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Trinity Test, July 16, 1945 - Radiation Monitoring

Radiation Monitoring, Trinity Test, July 16, 1945, page 1 - small image

Source: U.S. National Archives, Record Group 77, Records of the Office of the Chief of Engineers, Manhattan Engineer District, TS Manhattan Project Files, folder 4, “Trinity Test.”

The Trinity test, on July 16, 1945, was a spectacular success. A 6 kilogram sphere of plutonium, compressed to super-criticality by explosive lenses, exploded over the New Mexico desert with a force equal to approximately 20,000 tons of TNT.

This report, by Col. Stafford Warren, Chief of the Manhattan Project’s Medical Section, shows that the potential for radioactive fallout from the test was an important concern.

Warren’s report shows that fallout from the test exposed a family living 20 miles from Ground Zero to dangerous levels of radiation. By July 27, General Groves’ office diary shows, the radiation monitors became so concerned that they asked permission to talk to the family “to see how they feel.”

Images of the first page of this document, and of its two attachments charting fallout patterns, are included below the transcription.


By Authority of the District Engineer
Per K D Nichols

21 July 1945

To: Major Gen. Groves
SUBJECT: Report on Test II at Trinity, 16 July 1945

1. The test was performed two days ahead of the tentative schedule because everything of importance to the test was ready.

2. A study of the weather indicated that a variety of wind conditions at slow speeds going on in general N.W. and N.E. could be expected with different directions and speeds at different levels for 16 and 17 July 1945. These slow winds would be advantageous in localizing the outfall of active material from the cloud to the site and nearby desert areas. They would also dilute the outfall most effectively in the early hours of the life of the cloud when it would help the most. The monitoring problem would be worse, however, because of the wide area covered.

3. In the two days available, the population of the surrounding areas was located by G-2 on large scale maps for a radius of 75 to 100 miles. The deserted areas corresponded fortunately to the most probable courses of the outfall from the cloud as predicted by the directions of the winds at the various altitudes. Troops under Major Palmer were available if monitoring indicated that evacuation was necessary.

4. At zero minus five hours, five cars with Dr. J. Hoffman in charge were stationed with Major Palmer and troops at the outlet road near the east-west highway #380. They were in radio communication with Base Camp and Post #2. Outlying monitor cars were in San Antonio, Roswell, Carrizozo and Fort Summer to cover these areas in case the speed of the cloud was greater than predicted.

5. Dr. Aebersold was in general charge of the monitoring at Base Camp and the three shelters at 10,000 yards, with local telephone and radio communication. There was a technician monitor and doctor in each shelter and at Base Camp.

6. Dr. Hempelmann in charge of all the monitoring program was at S 10,000, the center of communication and final decisions (also Brig. Gen. Farrell, Dr. Oppenheimer, Dr. Bainbridge, Mr. Hubbard, etc.)

7. This officer acted as liaison in a secondary communication center in Base Camp. Lt. Col. Friedell was located with G-2 at Albuquerque as another communication center via long distance for controlling the field monitoring in case Base Camp communications broke down. All groups were keyed in by identical maps showing preliminary locations of the monitors, their presumed course, the two possible paths of the cloud, WNW and NNE (depending upon the altitude which it reached) houses and nearby ranges, etc.

8. Accessory equipment and other preparations were in keeping with the preliminary plans submitted in the preliminary report.

9. The shot was fired at 0530 on 16 July 1945. The energy developed in the test was several times greater than that expected by scientific group. The cloud column mass and top reached a phenomenal height, variously estimated as 50,000 to 70,000 feet. It remained towering over the northeast corner of the site for several hours. This was sufficient time for the majority of the largest particles to fall out. Various levels were seen to move in different directions. In general the lower one-third drifted eastward, the middle portion to the West and northwest, while the upper third moved northeast. Many small sheets of dust moved independently at all levels and large sheets remained practically in situ. By zero plus 2 hours, the main masses were no longer identifiable except for the very high white mass presumably in the stratosphere.

10. By 0800 hours the monitors reported an area of high intensity in a canyon 20 miles northeast of zero. Since this was beyond the tolerance set and equally high intensities were expected in other areas, four more monitor cars were sent into this northeast area from Base Camp. The roving monitors in this area were each accompanied by a trooper in a 4 wheel drive and authorized to evacuate families if necessary. At no house in this whole north and northeast area between 20 miles and 40 miles from zero was a dangerous intensity found. The highest intensities, fortunately, were only found in deserted regions. The highest found is shown in detail attached #1. Intensities in the deserted canyon were high enough to cause serious physiological effects.

11. The distribution over the countryside was spotty and subject to local winds and contour. It skipped the nearby highway #380 (20 mi. N.E.) except for low intensities which were equalled at twice and three times the distance. It is presumed that the largest outfall occurred in the N.E. quadrant of the site. This can only be explored by horseback at a later date.

12. The monitors all took considerable risks knowingly and many have received exposures of considerable amounts, i.e. 8r total. This is safe within a considerable margin. They should not be exposed to more radiation within the next month.

13. The dust could be measured at low intensities 200 miles north and northeast of the site on the 4th day. (Attached #2) There is still a tremendous quantity of radioactive dust floating in the air.

14. Neither the Base Camp or the shelters were contaminated very much.

15. Partially eviscerated dead wild jack rabbits were found more than 800 yards from zero, presumably killed by the blast. A farm house 3 miles away had doors torn loose and suffered other extensive damage.

16. Details indicating blast, heat, and other effects cannot be worked out until the area around the crater “cools down”. It is this officer’s opinion, however, that lethal or severe casualties would occur in exposed personnel up to two miles from a variety or combination of causes, ie., blast, heat, ultraviolet and missiles.

The light intensity was sufficient at nine miles to have caused temporary blindness and this would be longer lasting at shorter distances. Several observers at 20 miles were bothered by a large blind spot for 15 m after the shot. The light together with the heat and ultraviolet radiation would probably cause severe damage to the unprotected eyes at 5-6 miles; damage sufficient to put personnel out of action several days if not permanently. All of the personnel obeyed the safety precautions during the test so that no such injury resulted.

17. A great deal of experience was obtained on the requirements for quick and adequate monitoring. Excellent radio communications, good transportation and better and more rugged meters are required.

18. It is this officer’s opinion based on the damage to “Jumbo” (2400 ft), the extent of the glazed sand area (up to 500 ft.), the extent of the cleaned off area (about 1 mile), the farm house (at 3 miles) that this explosion was a great many times more violent than the 100 ton test. “Conservative” estimates by the scientific groups put it at least equivalent to 10,000 tons of T.N.T.

19. While no house area investigated received a dangerous amount, ie, no more than an accumulated two weeks dose of 60r, the dust outfall from the various portions of the cloud was potentially a very serious hazard over a band almost 30 miles wide extending almost 90 miles northeast of the site.

20. It is this officer’s opinion that this site is too small for a repetition of a similar test of this magnitude except under very special conditions. It is recommended that the site be expanded or a larger one, preferably with a radius of at least 150 miles without population, be obtained if this test is to be repeated.

Colonel Stafford L. Warren
Chief of Medical Section
Manhattan District

  cc/ Maj. Gen Groves (2)
      R. Oppenheimer (1)
      Col. Warren (1)

Images of the two attachments, diagrams of radiation “hot spots” 20 miles from ground zero, are included below. The diagrams show radiation levels on U.S. Highway #380, 20 miles NE of ground zero, as “Intensity approximately 50 r total for 10 miles along this highway.”

The second attachment is titled “Location of Hot Canyon 20 miles from zero along N.E. path of cloud.” The canyon was 5 miles east of Bingham. This is the description of radiation levels:

“Hot Canyon” 1.1 miles east of road junction
15.0r/hr at 0300 hours after zero          )
14.0r/hr at 0330 hours                     )  total dose = 212
6.0r/hr at 0830 hours                      )  to 230 r at 3600 hours (after rain)         )

One mile east of the canyon, the diagram notes the location of a house with “family with 1 child,” with radiation dose described as follows:

House (with family) 0.9 miles beyond “Hot Canyon”
0.4 r/hr at 3600 hours after zero & after a rain
Accumulated total dose 57-60r.

By July 27, the radiation monitors had become so concerned about the radiation exposure of this family that they asked permission to visit them to “see how they feel.” The office diary of General Groves, which was kept by his secretary, includes this entry for Friday, 27 July 1945 (emphasis added):

4:15 PM. General Groves called Col Warren re Friedell making measurements on certain family out there. Gen Groves suggested that Col Warren call Davies in New York and get the matter straightened out because there seems to be a difference in stories between Daley, Hempelmann and Davies. Gen Groves also asked Col Warren to tell Davies that he was released as far as the General was concerned. Lt. Davies from MSA, N.Y. called Gen Groves and advised Gen Groves that Lt. Daley called him and informed him that Friedell’s boys had made some further observations and are concerned about one family to the extent that they want to get in touch with that family to see how they feel. They called me to ask about the legal end -- told them there was nothing I could do about it. Told them to confer with Col Warren and you. I am sure nothing has been done yet. The call came in a few minutes ago and thought I would relay the message to you. Davies advised the General that he would be at Maj Tandy’s office at MBA and will stay there until Gen Groves calls releasing him.

(Source: U.S. National Archives, Record Group 200, National Archives Gift Collection, “Diary of Lt. Gen. Leslie R. Groves,” Microfilm roll #2.)

This incident was mentioned briefly in a government-sponsored historical study published in 1987. See Bart Hacker, The Dragon’s Tail: Radiation Safety in the Manhattan Project 1942-1946 (Berkeley: University of California Press, 1987) pp 104-105. According to this source, the family was named Raitliff; another house was discovered nearby with a couple named Wilson.

Radiation monitoring, Trinity Test, July 16, 1945 - page 1

Images of pages 2 and 3 not included here.

Attachment 1a:

Radiation Monitoring, Trinity Test, July 16, 1945 - attachment 1a
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Attachment 1b:

Radiation Monitoring, Trinity Test, July 16, 1945 - attachment 1b
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Copyright © 1995-2015 Gene Dannen
Created July 31, 1995 Last modified August 4, 2015
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