Spectrometric Analysis of Gamma Radiation from Fallout from Operation Redwing (Research and Development Technical Report Usnrdl-TR-146, April 29, 1957) (Deleted)

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SPECTROMETRIC ANALYSB OF GAMMA RADIATION * FROM FALLOUT FROM OPERATION REDWING Research and Development Technical Report USNRDL-TR- 146 S~nitized by Chief, ISIS, DNA

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.. From: Commanding Officer and Director To: Department of Defense Agenciw on SMstribution List for Report RESTRICTED DATA Eeport USNRDL-TB-Y46; forwarding of Ii Analysis of GammaRadiation from Fallout from Operation h 1. Enclosure (1) is the fotih h a series of reports de~g with analyses of gamma radiation from fallout samples at various weapons tests. Previous reports were made for Operations UPSHOT-KNOTHO~ (USMUW420, 31 August 1953), CAST~ (USNRDk~-32, 26 January 1955), and TEAPOT 2, Efficient arrant: nments for transportation of samples to the Laboratory allowed analyses to begin as early as 51 hours after shot time — L . a notable tiprovemnt over the earliest-time receipt of four days accomplished at Operation CASTLE. Other improvennnts have been made h the way of refinements in instrumentation and analyzing techniques. 3. Absolute photon titensities as a function of time are given for each detectable gamma line from each sample. Since the sample constituted an accurately determined portion of the material. taken from a fallout collector of known area, an estimate can be made of the total gamma source strength per unit area in the vicinity of the collecting site. Dose rates calculated for two samples agree within experhental e~or with dose rates actually measured at the site. —

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SPECTROMETRIC ANALYSIS OF GAMMA RADIATION FROM FALLOUT FROM OPERATION REDWING Nuclear Radiation Characteristics Commanding Officer and Director Captain Richard S. Mandelkorn, USN U.S. NAVAL RADIOLOGICAL DEFENSE LABORATORY San Francisco 24, California

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Fallout samples collected at Operation REDWING were measured by a scintillation type gamma-ray spectrometer. Absolute photon intensities as a function of time are listed in tabular form for each detectable gamma line from each sample. Most samples represent known fallout collection areas so that dose rates in the vicinity of collection sites can be calculated therefrom. Cloud samples from Shots Zufii and Navaho show the presence of NaU- induced activity and possible mechanisms for its production are suggested. This report is one of a series of similar reports issued by the USNRDL Radiation Characteristics Section following major weapons test series: USNRDL-4Z0, 31 August 1953;

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Gamma radiation emitted by selected fallout samples collected at Operation REDWING was spectrometrically analyzed for the purpose of determining its characteristics (expressed in terms of absolute photon intensities for each discernible gamma line) as a function of time after shot. Since the absolute photon intensities are given and most -- samples represent known fractions of the area of the fallout collectors, one result is that dose rates in the vicinity of f the collecting site can be calculated. Another important result of the analysis suggests that NaU becomes an increasingly important contributor to the

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This report is one of a series of technical reports on the gammaray spectra of fallout samples collected at various weapon tests. Previous data has been reported in tMs labo-toryrs USNRDL- 420, 31 August 1953, USNRDL-TR-32, 26 January 1955, and USNRDL-TR-106, 27 August 1956. The sample study reported here was conducted in conjunction with Project 2.6.3, Operation REDWING. The work was done under Bureau of Ships Project No. NS 081-001, Technical Objective AW- 7, and is a continuation of that described * as Subtask 6 in this laboratory’s report to the Bureau of Ships, DD Form 613, July 1956. Similar work is described under Program 9, Problem 1, in this laboratory’s Semi-annual Progress Report, 1 July to 31 December 1956, Progress Report USNRDL-P- 1,. January 1957.

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~~e~ of the report were greatly appreciatecl: The excellent liaison both inregard tq 8ppples and information Members of the Electronics Sectioq ~g~~ cooperative at all

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radioactive fallout sanples collected by ?roject 2.6.3 at the Sprin~, 1956, weapons test series known as Cperation istics of the fallout radiatidnversus time an~ to compare these characteristics for consistency among sariplesfrom the same and different shots. The present work is a continuation of the spectrometric analyses of fallout ~anma radiation which have been made at Operation UPSkOT-iC!OTi{GEat the Nevada Operation CASTE at the Pacific 2roving samples of various types at and Operation TEAPCT at the takin~ advantage of further and analyzing technique. several ship, bar~e anillan~ stations and returned them to NRDL by air. This rapid return allowed analysis to begin as early as 51 hours after shot time--a notable improvement over the earliest-time receipt of 4 days accom- - plished at Operation CASTLS. Other samples requiring

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lqia as ten tin;sQftcr snot ti::;.. ___—..—___—- . for each enerGY from the various samples, while in previous reports only relative line intensities were ~iven. Since each sample constituted an accurately ~etermine~ portion of the material talcenfrom a fallout collector of known area, an estimate can be made concerning the total gamma source strength per unit area in the vicinity of the collectin~ site. . Other than reportin> quantitative information on photons emitted per square inch of fallout collection . area, this experiment exemplified better organization and preparation in other ways. The samp~es were pretion throughout the cburse of the experiment. The instrument was improved by the substitution of a Moseley X-Y plotiterfor the Brown recor~er. Finally the analysis itself was made simpler an3.more consistent by the use of a better way of measurin> peak areas.

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A gamma-ray photon is cletectableonly by means of the ener~y it loses in an interaction or interactions with matter. Gamma-ray photons lose ener~y by three main processes: photoelectric effect, Compton effect, an3 each effect is dependent on the ener~y of the g--ray photon and the atomic number of tie detecting material. going any of these processes ultimately manifests Itself in the molecular excitation of the 5etectinS matertal. this excitation in the form of useful scintillations of light whose pre~ominant color depends on the crystal. -TheIntensity of the liGht flashes produced in these materials is dependent (in some cases, linearly) on the energy of the Incident photon (provided it is totally absorbed). These li~ht pulses are detected by a photomultiplier tube which converts them into electrical pulses.

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These pulses are amplified then analyzea accoraln~ LOl ray detectors because of their high efficiency ard linearovercome by sealing them hermetically in thin-walled alumiFor this experiment a special sample holder was made for the ”spectrometerwhich allowed the bottoms of the l sample containers to be place5 2 inches above the top of the collimator (a l/2-inch hole through 6 inches of lead) s-inch Du Mont photomultiplier tube, preamplifier and housing (Fig. 1) were the same as those used in the analysis of the data from Operation TEAPOT3 an~ are more fully The output of the preamplifier was fed into a Tracerlab non-overloading linear amplifier whose output pulses were analyzea by an Atomic Instrument Company Model 510 was provided by a helipot, motor driven throuyh its renzeo The output pulses from the analyzer were fed Into

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meter has 7 scales ran~in~ in sensitivity per second full scale to 1000 counts per secon~ full was varied by the base line bias of the modification in the form of apotentiometer 1 in series with the X input allowed fine afijustmentsto be made in matchin~ pulse height to particular li,neson the recoriin: paper. A ‘;ypicalrecor3ing made plier tube was constructed from a UCRL The fallout samples were Supplied in 3/~-inch in diameter by 2-incfiin length polystyrene curies total activity. Descriptive information about sample, the effective fallout area it represents, and associated shot is listed in Tablel-~ Samples were obtained from 2 types ~i’c~llectors. - through which a representative sample of air was drawn as

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minirmm tb.eamount of extraneous material deposite~ both either by an increase in radioactive back~round or by han3, the cover of the 5evice opens and a collectin~ tray rises into the wins stream for the preset exposure time (variable in Is-minute increments between O an~ 81 hours). The tray has an effective collectin~ area of about 2.5 square feet divide5 into numerous cells to reduce loss of material ~ue to wind. Samples, other than C1OU3.samples, were obtained. f’r’omthts type of collector mounte5 on convertea liberty After collection the fallout material was flown back to the Laboratory and aliquote5 by Chemical Technology Division. The aliquots su~pli~d to this section for analysis were place~ in the polystyrene containers an~ prevented from shifting, in the case of’granular or pulverized samples, by the ad5ition of a small amount of paraffin. The machine viascalibrated so that on “standard” gain the total absorption peak of a 100, 200, or 300, etc., kev Inch, etc., line on the rec?r~ing paper. This calibration was accomplished first by careful zero-settin~ of the X-Y

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recoraer an~ analyzer, an~ ti~enby varyin: the Cain of the priate positions on the retarding paper (Fig. 2). It was found that this calibration was very stable, requirin~ only an occasional small a~justment in the zero-set control of. the recorder to allow for variation in paper placement and rulinG defects on the paper. That the equipment was essentially stable in the Y direction was shown by perio~ic checks with the 60-cycle test position of the count-rate meter an3.by the linearity of the decay curves on semiPulse height spectra were recoriiedfor each sample on amplifier gain increase3 by a factor of four (0.75 Mev full scale)--in those cases where the strength of the s~ple How long the taking of perio~ic recordings of coul~ continue after shot time before the count too low for statistically significant results course, on the original strength of the sample. On nearly all of the stan~ard-gain records, a calibrate~ ~a22 stan5ar5 source was recorded on the same sheet different color ink, while on the high-gain records

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203 source was used.calibrate HZ Tnis use of the calibrated standards allowed a constant check to be made on the stability of the equipment (see Appendix I). ANALYSIS OF DATA The pulse height spectrum of a monoenergetlc g- ray ‘ consists of a total absorption peak and continuous “Compton” distribution. If the energy of the gamma ray is above 1.02 Mev, psir production begins ‘0 ‘Ske place and one of two secondnr~_pe~ks is contributed to, according to whether one or both of the annihilation quanta escape from the crystal. The peaks all have the shape of normal distribution functions or “Gaussians” because of the statistical nature of the inherent in the operation of a photomultiplier tube. If two incident gamma energies are present, the lower is superimposed ad~itively on the continuous Compton distribution of the higher. An example of the idealized addition of three different monoenergetic gamma rays’is s’nownin Fig. 3* As more gamma energies are added the resulting confusion becomes progressively worse. (If the higher energies are above 1.02 Mev, the presence of annihilation radiation escape peaks further complicates the spectrum and thus the analysis.)

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Fig. 3 Addition of Three Total Absorption Peaks at Different Energies and Their Associated Compton

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In or~er to untangle these overlapping ~istributions and arrive at the basic gamiialines, a metho~ of graphical unfolding is usefi. Gne starts at the highest-energy peak present (assuming that it is not superimposefion the Compton distribution of a still higher ener~y) and measures the area “ total ratio (the method of ascertaining this ratio is 5escribed later) for each energy, one can calculate the area of the Compton distribution. After drawing in the Compton distribution of the highest energy, the process is repeated for the next highest-energy peak using the Compton of the energies present are analyzed. the instrument for each energy, is only one energy contributing one can tell width of the peak. The resolution (width of a peak in ‘ highest energy as a base line. The process is continued until all the lar peak, “or two or more unresolved energies present, by kev at half-maximum height) was determined by measuring the resolution for several total absorption peaks of and plotting the results (Fig. 4). In order to facilitate the measuring of the areas and to aid in estimating the relative size unresolved peaks,a series of Gaussian curves of pre5etermine5 witlths,various heights anfi,thus,known areas, were drawn by means of a Gaussian curve generator (see Appendix II). BY

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superimposition of the suitable Gaussian on the pulse height epectrurn with the aM of a frosted-glass light table, the each total absorption peak was Aetermlned without The ratio ot tion peak area ts with a planimeter. For errors incurred system see the sectton on errors. l some function of We energy of the gamma For the ae$xxmtiat- of the graphical ascertain further machine parameters and other energyIs simply the fraction of incident @otons of a particular energy which interacts In the crystal and is tihere~ Is the total linear abso~ptlon coefficient v versus E Is shown plotted In Fig. 5, mined exper.i.ntentally by-=cotiing the spectra of several sources af ‘knowngamma energy and measuring am?as of the total abaorpttaanpeaks anflthe azwas under the total speetrum. ThemWlo8 of~hese areas platted against energy

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furnisila fairly smootlicurve. The peak-to-total curve characteristic of’the machine used In experiment is shown in Fig. 6. (3T cOUn~S per second per square Inch total absorption peak, C. This number is a of the machine and is foun~ by tiivi~ingihe counts per secon~ per Inch of deflection in tion by the channel width In chart Inches. 0.1 inch on the chart paper and the total Y deflection Is 10 inches. Then for the 200 c/s scale, C is 200. (4) Source re5ucti6n factor, ~. This factor is the fraction of photons leaving the source which enter the crystal an~ takes into account effects of collimator point source ancl 9 Derived from informa-R. L, Mather of this laboratory. tion contained in this paper l-sFig. 7, showing K, the source re~uction factor for a point source, versus r, the distance of the source from the collimator axis. To find the average source retiuctionfactor, %, to account crystal from the source,assuming a an opaque collimator. These effects some detail in a theoretical paper by

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Fig. 6 Ratio, W, of the Area Under the Total Absorption Peak to the Area Under the Entire Pulse-Height Distribution Versus Gamma Energy

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for the finite extension of the source; it was necessary to perform the following numerical inte~ration: where rs is the raiiiusof the extended source. Tne where a is the collimator diameter, b the collimator thickness anfiz the distance from the center of the collimator to the source. When r is larger than a/2 and smaller than az/b, K is some graphically by interpolation and Natherts paper. For values of r function of r determined ~erived from Fig. 9 of equal to or larger than Is use~. Penetration is a function of Gamma-ray energy an5 the increased aperture due to increased energy is approximately the same as the geometrical aperture of an opaque collimator two mean-free-paths less thick. For the particular Seometry involved in the present ‘ experiment, the values of the’machine parameters are as

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where l/LLis the mean-free-path of gamma radiation b) at 200-kev intervals between O and 2 Mev and at The absolute photon intensity is a combination of these various correction factors, i.e.: the energetically appropriate I/A from the graph, the number was …