Background: The development of measuring technologies of radioactive contamination becomes more and more important. Long Range Alpha Detector(LRAD) can effectively solve some problems such as the short ranges of alpha particles

weak penetration ability and so on. LRAD also has advantages that the alpha radioactivity of interior surface of irregular pipes or just irregular surface can be measured nondestructively and precisely. Purpose:This paper attempts to obtain the measured value of multi-point sources and large contamination areas in pipes. And the comparative experiments of impact factors of two different radioactive surface sources(different nuclides and radioactivities) have been done. Methods: In the experiment

a new domestic LRAD is utilized to measure the alpha activity of interior surface of steel pipes. Here

the inner diameter of pipes is 78 mm and there are two kinds of pipes:straight pipe and U-bend pipe. The radioactive sources include241Am(3 772.2 Bq) and239Pu(224.4 Bq). The distance between different sources takes values of 0 cm

10 cm

20 cm and 40 cm

respectively

and the distance between the whole mixed radioactive source and detector increases by step of 20 cm. Results: When the relative distance of the two surface sources is 10 cm

the detection efficiency of LRAD is higher than that of relative distance of 0 cm

20 cm and 40 cm

which matches the result of theoretical analysis. When241Am is closer to the detector than239Pu

measuring results are much higher than those of the opposite situation. And in the same position

when the two sources are put together

the measured results are lower than those of the sum of each single source. Conclusion:Based on the experimental results

a self-developed new LRAD measurement system has been used to measure two non-point sources in pipes.