Project Trinity, 1945-1946

by Carl R. Maag; Steve Rohrer

CHAPTER 3

RADIATION PROTECTION AT PROJECT TRINITY

The TR-7 or Medical Group, shown in the figure 1-5 organizational chart, was responsible for radiological safety at Project TRINITY. Many of the physicians and scientists in the Medical Group had worked with radioactive materials before and were trained in radiological safety procedures. The Chief of the Medical Group supervised the radiological safety operations and reported to the TRINITY director. In addition to providing medical care to TRINITY personnel, this group established radiological safety programs to:

o Minimize radiation exposure of personnel on the test site and in offsite areas

o Provide monitors to conduct radiological surveys onsite and offsite

o Provide and maintain radiation detection instruments

o Provide protective clothing and equipment.

An exposure limit of 5 roentgens during a two-month period was established. Personnel were provided with radiation detection instruments to determine their exposures (1).

3.1 ORGANIZATION

The Medical Group consisted of physicians, scientists, and administrators, as well as radiological monitors. Many of these personnel were nonmilitary, but all worked on the Manhattan Project under the administration of the Army Corps of Engineers Manhattan Engineer District.

The Medical Group was divided into two monitoring groups, the Site Monitoring Group, which was responsible for onsite monitoring, and the Offsite Monitoring Group. Each reported to the Chief of the Medical Group, and each communicated with the other during the monitoring activities. In addition to these two groups, a small group of medical technicians provided radiation detection instruments to Medical Group personnel (1; 10).

3.2 SITE MONITORING GROUP

The Site Monitoring Group consisted of a chief monitor, three other monitors, and several medical doctors. This group had the following functions (1; 10):

o Conduct ground surveys of the test area and mark areas of radioactivity

o Conduct surveys of the Base Camp and roads leading into the test area

o Provide protective clothing and equipment, including film badges and pocket dosimeters, to personnel

o Monitor all personnel for radioactive contamination and provide for their decontamination

o Maintain a record of radiation exposures received by personnel.

The Site Monitoring Group monitored the radiation exposures of personnel in the test area. The time spent by personnel in radiation areas was limited, and radiation detection instruments were provided to permit continuous monitoring of exposure rates. In many cases, a monitor from the Site Monitoring Group accompanied project personnel into the test area to monitor exposure rates (1; 10).

Two members of the Site Monitoring Group, a monitor and a physician with radiological safety training, were assigned to each shelter. The supervising monitor was stationed at the Base Camp and was in radio and telephone communication with all three shelters and the offsite ground and aerial survey teams. Before any personnel were allowed to leave the shelter areas, a radiological safety monitor and a military policeman from each shelter advanced along the roads to Broadway to check radiation levels. They wore respirators to prevent them from inhaling radioactive material (1; 10).

Since it was expected that any dust from the cloud would fall on one of the shelter areas within 30 minutes of the shot, plans had been made to evacuate personnel as soon as the monitors completed their initial survey. Because the cloud moved to the northeast, the south shelter (the Control Point) was not completely evacuated, although nonessential personnel were sent to the Base Camp. The west shelter was emptied of all personnel except a searchlight crew spotlighting the cloud as it moved away (1; 10).

Only at the north shelter did an emergency evacuation occur. About 12 minutes after the shot, a detection instrument indicated a rapid rise in the radiation levels within the shelter. At the same time, a remote ionization monitoring device detected a rapid increase in radiation. Because of these two readings, all north shelter personnel were immediately evacuated to the Base Camp, 25 kilometers to the south. Film badges worn by personnel stationed at the north shelter, however, showed no radiation exposure above the detectable level. It was later discovered that the meter of the detector in the north shelter had not retained its zero calibration setting, and radiation at the north shelter had not reached levels high enough to result in measurable exposures of the personnel who had been positioned there. However, fallout activity was later detected in the north shelter area, proof that part of the cloud did head in that direction. This also explains why the monitoring device detected rising radiation levels (1; 12).

After ascertaining that radiation levels along the roads leading from the shelters to Broadway were within acceptable limits, the radiological safety monitors and military police established roadblocks at important intersections leading to ground zero. The north shelter monitor and military police set up a post where the North Shelter Road ran into Broadway. The west shelter monitor and a military policeman blocked Vatican Road where it intersected Broadway. The south shelter monitor and military police set up a roadblock where Broadway intersected Pennsylvania Avenue (1).

The monitor assigned to Guard Post 4 surveyed the Mockingbird Gap area to ensure that it was safe for the guards to return to their post. This position controlled access to the McDonald Ranch Road, which led directly to ground zero (1).

At 0540 hours, the chief monitor departed from the Base Camp with a military policeman to monitor the entire length of Broadway. They first checked the roadblock at Pennsylvania Avenue and Broadway. Next they drove to the roadblock at Vatican Road and Broadway. Upon the chief monitor's arrival, the west shelter monitor traveled about nine kilometers west on Vatican Road to monitor Guard Post 1 so that the military police could reoccupy the post. The monitoring excursion to Guard Post 1 continued until the chief monitor had returned from Guard Post 2, located 17 kilometers northwest of the Vatican Road roadblock on Broadway (1; 18).

The chief monitor arrived at Guard Post 2 at about 0550 hours and found the post empty. He then continued five kilometers north along Broadway to the foxholes from which the military police had watched the detonation. There he found the guards, the five radiological safety monitors assigned to the evacuation detachment, and the Commanding Officer of the evacuation detachment (1; 18).

The military policemen refused to return to Guard Post 2, insisting that they had received orders over their two-way radio from the Base Commander to evacuate their post and head for San Antonio, New Mexico, a town 28 kilometers northwest of the Guard Post. The Base Commander had noted that portions of the cloud were heading northwestward and, fearing that fallout from the cloud would contaminate Guard Post 2, had ordered the military police to evacuate. The chief monitor, however, had found no significant radiation levels anywhere along the northern part of Broadway nor around Guard Post 2. The Base Commander, after being contacted by the chief monitor, drove to the foxholes and ordered the guards to return to their post. This was the only unplanned incident during the onsite monitoring (1).

After Guard Post 2 was reoccupied, the chief monitor returned to the roadblock at the intersection of Broadway and the North Shelter Road. The north shelter monitor informed the chief monitor of the sudden evacuation of the north shelter, whereupon the chief monitor surveyed the north shelter area and found intensities of only 0.01 and 0.02 roentgens per hour (R/h). The chief monitor then contacted the south shelter and informed Dr. Bainbridge that the north shelter region was safe for those who needed to return, that Broadway was safe from the Base Camp to Guard Post 2, and that Guard Post 2 was now manned so that personnel leaving for LASL could be checked out (1).

The chief monitor then returned to the south shelter and assembled the monitors from the three roadblocks and Guard Post 4 to prepare for entrance into the ground zero area. The time was about 0815 hours. The military police at the roadblocks were given radiation meters to survey the adjoining area. Broadway from the south shelter to Guard Post 2 was remonitored occasionally to reassure the military police that there was no radiation problem. Monitors also surveyed the Base Camp for 24 hours after the detonation. No radiation above background levels was detected there (1).

The following brief description of the radiological environment in the TRINITY test area is based primarily on the results of the remote gamma recorders situated in the test area and on results of the road surveys conducted after the detonation (1).

Within about 1,400 meters of ground zero (except to the north), radiation intensities between 0.2 and 1.3 R/h were detected during the first few minutes after the detonation. These readings decreased to less than 0.1 R/h within a few hours. At greater distances to the east, south, and west, radiation levels above background were not detected (1).

The cloud drifted to the northeast, and higher gamma readings due to fallout were encountered in this direction. About five minutes after the detonation, a reading of 3 R/h was recorded 1,400 meters north of ground zero. Several minutes later, the intensity there had increased to greater than 7 R/h, and it continued to increase for several more minutes. Gamma detectors 9,150 meters north of ground zero, however, recorded no radiation above background levels. This indicated that the cloud had passed over or near the 1,400-meter area and only partially over the 9,150-meter area where the north shelter was located. Subsequent ground surveys of this area found no gamma intensities higher than 0.02 R/h (1).

Gamma radiation levels at and around ground zero were much higher than in other onsite areas because of induced activity in the soil. Twenty-four hours after the detonation, the gamma intensity at ground zero was estimated to be 600 to 700 R/h. This estimate was based on data provided by the tank crew that drove to ground zero to obtain soil samples. The intensity decreased to about 2 R/h at 725 meters from ground zero. Gamma intensities of 0.1 R/h or more were confined within a circular area extending about 1,100 meters from ground zero (except in areas of fallout). One week after the shot, the gamma intensity at ground zero was about 45 R/h. After 30 days, intensities at ground zero had decreased to 15 R/h, and intensities of 0.1 R/h or more were not encountered beyond about 365 meters from ground zero. Gamma intensities of 3 to 10 R/h were found at ground zero three months after the detonation (1; 19).

3.3 OFFSITE MONITORING GROUP

Four two-man teams and one five-man team supervised by the chief offsite monitor constituted the Offsite Monitoring Group. Before the detonation, the four two-man teams established monitoring posts in towns outside the test area. These towns were Nogal, Roswell, Fort Sumner, and Socorro, all in New Mexico. The five-man team remained at Guard Post 2 to assist in evacuation of nearby residences if the TRINITY cloud drifted in that direction. These residences, the Fite house and the homes in the town of Tokay, were 24 and 32 kilometers northwest of ground zero, respectively. Since the cloud drifted to the northeast, evacuation was not required. All offsite monitoring teams were in radio or telephone contact with personnel at the Base Camp (11).

Offsite monitoring teams in areas northeast of ground zero encountered gamma readings ranging from 1.5 to 15 R/h two to four hours after the detonation. Three hours after the detonation, surveys taken in Bingham, New Mexico (located 30 kilometers northeast of ground zero) found gamma intensities of about 1.5 R/h. Radiation readings at the town of White, nine kilometers southeast of Bingham, were 6.5 R/h three hours after the detonation and 2.5 R/h two hours later. Another team monitoring in a canyon 11 kilometers east of Bingham found a gamma intensity of about 15 R/h. Five hours later, the intensity had decreased to 3.8 R/h. It was estimated that peak intensities of gamma radiation from fallout on shot-day were about 7 R/h at an occupied ranch house in this canyon area (1; 11; 19).

Monitoring teams resurveyed these towns about one month after the TRINITY detonation. At Bingham, gamma readings of 0.003 R/h and 0.0001 R/h were found at ground level outdoors and at waist level inside a building, respectively. At the town of White, the highest outdoor gamma reading was 0.008 R/h. Inside a building, the highest reading was 0.0005 R/h (11).

Surveys taken in the canyon area one month after the detonation indicated that gamma intensities at ground level had decreased to 0.032 R/h. The occupied ranch house was also surveyed, both inside and outside. The highest reading outdoors was 0.028 R/h, and the highest reading indoors was 0.004 R/h (11; 19).

Monitoring was also conducted in offsite areas other than those to the north and northeast of ground zero. Monitors found no radiation readings above background levels (11).

Significant fallout from the TRINITY cloud did not reach the ground within about 20 kilometers northeast of ground zero. From this point, the fallout pattern extended out 160 kilometers and was 48 kilometers wide. Gamma intensities up to 15 R/h were measured in this region several hours after the detonation. One month later, intensities had declined to 0.032 R/h or less (11).