Unusual ²³²Th and ²³⁸U contamination on some road surfaces in Taoyuan, Taiwan

At least eight bustling streets or approximately 3-5% of all the road surface of civilian utility in the downtown area of Taoyuan City, Taiwan, were inadvertently found to contain unusual levels of radioactivity during a routine environmental radiation surveillance in mid-1994. Crushed rock debris and coarse sands separated from the asphalt pavement were identified to be the source of excessive radioactivity. By employing gamma spectrometry, we have measured ²³²Th activity (via ²²⁸Ac) and ²³⁸U activity (via ²¹⁴Bi) in some of the samples to be up to about 4,000 and 1,000 Bq kg^-1, respectively. The dose rate on the road surface reached about 1.3 μSv h^-1, as compared with the background level of 0.08 μSv h^-1 in much of Taiwan. This unusual radioactivity was due to accidental mixing of road construction materials with materials enriched ²³²Th and ²³⁸U.