Casebolt, H H

Mccrary, J H

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Particle Accelerators1976, Vol. 7, pp. 111-117© Gordon and Breach, Science Publishers Ltd.Printed in the United KingdomRESIDUAL RADIOACTIVITY IN THE FERMILABACCELERATOR ENCLOSURESJ. H. McCRARY and H. H. CASEBOLT, Jr.Fermi National Accelerator Laboratory, Batavia, Illinois 60510, USA(Received February 3,1976; infinalform March 18, 1976)A self-propelled radiation survey vehicle is used to make periodic measurements of the radiation levels in the variousFermilab accelerator enclosures resulting from residual radioactivity induced by losses from the proton beam. Thevehicle is equipped with instrumentation which provides -a' strip chart record ofthe radiation intensity as a function ofposition and the mean radiation intensity of a given segment of the accelerator. Plots of the mean radiation intensity asa function of time for the past year and a half are presented here for several of the accelerator enclosures.FIGURE 1 Schematic diagram of the Fermilab accelerator.ABORTTARGET"""""(0-0) 8"t}IiJ<:>B SECTORE SECTORMAIN RINGTRANSFERHALL (A-O)BOOSTERTO TARGET AREASSWITCHYAROsectors which are designated by the letters Athrough F. The upstream end of each of thesesectors contains a 52-meter straight section whichbears the additional designation of "zero." Thus,Main Ring injection and extraction take place inthe Transfer Hall which is located at A-O. The beamabort target, located at D-O, is a series of beamstops located radially outward onto which a smallfraction of the beam is dumped at the end of slowspill extraction. The 400-GeV protons are ex-tracted from the Main Ring in the Transfer Halland distributed to the three major experimentalareas through the Switchyard which is comprisedof Enclosures B, C, D, and E. (For a more detailedThe residual radioactivity present in the Fermilabbeam-line enclosures is induced by high-energyprotons lost from the primary beam. These protonscause spallation reactions and nuclear cascades inthe steel, copper, concrete, and other materialscomprising the vacuum system, magnets, suppor-ting structures, and enclosure walls of the accele-rator. Virtually all isotopes lighter than theelements present in these materials can be formedthrough nuclear reactions initiated by the primaryprotons. Thus, the radiation in the acceleratorenclosures consists of fJ- and y-ray emissions frommany radioactive isotopes. While the resultingradiation spectrum is complex and is a function ofboth time and position, its combined interactionwith matter resembles that of 1-MeV y-radiation.The pattern of build-up and decay of residual radio-activity as functions of position and time is im-portant to: (1) both the short-term and long-termimprovement of accelerator operating efficiency,(2) the consideration of proposed changes in modesof operation, and (3) the determination and predic-tion of safe working conditions within the accel-erator enclosures.Figure 1 is a schematic drawing (not to scale)showing the relative location of the severalFermilab accelerator enclosures. Arrows in thefigure denote beam direction. 200-MeV protonsare injected from the Linac into the Booster. Fromthe Booster, 8-GeV protons are injected into theMain Ring where their energy is increased to400 GeV. The Main Ring is divided into six equalINTRODUCTION111112 J. H. McCRARY AND H. H. CASEBOLT, Jr.description of the Fermilab accelerator and itscomponents, see Reference 1.)To provide an indication of the scale of theaccelerator system: the Linac length and theBooster diameter are both 150 meters; the MainRing diameter is 2000 meters; Enclosure B is 130meters long and Enclosure C is 330 meters long.PROCEDUREThe Fermilab radiation survey vehicle is a modi-fied battery-powered golf cart. 2 The cart isequipped with a Geiger-Mueller tube, a logarith-mic count-rate meter, a strip chart recorder and ascaler. A permanent magnet attached to the rim ofone of the wheels trips a magnetic switch as thewheel rotates. The signal thus obtained is used bothto advance the strip chart and to gate on the scalerfor a 50 msec time period. Over the speed range(0-15 km/h) of the golf cart the chart drive speedis directly proportional to the vehicle speed. Onecentimeter on the chart paper corresponds to 43meters of vehicle travel. Although an effort is madeto maintain a fixed distance of 30 cm between theG-M tube and the beam-line devices during theconduct of the survey, experience has shown thatthis parameter is not critical. The number of countsregistered on the scaler from the G-M tube is, to afirst approximation, independent of the v€hiclespeed, and is proportional to the radiation intensityaveraged over a given length of enclosure. Theapproximation is good since the vehicle moves only20 cm per counting period at its top speed. Theequipment has been calibrated and the propor-tionality constants which relate total counts tomean radiation intensity have been determined forall segments of the various enclosures. Correctionsfor detector dead time are made to the observedcount rates.During normal accelerator operations, one dayof each week is devoted to routine maintenancework which requires access to beam-line enclosuresby work crews. Immediately prior to these accesses,BOOSTER PERIOD1000I I I I0 50 100 150METERS100.s::=."-a::E101.00.1 L...---.....:.:.....- ...:...-... ----IFIGURE 2 Radiation survey vehicle strip chart of the Fermilab Booster Accelerator showing a plot of radiationintensity vs position. The numbered marks at the top of the figure indicate the position of the short straight sectionin each Booster period. This survey was made on October 23, 1975.RADIATION LEVELS AT FERMILAB 113radiation surveys are conducted in all of the en-closures in order to establish safe working con-ditions and maximum occupancy times. Theenclosures in which the radiation survey vehicleis used include the 8-GeV Booster Accelerator,the 400-GeV Main Ring, and the Switchyard.RESULTSFigures 2, 3, and 4 are reproductions of strip chartsobtained with the survey vehicle in the Booster, theMain Ring, and the Switchyard, respectively. Itshould be noted that due to the highly com-pressed distance scale on these strip charts, theextremely narrow peaks which appear as "finestructure" on the broader features are real and arereproducible.The Booster chart shown in Figure 2 was madeon October 23, 1975, I! hours after the beam hadbeen turned off. The important features on thisgraph are the injection (period 1) and extraction(period 13) regions of the accelerator. Also clearlyvisible in this plot are the increased levels of radio-activity present in all of the 24 short straight sec-tions, located at the period markers. The MainRing chart shown in Figure 3 was made on October16, 1975, 24 hours after the beam had been turnedoff. The major loss points in the Main Ring are theTransfer Hall where both injection and extractiontake place and the beam abort target located in Dsector. A typical Switchyard chart is shown inFigure 4. This survey was conducted on November3, 1975, 7 hours after the beam was turned off. Thehighest loss point in the Switchyard is the shield atthe upstream end of Enclosure C. This devicelimits the size of the beam entering the beam split-ting (Lambertson) magnets. Other loss. points inthe Switchyard include the Lambertson magnetslocated in the upstream--ends of Enclosllres·B ,andC SECTORF SECTORIe-ojB SECTORE SECTOR100 300 500I I I , I Io 200 400D SECTORA SECTORIA-O I1000100..r:."'-crE 101.00.1ID-ol1000100..r:. 10"'-0:::E1.00.1L....L...iL.--.1" '" 1"'1 II It., IeFIGURE 3 Radiation survey vehicle strip chart of the Fermilab Main Ring. The event marker at the top of the chartindicates the location of quadrupole magnets ("" 30 m apart) for horizontal axis calibration. This survey was madeon October 16, 1975.114 J. H. McCRARY AND H.H. CASEHOLT, Jr.I I Io 50 100METERS1000100~ 100::E1.00.1ENCLOSURE B ENCLOSURE C ENCLOSURE E I ENCLOSURE 0 IFIGURE 4 Radiation survey vehicle strip chart of the Fermilab Switchyard showing radiation intensity vs positionon November 3, 1975.FIGURE 6 Radioactivity decay plot for the Main Ringshowing relative radiation intensity vs hours after beam off.Hours after beam offC and the electrostatic septa in Enclosures Band E.The scaler data permit the study of long-termchanges in residual radioactivity throughout theaccelerator. To make these data more meaningful,corrections are made to the scaler readings for theshort-term decay in activity which takes placeduring the first few hours after the accelerator hasbeen turned off. Figures 5, 6, and 7 show typicaldecays of residual radioactivity in the Booster,Main Ring, and Switchyard enclosures, respec-tively.Figures 8 and 9 show the radiation intensityresulting from residual radioactivity in the BoosterAccelerator as a function of time. Figure 8 containsplots of the radiation intensity, measured at a1.0~'(j').8c(1).~§ .6'B:.0e.4Q)>~&.20 2Main Ring4 6 10 20 40 60 100 200 400Hours after beam offoL----2.L...----L..-....L4---J-6.J.....-1...-L...1-L.1o---'2o-...J.--4...L.-o--L-..I6o~........10.L-0--2.....L.0-O---J---J1.0~ .8'(i;c(1)~ .6co'"5:0 .4EBooster1.0~.8'(j')c~.£c .60~:.0e.4(1)>~.2Q)et:0 2Switchyard4 6 10 LD 40 60 100 LDOHours after beam offFIGURE 5 Radioactivity decay plot for the Booster acceler-ator showing relative radiation intensity vs hours after beam off.FIGURE 7 Radioactivity decay plot for the Switchyardshowing relative radiation intensity vs hours after beam off.RADIATION LEVELS AT FERMILAB 115o 10 20 30 40 50 60 70A SON D I J F M A M J I J I A SON DIJ1974 1975 1976FIGURE 8 Mean radiation intensity vs time for the totalBooster enclosure and for the 200-MeV proton injection seg-ment of the Booster enclosure. For Figures 8 through 14 allintensities have been corrected to their value at 8 hours afterbeam off, and dashed lines indicate no surveys during theseweeks.I~101 f-Booster Injection ,:.f\\ _:. nJLr-~.-' \ J-l,i1'unIlf..J ~r\JU lr ~\ ;1 UlJ1\ V lr'0t "!\:1r11_ JLf\ (" \ J/ 'VI \1\ 'IS. ' \Total Booster . \"jU--Time (weeks)distance of 30 cm from the Booster magnets,averaged over the injection segment and over the471 m circumference of the accelerator. All radia-tion intensities have been corrected to indicatetheir value at eight hours after the beam was turnedoff. Figure 9 contains similar plots of data for theextraction segment of the Booster Ring and for thetotal Booster less injection and extraction. Figure10 shows the radiation intensity (8 hours afterbeam-off) averaged over the 6.28 km circumferenceof the Fermilab Main Ring and over the 150-m-long Transfer Hall. Figure 11 contains a similarplot of radiation intensity for the Main Ring lessthe Transfer Hall and the beam abort segments.Figures 12, 13, and 14 show the mean radiationintensities in the Switchyard enclosures. Meanintensities were not measured in the Switchyardprior to January 1975.Errors in the measurements shown in Figures8-14 are estimated to be within ±20 %. Thisdegree of accuracy is sufficient since the primaryfunction of these data is to show long-term trendsin residual radioactivity. Figure 15 is a graph of thetotal number of protons accelerated per week inthe Main Ring as a function of time. Prior to July1975, the Main Ring was operated at 300 GeV.400-GeV operation was begun in late July 1975,and continues until the present writing. To someI~10'Booster Extractionfl" rui~1f'\ \ l\jUVBooster-(In~o a ExL) flJ\rJL/I J\v" '11fI \ , ! \, I '; , :'V : Lol./l.) 0' ,Time (weeks)10'-o 10 20 30 40 50 60 70I A I S I 0 I N I D I J I F I M I A I M I J I J I A I S I 0 I N I DIJI1974 1975 1976FIGURE 9 Mean radiation intensity vs time for the 8-GeVproton extraction segment of the Booster enclosure and for thetotal Booster less injection plus extraction.Time (weeks)o 10 20 30 40 50 60 70I I I I I I I I I I I I I I I I I I IASO NO J FMAMJJ A SO NDJ1974 /975 1976FIGURE 10 Radiation intensity vs time for the FermilabMain Ring and for the Transfer Hall.116 J. H. McCRARY AND H. H. CASEBOLT, Jr.I~Main Ring - (AO 8 DO) !'if10° N'''/ :, ~'I '" II !' r'i . '. 11,1I~ 10'-Time (weeks)IAISI O INlol J IFIMIA'M'JIJ I A IS'O INIOI J '1974 1975 1976o 10Tim (weeks20 30 40 50 60 70o 10 20 30 40 50 60 70III IIIIII1 III II II ~ASONOJ FMAMJJ ASO NOJ1974 1975 1976FIGURE 13 Mean radiation intensity vs time for the Switch-yard Enclosure C.FIGURE 11 Mean radiation intensity vs time for the MainRing less Transfer Hall and Beam Abort.101Enclosure E \~~l)~\J\I~ i------_~ \ ' : U10°10 1I~ Time (weeks)'AISIO 'N10IJ IFIMIAIMIJIJ.lAlslo'N'oIJ I1974 1975 1976FIGURE 12 Mean radiation intensity vs time for the Switch-yard Enclosures Band D.20 30 40 50 60 70o 10 20 30 40 50 60 70IA I S I 0 I N10 IJ I F I M' A I M I J I J I A I S 10 IN 1D IJ I1974 1975 1976FIGURE 14 Mean radiation intensity vs time for the Switch-ya~d Enclosure E.extent the trends in Figure 15 are also present inthe residual radioactivity graphs (Figures 8-14).It should be pointed out that during the Summerof 1975 several important changes were made inthe alignment of hardware in the Booster injectionregion. These resulted in the pronounced peaks inthe two curves of Figure 8.,\.s,~::::~:__JV~"JJTime (weeks)10oRADIATION LEVELS AT FERMILAB 117CONCLUSIONSo 10 2r30 40 50 60 70I A IS I 0 'N I D J 'F I M' A' M I J I J I A IS ' 0 IN I Drr:1974 1975 1976FIGURE 15 Main Ring beam intensity (protons/week) vstime. "Off" indicates periods when the accelerator systemswere shut down for extended maintenance and developmentwork.Perhaps the most important feature of the datapresented here is the stability of the residualradioactivity as a function of time. Effects whichmight influence the long-term residual radioactivitylevels are beam-line losses, injection and extractionefficiency, proton energy, and proton beam inten-sity. The efficiency with which 8-GeV protons areinjected and captured into the Main Ring is ap-proximately 85 % to 90 % and is fairly stable intime. About 5% of the proton beam is lost atvarious points in the Main Ring during the three-second acceleration period. Main Ring extractionlosses vary between 1% and 1.5 %. The residualradioactivity present in the Main Ring enclosureis directly proportional to these losses.It may be noted that the change in Main Ringoperation from 300 GeV to 400 GeV in late JulyThe authors wish to express their appreciation to Dr. D. D.Jovanovic for many helpful discussions and to J. C. Baumanfor performing the computations involved in this work.ACKNOWLEDGEMENTS1975 is not noticeable in any of these plots. Itwould, therefore, seem reasonable to predict thatadditional increases in proton energy would notresult in marked increases in residual radioactivity.During August, September, and October 1975,there was a steady increase in beam intensity(Figure 15). A corresponding increase in residualradioactivity during this period is discernable inthe graphs shown here, but the radioactivity in-creases at a much lower rate than the beam inten-sity, indicating that beam losses during this periodrose more slowly than beam intensity.In conclusion, it can be said that since August1974, when measurements of mean radiation in-tensity began, the long-term trend in residualradioactivity has been rather stable in most partsof the Fermilab accelerator. Over periods of weeks,excursions related to accelerator output and effi-ciency have been noted, but most enclosures havenearly the same level of residual activity in late1975, as they had in mid-1974. The long-termresidual radioactivity data of the type presentedhere are important in predicting the conditionswhich will attend projected changes in acceleratorconfiguration and operation. From the past historyof residual radioactivity build-up as a function ofknown proton losses an estimate offuture radiationlevels within beam-line enclosures can be made,and safe working conditions and procedures inthese areas can be established.REFERENCES1. R. R. Wilson, Scientific American, 230, No.2, 72 (1974).2. R. E. Shafer and D. D. Jovanovic, IEEE Trans. Nuc. Sci.,NS-20, No.3, 499 (1973).OFF-I t- -i I-OFF --II- OFFTime (weeks)OFF -I I-10 17.:::c:.Q)Q)3..........(/)co-0et

Residual Radioactivity In The Fermilab Accelerator enclosures

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