During TRINITY, the film badge was the primary device used to measure the radiation dose received by individual participants. The site monitoring plan indicates that film badges were to be issued to participants. The film badge was normally worn at chest level on the outside of clothing and was designed to measure the wearer's exposure to gamma radiation from external sources. These film badges were insensitive to neutron radiation and did not measure the amount of radioactive material that might have been inhaled or ingested (1). Personnel from the Medical Group had responsibility for issuing, receiving, processing, and interpreting film badges for Project TRINITY. The Site Monitoring Group compiled the film badge records for both onsite and offsite personnel. Radiological safety personnel and military police recorded the names and identification numbers of individuals as they entered the test area. This information was recorded in an entry logbook and on a personal exposure data card. Upon leaving the test area, individuals returned their film badges to the check station. When the film badges were processed and interpreted, the reading was entered on the individuals exposure data card. In this manner, the number of times an individual entered the test area and his cumulative exposure history were recorded and maintained (1). 4.2 GAMMA RADIATION EXPOSURE The safety and monitoring report lists film badge readings for about 700 individuals who participated in Project TRINITY from 16 July 1945 to 1 January 1946 (1). This list includes both military and nonmilitary personnel who were involved with the TRINITY operation and postshot activities. However, records are available for only 44 of the 144 to 160 members of the evacuation detachment (1). In addition, some of these film badge listings may be for personnel who were only peripherally involved with TRINITY activities, such as family members and official guests who visited the site. According to the safety and monitoring report, by 1 January 1946, 23 individuals had received cumulative gamma exposures greater than 2 but less than 4 roentgens. An additional 22 individuals received gamma exposures between 4 and 15 roentgens. Personnel who received gamma exposures exceeding 2 roentgens represent less than six percent of the Project TRINITY participants with recorded exposures. As described below, these exposures generally resulted when personnel approached ground zero several times (1).