Operation Ivy, Pacific Proving Grounds, November 1952, Project 5.3, Fall-Out Gamma Ray Intensity

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NUFICE: wheh govemmzmt or other drawings, specificatfons or o*&er data are used for any purpose other t-canin connection with a definitely reJ.ated Government thereby Incurs no responaibillty,nor any obligation whatsoever; and the fact that the Government raayhave fomulated, furnished, or in w way supplied the said drawings, epeclficatlons,or other data ie not to be regarded by implicationor othervise as in any mnner licensing the holder or any other person or corporation,or conveying any rights or pennlssion to mnvfacture, u8e or sell w patented invention that may in any way be related

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I Report to the Scientific Director FALL-OUT GAMMA RAY INTENSITY University of CalXornia Radiation Laboratory

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FALL-OUT GAMMA RAY INTENSITY by: Mclti P. Klein hfeasurements of the intensity of fall-out gamma radiation as a function of time and distance from the Ivy Mike and King shots are reported. Instruments were placed around Eniwetok Atoll and at eeveral atolls in a southerly semicircle with respect to Eniwetok. Contrary to expectations the winds prevailing at the time of the Mike shot were south or southeasterly and no fall-out gamma radiation with the limito of detectibfiity was measured at any location other than Eniwetok. The gamma radiation measured on Eniwetok Atoll was conoidered normal fission fragment radiation which decayed with Other people beside the author who worked and participated in Projact afl of UCRL, Livermore, California.

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Circuit diagram of logarithmic electrometer and accessories. View of assembled chamber with tilectrometer housing: View of interior of housing containing the equipment. . . View of apparatns completely enclosed. ....

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/Knowledge of the fall-out pattern of a large thermonuclear detice is essential for proper evaluation by military as well as teat planners and personnel. In order to learn vomething of the fall-out characteristics of the first thermonuclear device an experiment was designed to measure the fall-oat gamma ray intensity as a function of time and spatial disposition, with respect to ground zero. Implementation of this experiment was achieved by placing completely self-contained measuring instruments at a large number of locations distributed around the shot island and varying in distance from approximately one mile to several hundred miles. Instruments were installed on the following islands of Eniwetok Atoll: Bogallua, Ruchi, Tziteiripucchi, ‘Engebi, Bijiri, Aniyaanii, Parry, Eniwetok and Ftigili. Additional stations were established at the following aitec: Roi, Kwajalein, Majuro, Ponape, Bikiti, Ujelang and Kusaie. These latter locations were selected as a compramize between the most desirable distribution of sites and logistic acceesibility for installation and servicing of the instruments. In order that the equipment be as versatile as possible with respect to both gamma intensity and measurement locations the following features of the equipment were considered to be essential: (1) A lower limit of 5 mr/hr sensitivity with a dynamic ra.:ge of six orders of magnitude since no previous data were avaf;able ao a guide to the intensities to be expected. ment could operate in the open without attendance for an exrended period. The manner in which the&e requ!rementa were satisfied w!Y be give; below with a description of the equipment. The central feature governing the design of the equipment was that of the extended dynamic range, This immediately suggested that the recording instrument plot the logarithm of the radiation intensity. The electrometer employed was based on a circuit developed by C. W. Johnson o:f LASL. A circuit diagram of rhia devi.ce is given in Fig. I. The circuit consists of a differential amplifier the output of which is proportional to the logarithm of the ionization chamber current. The limiting sensitivity of the circuit amperes. Th? output of the unit was linearly pro-

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portional to the logarithm of input current over a range in excess of six decade 5. Since the, units were to be self-contained and to operate unattended at remote locations they were powered by batteries. Variations in operating characteristics, due to gradual decrease of battery voltage were accounted for by prbviding for periodic self-calibration. This was achieved by injecting known currents into the circuit at four-hour intervals by a camoperated switch on ‘the clock-driven recording milliameters. ’ IiThe units opt ,ated satisfactorily for approximately five days which was Yhe maximum length of time the clock motors operated. The low currents to be measured together with the high values of rosistance in the circuit necessitated exclusion of all water vapor. This was achieved by containing the electronics in an air-tight housing pressurized slightly with dry nitrogen gas. The limiting sensitivity of the electrometer, 10 -12 amperes, together with the requirement that the apparatus possess a sensitivity to gamma radiation of 5 fla r/hr, dictated that the ionization chamber have a sensitivity of about 10 ampere/r/hr. Since nc commercial ionization chambers of this sensitivity were available it was necessary to develop one with the appropriate characteristics. The resulting chamber was a high pressure multielectrode Argon chamber with a sensitivity of approximately 2 x 10 -9 amp/r/hr. Figs. 2 and 3 show, respectively, an exploded view of the chamber, and a view of the assembled chamber together with the electromThe electrometer, ionization chamber, recording miliiameter and b.rtteries were contained in rugged aluminum boxes which could be handled by two men and could withstand exposure to any weather which might be encountered at those locations wh-_te no protective shelter was available. Aluminum radiation shields placed over the boxes precluded any “greenhouse” effect from occurring. An undue temperature rise in the boxes could have undesirable effects; a high temperature could create irreversible changes in values of the components in the electrometer leading to instability or undue drift, seriously decreasing the life of the batteries and an increase in the ionization chamber pressure beyond the limit of the protective pressure seal. Figure 4 shows the interior of the housing containing all equipment. Figure 5 is a view of the apparatus completely enclosed. Provision was made to allow operation with the ionization chambers at a location remote from the electrometer.proper and many of the units on Eniwetok were so employed with the electrometers inside blockhouses and the chambers inside protective canisters located outside. Calibration of the ionization chambers was carried out by comparison

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with Victorcen Wr!irnble~~ ionization chambers which had been prevfously calibrated. The saturation behavior of the chamber6 was studied and at i the operating voltage of. 900 volts the chambers were well onto the plateau. , The current collected at 900 volts was in excess of 95 per cent o.f that , collected at 5,000 volts. At the low and intermediate radiatio intensity ranges the sources consisted of radium and a several-curie Co 60 source. For the high intenoity region a one-Mev industrial x-ray machine was employed. By comparing the response to the collimated and uncollimated beamr of the x-ray machine it was possible to deduce the solid angle over which the chamber was reAs stated above in the section on the electrometer, at four-hour interYais the sensitivity of the electromet r wa checked. lzhi# was done by electrometer in place of the ion chamber currtnt. of the equipment was from 5 mr/hr to 5 kr/hr. The range of .pensitivity were serviced in the field a 25-mc Co60 Each time the inlrtruments source was used to check the overall operation of the system. The distribution of off-atoll measuring locations in a southerly semicircle about hziwetok was chosen because the prevailing windu at higher altitudes at the pertinent time of year were coneidersd to be northerly. The perversity of nature revealed itself on shot day au the upper air motion was predominantly toward the north. Accordingly, none of the fall-out was carried over the measuring sites off-atoll. No radiation, within the detectability of the instruments, was indicated at any of the off-atoll sitee. There are plotted on Figure 6 the gamma-ray intensity versus time &ta for those islands on Eniwetok where data were obtained: Engebi, Runit, Bijiri, Aniyaanii, aLd Rigili. Parry and Eniwetok suffered no fall-out within the limit8 of detectability of the equipment. Complete data around Eniwetok Atoll were not obtained because the ionization chambers located on Bogallua, Ruchi and Bogon ielands were destroyed by blast and thermal damage. Landline telemetering from Engebi to Parry was installed but did not survive the The only significant results from the King shot were the Slowex data which have been incorporated into the Project 5.2 report. (See report WTAlthough the lack of conclusive results from this experiment ia in no way a reflection on the operation of the equipment but rather on an unfortunate distribution of measuring sites, the participants in this experiment Hould not recommend duplicating the equipment for any future experiment6 of this type.

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:hct advent of the solid-state electronic devices would allow a vastly more compact ana less expensive apparatus to be constructed if ‘an experrment of this type were to be repeated. Scintillating phosphors and photomultiplier tubes in conjunction with transistorized amplifiers together with the requisite battery supply would occupy only a fraction of the volume requireid by the units employed in this experiment.