DOI: 10.14489/td.2015.010.pp.053-060

Bogolubov E.P., Koshelev A.P., Mikerov V.I.
MEASUREMENTS OF THE MOISTURE LOAD IN AN ASPHALTIC CONCRETE PAVEMENT USING PULSED FAST NEUTRON GENERATORS
(pp. 53-60)

Abstract. According to existing standards the moisture load in various asphaltic concrete pavements should not exceed 9% (vol.). At the moment humidity measurements of various materials are carried out mostly with the devices produced by Troxler (USA) company. For this purpose the models Troxler 3430 and Troxler 3440 are provided with ampule sources of gamma rays (Cs-137) and fast neutrons (Am-Be or Cf-252). Prospects of the replacement of ampule sources with neutron generators are in general based on the following: –  neutron generators emit radiation only in operation and therefore they are safe during storing or transporting; –  pulsed mode of the generator provides auspicious background conditions; –  there is a great variety of portable neutron generators with a higher neutron yield than any of the ampule sources used at the moment in portable devices. The paper contains a comparative assessment of possible methods intended for measuring humidity within an asphaltic concrete pavement which are based on the applica ion of a pulsed neutron generator. The essence of these methods consists in detecting the radiation emitted out of the pavement under fast neutrons irradiation. In the pavement fast neutrons produce in large quantities thermal and epithermal neutrons, gamma rays of the neutron capture on hydrogen (Е = 2,22 МэВ) as well. The selected method should fulfill the following requirements: non locality, i.e. possibility to perform measurements when driving, necessity to provide a priori given spatial resolution alongtrack direction, remoteness of the controlled object from the measuring device, provision of the sanitary code not only for the personnel but for the population too. Technical conditions of humidity measurements accepted in the paper were formulated in the following way: amount of clearance between the device and the pavement – 20 cm; transportation velocity – 10 km/h; spatial resolution in along-track direction – not worse than 10 m; absolute error of humidity measurements – 0.5%; confidence probability – 0.95. The methods based on detecting various radiations were analyzed by modeling the processes accompanying fast neutrons interaction with the pavement using a MCNP code. The results of simulations showed that the method based on detecting thermal neutrons was characterized by the highest sensitivity and by a comparatively simple technical implementation. When the generator output is constant or effectively monitored the stated above technical conditions are satisfied with the device containing the pulsed neutron generator with fast (2.5 MeV) neutron yield not less than 107 neutron/s and the thermal neutron detector with the detecting area not less than 600 cm2. It is obvious that an increase of the detector area promotes a corresponding increase of the transportation velocity. The calibration procedure of the device is one of the most important issues emerging in the practical application of the method. The point is that the results of humidity measurements depend on the pavement density. In case of unknown density the problem can be solved using calibration curves obtained in advance for standard samples with definite humidity and densities. Another solution is based on density measurements performed simultaneously with humidity measurements using in addition, for example, an X-ray source. Simultaneous detection of thermal neutrons and gamma rays ensures a higher reliability and accuracy of the measurements, as well.

Keywords: moisture load, asphaltic concrete pavement, pulsed neutron generator, radiation.

 

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