IEEE Transactions on Nuclear Science, Vol. NS-30, No. 2, April 1983

RADIATION SAFETY SYSTEMS AT BROOKHAVEN NATIONAL LABORATORY'SLOW-ENERGY ACCELERATORS

Charles W. Flood, Jr.Safety & Environmental Protection Division

Brookhaven National LaboratoryUpton, New York 11973

Brookhaven National Laboratory has severallow-energy accelerators in use at the present time.I intend to discuss the radiation safety systemsinstalled at five of these accelerators. Theaccelerators included are a Dynanitron, 3.5 MeV Vande Graaff, 60" Cyclotron, 41" Cyclotron and TandemVan de Graaff facility. All of these acceleratorsare capable of producing radiation levels in excessof 100 rem/h and the radiation safety systems aredesigned to prevent personnel from being exposed tohigh levels of radiation. For the purposes of thistalk I would like to place the accelerators in twodifferent categories. In the first category are theaccelerators which have safety systems that preventoperation unless the radiation facilities arecompletely enclosed and interlocked, thus preventingany personnel access. Included are the Dynamitron,the 60" Cyclotron, and the 41" Cyclotron. In thesecond category are the accelerators with safetysystems which allow access to any part of theaccelerator facility when the radiation levels arelow, but require complete personnel restrictions whenthe radiation levels are high. Included are the 3.5MeV Van de Graaff and the Tandem Van de Graaff.

In addition to the different requirements foreach category there are many requirements that arethe same. These include: access to the acceleratorarea or target area must be controlled either bydoors with interlocks or by light beams; inspectionstations throughout the facility which must beactivated before the accelerator can produce highradiation levels; scram buttons that when pushed willshut the accelerator down must be available in allactive areas; and reinspection is required if anaccess interlock is violated or a scram button ispushed.

Dynamitron

The Dynamitron is an electron accelerator with ausable energy range of .3 to 2.5 MeV. The maximumbeam current is 5 mA. Typical operating perimetersare energies from 1 to 2 MeV with beam, currents inthe order of 200 p A. This facility is used to studythe interaction of electrons with solid matter. Theaccelerator is mounted in a vertical position withthe beam tube connected to a 900 bending magnetlocated in the magnet room (Fig. 1). The magnet canbe rotated in the plane of the exit beam line todirect the beam through a vacuum line to one of fourexperimental target areas. The combination ofaccelerator area, magnet room and the experimentaltarget area to which the beam line is connectedconstitutes the active accelerator facility. Thethree remaining target areas are not part of theactive interlock system and can be entered withoutshutting down the accelerator. Each active area mustbe secured. That is, entrances must be properlyclosed within two minutes after activation of theinspection station. The first area to be secured isthe machine area which is immediately above themagnet room. Two keys are necessary to secure thisarea, the beam key which is captured outside themagnet room door and the power drive key which iscaptured on the upper level. When the double doorson the upper level have been closed and secured, theinspection station activated, the power sir-ive key

removed, and the gate covering the lower part of theladder closed, the interlock system for the machinearea is secured.

Figure 1. Dynamitron

The magnet room is the next area to be se-cured. An operator using the beam key to operate theswitch outside the magnet room door, confirms thatthe ladder gate is closed and that the magnet isconnected to the correct target area, activates themagnet room inspection station, closes the magnetroon door, and removes the beam key from the switch.The interlock system for the magnet room is nowsecure.

To secure the target area, the area selectorswitch in the control room and the bending magnetmust be set to the same target area. The beam key isused to operate the switch outside the proper targetarea, the inspection station in the target area isactivated, the door is closed, and the key removedfrom the switch. The interlock system for the targetarea is secured. Both the beam key and the powerdrive key must be captive in the console before theaccelerator can start. Throughout the facility thereare radiation monitors with alarm points which turnoff the accelerator when exceeded. If this occurs,the system can be reset at the console, andreinspection is not required. The monitor in theactive target area does not alarm.

Cyclotron

The 60" Cyclotron can accelerate protons toapproximately 35 MeV, deuterons to 23 MeV, 3He to40 MeV, and 4He to 60 MeV, with currents of approx-

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imately 100 pA. The 41' Cyclotron, which is stillbeing tested, can accelerate protons to 17 MeV anddeuterons to 10 MeV. Both these facilities are usedfor the development and production of short-livedisotopes for medical research.

The 60" Cyclotron has two main radiation facil-ities, the Cyclotron vault itself and the target roomlocated north of the vault area (Fig. 2). The vaultis provided with two concrete doors, one large, forCyclotron disassembly (seldom used) and the othersmall, with a modest labyrinth inside which is usedfor normal access. There is a light beam across theentrance at the end of the labyrinth and two inspec-tion stations which must be activated in the vaultarea. This may occur in any order or time frame.Near the labyrinth entrance is a third station thatdeactivates the light beam for 5 seconds so exit fromthe vault without tripping the interlock system ispossible. If the light beam is broken it is neces-sary to reset both inspection stations. Both doorshave interlocks and must be closed in order to startthe Cyclotron. If either door is opened the inter-lock system is broken and reinspection is required.There is a radiation monitor in the vault but it isnot part of the interlock system. After the roomlights have been extinguished, which provides avisible signal that the Cyclotron is ready to start,the door can be closed. Auxiliary lighting is pro-vided which is adequate for egress. While the dooris closing a gong sounds to provide an audible warn-ing. The target area door has interlocks and thereis an inspection station inside the target area. Theinspection station must be activated, the lights shutoff, and the door closed before the Cyclotron canstart. If the door is opened the Cyclotron will shutdown.

Figure 2. Cyclotron

The 41" Cyclotron has two inspection stationsin the vault area, that can be activated in anyorder, also the door must be closed before the Cyclo-tron can be started. A light beam will be installednear the entrance doorway. If this beam is brokenreinspection will be required. The area of the 41"Cyclotron was part of the original target area of the60" Cyclotron. The target area was decreased in sizeto make room for the 41" Cyclotron. Shielding is de-signed such that the area between the target area andthe 41" Cyclotron may be occupied by personnel wheneither or both are running.

3.5 MeV Van de Graaff

The 3.5 MeV Van de Graaff is a horizontal typemachine that can accelerate beams of protons, deuter-ons, tritons, 3He and 4He (Fig. 3). The maximum

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Figure 3. 3.5 MeV Van de Graaff

beam currents on target are 100 pA for protons anddeuterons and 30 lpA for tritons and singly ionizedhelium ions. There are light bean barriers atEntrances 1 and 2, and door interlocks at Entrances3, 4, and 5. The facility is divided into two zones,the machine area and the target area. The machinearea has ganna ray monitors while the target area hasboth ganma ray and neutron monitors. The informationfrom these monitors is fed into the radiation protec-tion control unit which is located at the console.Three conditions are indicated by the radiation moni-toring system: the green condition, radiation levelsbelow 1 mrem/h free access allowed; the yellow condi-tion, radiation levels between 1 mrem/h and 10mren/h, warning but no interlock requirements; thered condition, radiation levels above 10 mrem/h, noaccess, activation of interlock system required.

There are five inspection stations in thefacility, two in the machine area and three in thetarget area. In order to set a zone all entrancesmust be closed and the inspection stations activated.To complete activation of a zone it is necessary toactivate an inspection station in the other zone orthe one at the console within 30 seconds of breakingthe related light beams.

Most of the experiments performed on thisaccelerator do not produce high levels of radiationso that free access is normally permitted. Thissafety system is a simplified version of that used atthe Tandem.

Tandem Van de Graaff

The Tandem Van de Graaff facility consists oftwo large horizontally installed Tandem, Van de Graaffaccelerators, MP 6 and MP 7. Each is rated for thir-teen MV on the high voltage terminal. The two ma-chines are installed in series so that the firstaccelerator can be used to inject a bean into thesecond accelerator, thereby achieving higher energiesthan can be achieved with either of the two machinesoperating independently. This mode of operation,where one machine injects into the other is called

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"Three-Stage Acceleration". Figure 4 shows the floorplan for the Tandem Van de Graaf f facility. As canbe seen it is rather complex, two accelerators in one

large roon with a basement area under the accelera-tors, four target rooms and several possible beamlines in each room.

The Tandem safety system allows access to theaccelerator areas while the machines are in operationwithout breaking the integrity of the interlocks.This is accomplished by subdividing the area intozones. Interlocks are established where needed butnot bypassed or defeated in a conventional sense.Perineters which resemble more conventional door in-terlocks, guard the many entrances to the acceleratorarea and thereby provide a second level of indepen-dent protection. Because all safety systems relayultinately on the good will of the staff involved,every effort has gone into making this system easierto use than abuse.

The accelerator complex has been divided intoeleven zones. Four on the main floor, four in thebasement area under the accelerators, and three forthe infrequently used passageways which lead into theaccelerator room itself. Each zone contains one ormore inspection stations. Each inspection stationincludes a push button switch for activation and anamber indicAting light for acknowledgement. Stationsare located wherever inspection is advisable and inparticular at all zone entrances or exits. Violationof the barrier (door, gate or light beam) either bysomeone entering or leaving a zone deactivates theassociated stations unless an inspection station inthe next zone is activated to set the zone. Theactivation of an inspection station is acknowledgedby a flashing of the amber light at the station. Theoperator proceeds under the time constraint of 40seconds to activate the next station.

Four radiation monitors are mounted in theaccelerator room, at the low energy end and at thehigh energy end of each machine. Radiation monitorsare also located in each of the four target rooms.These monitors are capable of monitoring gamma raysas well as fast neutrons. Information from the moni-

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Figure 4. Tandem Van de Graaff

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tors is fed back into the control room and is dis-played on the master display panel. The radiationmonitors activate a three-level system. The lowlevel, which is called the green or unlimited accesslevel, corresponds to radiation levels from 0 to 2mrem/h. The yellow level, or the conditional accesslevel corresponds to radiation levels from 2 to 50mrem/h and the red level or no access level is 50mrem/h or greater. When a radiation monitor indi-cates either a red or yellow condition, rotatingbeacon lights of either red or yellow are activatedin that zone to warn people that the adjacent zone isreceiving radiation. If the zone has not been se-cured and the radiation monitor goes into a reedcondition, the accelerator or both accelerators willbe shut down preventing any further production ofradiation until the zone has been secured.

To work in conjunction with the zones and theradiation monitors in each zone there is a require-ment that a perimeter control be set up. This issimilar to a standard interlock system seen at mostaccelerators. Essentially, interlocks or light beamsare placed on all entrances to the accelerator room.If these entrances are violated, Faraday cups are

automatically inserted into the ion beam. Theseinterlocks are independent of the radiation monitor-ing system and the zones that have been secured. Thislimits the ability of people to gain access to thefacility. The only door that is not interlocked inthis regard is the heavy shielded door between thecontrol room and the accelerator room itself.

The target rooms have their own safety sys-tems. Each target room has a radiation monitor. Ifthe machines are running in a three-stage fashiononly one target room can be functioning. If they are

running in a single- stage fashion, Target Room Icould be used with MP 6 and Room 2, 3 or 4 could beused with MP 7. Shields in the beam lines to Target

Rooms 2, 3 and 4 are designed to permit removal ofonly one at a time. The radiation monitor must beplugged into the desired line in order to have theshield plug removed. If the radiation monitor acti-vates the red level, it is required that the heavyshielded access door to the target room be closed.Inspection of the target room to insure that no oneis there is required and the door must be closedwithin the time restraint.

Figure 5 shows the master display panel whichis located in the control room. It indicates theradiation condition by green, yellow or red lightsfor each of the monitors, in the accelerator or tar-get rooms. It indicates whether the zone has beensecured by a purple light and whether the perimeteraccess point is secured by a white light. In thelower right-hand corner is the emergency stop infor-mation for the emergency stops around MP 6, MP 7 andin each of the target rooms. With this informationat the console, the operator can determine whichareas are secured, which perimeter has been set, whatthe radiation levels are in any of the areas whereradiation monitors are located.

Even though this facility is a very complexsystem the interlock system is designed to make it asversatile as possible, such that it should never benecessasry to bypass the interlock system. The sys-tem is designed to be failsafe and redundant and notdependent on any one particular monitoring mode todetermine if it is safe for access.

Therefore, at Brookhaven National Laboratory wehave gone from the simple straightforward - keepingpeople out of an area - to the much more complex sys-tem which allows access under safe conditions andwill enforce non-access when the conditions arehazardous.

Figure 5. Master Display Panel