.
  • In the radon business since 1968, our instruments are updated regularly with the most recent major re-design in 2015. Modern, low-power, field-rugged electronics. Some earlier versions still working after 40 years.
  • Continuous real-time monitoring and data recording.
  • RS232 port/pc software.
  • User programmable measurement intervals, sample and count periods and alarm level settings.
  • Can work in a tent without electricity or be carried from point to point in the field.
  • 50 readings per day. Results available immediately.
  • Portable. Rechargeable battery pack good for a long day in the field and recharges in a few hours.
  • Can be operated by junior personnel if carefully supervised.
  • EPA and CE Mark compliant.
  • Click here ( for more details of our radon instruments, and for other instruments, components and accessories we provide.
  • Technical specification sheets and pictures of our instruments provided on request.
  • Multilingual consulting and training (if required).
For instruments contact

Click here for technical details and other applications of our radon instruments.
Bateman, H., 1910: The solution of a system of differential equations occurring in the theory of radioactive transformation, Proc. Cambridge Phil. Soc. 15:423-427. These equations help us understand the amount of various nuclides present after isolation of their radioactive precursor, including what happens in the Lucas cell in the first few minutes after filling, and how nuclides grow into radioactive equilibrium in the earth over the billions of years of its history.
Lucas, Henry F., 1957: Improved low-level alpha scintillation counter for radon, Rev. Sci. Instruments, vol 28, No. 9, pp 680-683. Credited with invention of the Lucas cell, a hollow cylinder sealed at one end with a glass window and coated on the inside with silver-activated zinc sulfide. This material emits a photon of light (scintillates) and this scintillation is detected by a photomultiplier tube and counted by an electronic apparatus. The Lucas cell is the size of a drinking cup because that is the range of an alpha particle in air.
Dyck, W., 1969: Radon determination apparatus for geochemical prospecting for uranium, Geol. Survey of Canada, paper 68-21.

click here for paper in pdf
Developed a portable Lucas cell system which is the predecessor of our instrument. We have been providing battery operated radon instruments since 1972.
Morse, R.H., 1976: Radon counters in uranium exploration, in Exploration for Uranium Ore Deposits, International Atomic Energy Agency, p. 229-239. Developed a formula for discriminating between radon (half-life = 3.8 days) and thoron (half-life = 55 seconds) based on the alpha counts counts in each of the first three minutes. This method offers the advantage of short counting times, useful in mineral exploration and in sniffing for entry points of radon and thoron into buildings.
Coleman R.L., 2002: A method for concurrent and continuous measurement of Rn-222 and Rn-220 using scintillation cells. Report ORLN/TM-2002/37. Oak Ridge National Laboratory

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A method is described for the continuous and simultaneous measurement of both 220Rn and 222Rn in air. Two scintillation flasks are arranged in a serial configuration and the concentrations of 222Rn and 220Rn are determined by making use of the difference between the half-lives of the two radon isotopes. The method was developed for directly measuring 220Rn in occupied areas where fuel materials containing 228Th were being used, but could also be useful for other applications. Since 222Rn is usually present from either naturally occurring materials or due to the presence of process material, the method was designed to allow measurement of the two isotopes at coincident times. The method is discussed for counting equipment using scintillation cells, but the approach would also be directly applicable for any type of pulse-counting radon monitoring equipment such as pulse-ion chambers. Although intermittent measurements with decay correction could be performed using a single detector, the use of two cells allows continuous monitoring and a higher degree of detection sensitivity. The approach makes use of isotope-independent calibration factors and could therefore easily be modified for use with a single detector when only one of the radon isotopes is expected to be present.
Shinji Tokonami, Mingli Yang, Hidenori Yonehara and Yuji Yamada, 2002: Simple, discriminative measurement technique for radon and thoron concentrations with a single scintillation cell, Rev. Sci. Instrum. 73, 69.

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A simple, discriminative measurement technique for radon and thoron concentrations is discussed. In this technique, a single scintillation cell is used for both radon and thoron measurements. It consists of two measurements that use the difference of the half life between the two isotopes. Alpha counting efficiencies for their associated radionuclides were estimated by a Monte Carlo calculation. When evaluating the conversion factor for concentration on two types of scintillation cells, both agreed well with experimental values. Optimum measurement conditions on the timetable are also discussed. This technique can provide two concentrations promptly. Although it is not highly sensitive, it is applicable to performance tests for radon/thoron monitors and simultaneous exhalation rate measurements for both radon and thoron.
B. Machaj, P. Urbanski, J. Bartak, 2007: Measurement of radon (222Rn) and thoron (220Rn) concentration with a single scintillation cell, NUKLEONIKA 2007;52(4):167-171

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Department of Nuclear Instruments and Methods, Institute of Nuclear Chemistry and Technology, Warsaw, Poland
They use Lucas cells.
A single scintillation cell (f54 × 74 mm) is used for the measurement of radon and thoron. The radon and thoron laden air is filtered and forced to flow at 1 dm3/min through the scintillation cell in the period of 1-10 min. The count number from alpha radiation is registered in the periods of 3-10 min and 20-30 min. Two values of detection and deposition efficiency of alpha radiation are used for radon (at air flow and air at rest in the cell) and for thoron. Measurements of radon laden air and thoron laden air showed good agreement between the reference concentration and the measured concentration, not worse than 1% for radon and not worse than 2% for thoron. Combination of radon + thoron concentration showed also a small interference (“cross talk”) not worse than 1%.
K.P. Eappen, R.N. Nair, Y.S. Mayya, 2008: Simultaneous measurement of radon and thoron using Lucas scintillation cell, Radiation Measurements 43 (2008) 91 – 97.

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Developed a method for the simultaneous measurement of radon (222Rn) and thoron (220Rn) using the Lucas scintillation cell.
A method has been developed for the simultaneous measurement of radon and thoron in a mixed environment using the Lucas scintillation cell (LSC). The method uses counts from two arbitrary counting intervals from zero time with respect to sampling hence, called two count method (TCM). The basis of the method involves formulation of simultaneous linear equations with two unknown variables. The unknown variables represent the radon and thoron activities in the cell. The coefficients of the equations are the integrated activities of radon, thoron and their alpha decay products per unit activities of the parent radionuclides. The method can be used to estimate the radon and thoron concentrations in a mixed environment accurately and quickly, as there is no need to delay the counting to achieve complete decay of thoron. The mathematical basis of the method along with few experimental results is presented in this paper. Results yielded, on comparison, good agreement with the delay count method (DCM) and double filter measurement (DFM) technique. Multiple regression analysis is also carried out to get the concentrations of radon and thoron from the counts obtained for various counting intervals. The corresponding growth factors of radon and thoron also showed good comparison with the average concentrations obtained from the two count approach for different counting intervals. Use of LSC for routine environmental measurements has its limitation due to high minimum detection level (MDL) values. However, LSC is commonly used in uranium mines, high background radiation areas and in calibration experiments. The present methodology is a very convenient approach to measure radon and thoron simultaneously especially in calibration experiments.
Lei Zhang, Jian Wu, Qiuju Guo, and Weihai Zhuo, 2010: Measurement of thoron gas in the environment using a Lucas scintillation cell, Journal of Radiological Protection 30. p597-605.

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A simple and fast method for measuring the concentration of 220Rn in the environment was developed based on the Lucas cell.
C. G. Sumesh, A. Vinod Kumar, R. N. Nair, R. M. Tripathi and V. D. Puranik, 2011: Estimation of thoron concentration using scintillation cell, Oxford Journals, Science & Mathematics, Radiation Protection Dosimetry, Volume 150, Issue 4, Pp. 536-540, Special article collection Fukushima.

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Measures thoron with Lucas cell.
Two counting techniques are proposed in this paper to estimate thoron (220Rn) concentration using a Lucas scintillation cell. The alpha activity build-up inside the cell is calculated theoretically by using Bateman equations. The first method is having a minimum detection limit of 325 Bq m-3 and can be used for thoron measurement in thorium-processing plants. In the second method, thoron concentration is calculated using the alpha counts from thoron progenies and is a reference to the first method. The results obtained by these techniques compare well with the double filter method.

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