JLAB-TN-04-012 27 April 2004

Technical Justification and Mitigations for Removal of Certain CARMs from the Personnel Safety System

Scott Schwahn and Erik Abkemeier,

Radiation Control Group, Jefferson Lab

Introduction The Controlled Area Radiation Monitors (CARMs) were designed into the Personnel Safety System (PSS) in the early days of the Continuous Electron Beam Accelerator Facility (CEBAF) machine. The intent of the monitors was twofold: to ensure that nominal radiation from machine operation was measured, and also to provide emergency shutdown of the machine in the case of a non-routine, and even catastrophic, loss of beam. After greater than ten years of CEBAF operation and changes to the facility including shielding penetrations, and the development of high reliability beam loss monitoring, it can be shown that several of the CARMs are no longer needed for protection to the workers or to the public. Continued use of these CARMs requires valuable personnel resources, and carries some increased risk for machine down time. This paper serves as a proposal to remove many of the CARMs from the PSS.

Scope Evidence supporting the removal of certain CARMs includes the following: calculations, experimental measurements, active monitoring records, passive monitoring records, and credible loss scenarios. Depending on the type of CARM being considered, there are different considerations. The CARMs to be considered for removal fall into six categories:

• LINAC CARMs (RM07, RM09, RM10, RM18, RM20, RM21) • Steered Beam CARMs (RM03, RM06, RM11, RM12, RM17, RM22, RM23,

RM24, RM27) • Exit Stair (NON-BSY) CARMs (RM08, RM13, RM14, RM19, RM25, RM26) • BSY Exit Stair CARM (RM 28) • Inter-Hall Protection CARMs (RM40, RM42, RM44, RM45) • Redundant CARMs (RM04, RM15).

Each of these categories of CARMs will be considered, and justification will be given on whether or not their continued use or removal is merited. The relative positions of each of the CARMs, while not exact, are displayed on Figures 1-4. It should be noted that some CARMs will not be considered in this paper, due to the complexity of justification and/or calculation or the simple fact that their utility is proven. The CARMs that will not be considered are the CARMs protecting the drop hatches (RM02, RM05, RM16), CARMs that protect workers in the end stations from radiation produced in the accelerator (RM29, RM30, RM31), and CARMs that monitor radiation

JLAB-TN-04-012 27 April 2004

from the end stations (RM41, RM43, RM46, RM47, RM48). Additionally, the counting house CARMs (RM 32, RM 33, RM 34, RM 35, RM 36, RM 37, RM 38, and RM 39) are not recommended for removal per se, but are recommended to be replaced with upgraded CARM ADM-616 units that have 3 probe capabilities instead of two, as is the case with the older CARM ADM-610 units. As funding becomes available, the remaining ADM-610 units will be upgraded to ADM-616s or better as technology permits. It should also be noted that CARMs are not identified as “critical devices” in PSS documentation, and therefore “three devices, two technologies” is not required.

Figure 1

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Figure 2

Figure 3

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Figure 4

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Summary Table The following table summarizes the findings in this report. Table 1

“Normal” Losses “Accidental” Losses

CARM Type Design

BLA (2.5 µA)

BLA (25,000 µA-µS)

Full Power, nominal PSS cycle time* (800 kW, 1 second)

Confirmed by Measurement?

Recommend removal?

LINAC 0.200 mrem/h (10 W)

2 mrem/h (10 kW)

0.00078 mrem

4.2 mrem Yes Yes

Steered - spreaders, arcs, recombiners

0.200 mrem/h1 (30-161 W)

12 to 67 mrem/h1 (10 kW)

1.4E-4 to 7.4 E-4 mrem

1.1 to 5.9 mrem

Yes Yes

Steered - injector

0.200 mrem/h1 (250 W)

N/A N/A N/A No Yes

Exit Stairs (Non-BSY)

0.0017 mrem/h (10W)

2.6 mrem/h (10 kW)

0.7E-3 mrem

0.04 mrem No Yes

BSY Exit Stair

0.029 mrem/h (10W)

43.2 mrem/h (10 kW)

1.2E-2 mrem

0.64 mrem No No

Inter-Hall 0 mrem/h

N/A N/A 0 mrem/h Yes Yes

Redundant N/A N/A N/A N/A Yes Yes It is interesting to note that in all areas, if the Beam Loss Accounting (BLA) system were to allow maximum beam current without tripping (below 2.5 µA), dose rates in the buildings would approach 1’s to 10’s of mrem/h. However, this level of beam loss is not reasonable as it is unlikely that the accelerator would be able to function with a sustained “point source” beam loss at just under the BLA setpoint at high energy.

*Nominal PSS cycle time is one second as identified in Jefferson Lab EH&S Manual Chapter 6310 T, and is considered a long time relative to the BLA cycle time. 1 Assumes 25% occupancy factor

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LINAC CARMs (RM07, RM09, RM10, RM18, RM20, RM21) The LINAC CARMs are identified as those that are directly above the cryomodules and no major steering components (save for the small steering available from the corrector magnets between the cryomodules and missteering due to cavity tuning effects). Between the LINAC and the aboveground CARMs are 125 cm (4 feet) of concrete and 300 cm (10 feet) of earth1. The density of concrete was assumed to be 2.35 g-cm-3 and the density of the earth was assumed to be not less than 1.92 g-cm-3. There are, however, penetrations built through the shielding to accommodate wave-guides and cables. These weaker points in the shielding are the areas to which these calculations refer.

Calculations A JLAB Tech note was written in 19952 that detailed the shielding necessary to reduce potential exposure from the machine through the penetrations. The calculations showed that, for the areas above the LINACs, the maximum exposure rates occur with the highest design energy beam. The normal power loss was assumed to be 10 watts, and the maximum credible loss was assumed to be 800 kilowatts. The more restrictive of the shielding requirements based on these two loss terms became the driving factor, and in this specific case, 800 kilowatt was the maximum credible loss scenario. The penetration shielding calculated to reduce the dose equivalent rate in the service building to 15 rem/h was 15-16” of pea gravel, and the penetrations were filled with at least this amount, measured by weight. The Machine Protection System (MPS) Beam Loss Accounting (BLA) system limits beam loss anywhere in the machine to 2.5 µA and the integrated beam loss to 25000µA-µs.3 For a 6.0 GeV machine, this is equivalent to a 15 kW distributed loss or an integrated point loss of 0.15 kW-s. If this entire loss occurred in the worst possible spot in the linac where a person was present, it would result in a dose of

µrems

hskWkWhrem 78.0

3600115.0

800/15

=⋅−⋅

A dose below 1 mrem is considered de minimus for the purposes of Jefferson Lab radiation protection. If one were to take into account even a conservatively long loss for one second at full power (assume 800kW-s), the total dose to an individual on the surface would be 4.2 mrem. This low dose cannot be accurately measured or distinguished from other occupational exposure using state of the art personnel dosimetry.

Measurements Initial measurements were made2 after prototype shielding was placed to verify the effectiveness of the shielding against both photon and neutron radiations. While the sources were isotopic rather than produced by the machine, the test verified the utility of the calculations. Additionally, a test procedure entitled “Radiation Measurements Near and Above Dumplets” was conducted on December 19, 2002 in order to verify calculations of radiation transport through thick shielding and penetrations. This test procedure was

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conducted by taking gamma and neutron dose rate measurements directly above ground with 7 microamp pulsed beam being delivered to the North LINAC Recombiner Dumplets. Using the same methodology as that from the 1995 Tech Note for determining penetration fillings, the predicted dose rates for 0.9 GeV beam were 0.130 mrem/hr neutron and 0.036 mrem/hr gamma. Actual observed dose rates were 0.017 mrem/hr neutron and 0.010 mrem/hr gamma. This additional margin of safety from calculated to actual radiation measurements (possibly due to some degree to the soil type and moisture content, which is more dense than the average soil densities used for calculational purposes) indicates that measured radiation levels in the event of an accident would be lower than calculated.

CARM Data An analysis of historical CARM data indicates that no missteering events were detected by CARMs in this region.

TLD Data In 1989, there were 2 instances in which TLDs adjacent to CARMs have shown measurable dose, both in the South LINAC, one on RM 18 at 19 mrem, and one on RM 20 at 13 mrem. It should be noted that the accelerator was undergoing commissioning at this time, and intermittent beam loss of greater magnitude may be experienced during commissioning.

Conclusions regarding LINAC CARMs While the LINAC CARMs were the only reliable devices for turning off beam in case of radiation exposure in occupiable areas in the early days of the CEBAF machine, they are no longer required. The CARMs are intended to shut off beam in the event of an unusual loss of beam, rather than either monitoring or shutting off beam during normal operations. The maximum credible loss was considered - one that would be catastrophic to the machine; total personnel dose at the worst possible point directly above a penetration was at worst 4 mrem. It is exceedingly unlikely that this situation could occur, and increasingly unlikely that the MPS Beam Accounting System would have failed and that the radiation probes for the unit would be in exactly the same spot for beam shutoff all at once. Because of the 3 probe capabilities of ADM-616 unit CARMs and the relatively high occupancy of the area, it is recommended that one CARM unit with a neutron probe at each end of the LINACs and the middle, directly over penetrations, be installed in each LINAC service building. For the North LINAC service building this would be in the region of RM 06, RM 09 and RM 11. For the South LINAC, this would be in the region of RM 17, RM 20 and RM 22. See Figures 7 and 8 for a rough approximation of the proposal.

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Steered Beam CARMs (RM03, RM06, RM11, RM12, RM17, RM22, RM23, RM24, RM27) The Steered Beam CARMs are identified as those that are potentially affected by loss of beam in the steering components of the machine. Specifically, these CARMs are above injector components, spreader and recombiner sections, east and west arcs, and the beam switchyard (BSY). Between these beamline components and the aboveground CARMs are also 125 cm (4 feet) of concrete and 300 cm (10 feet) of earth. In all but the BSY, there are penetrations built through the shielding to accommodate cables, and the calculations refer specifically to the penetrations, as these are the weakest points in the shielding.

Calculations JLAB-TN-95-026 also calculated dose rates from these steering areas, which were considered separately as high routine loss points. Detailed consideration was given to maximum credible losses, including normal beam trajectory, trajectory caused by total loss of power in a magnet (understeering), and trajectory caused by the loss of a magnet shunt (oversteering). For the spreaders, the limiting calculation was due to normal losses (with the resultant need to keep aboveground dose rates to less than 200 µrem/h) rather than the full loss scenario. The normal loss was assumed to be 161 W directly below the penetration where a person is spending 25% of the work year (see the tech note and supporting tech notes for further explanation of this number). The penetration shielding calculated to reduce the dose equivalent rate in the service building to 15 rem/h was 85” of pea gravel, and the penetrations were filled with at least this amount, measured by weight. For the recombiners and the arcs, the limiting calculation was also due to normal losses rather than the full loss scenario. The normal loss was assumed to be 30 W directly below the penetration where a person is spending 25% of the work year (again, see the tech note and supporting tech notes for further explanation of this number). The penetration shielding calculated to reduce the dose equivalent rate in the service building to 15 rem/h was 78” of pea gravel, and the penetrations were filled with at least this amount, measured by weight. The injector CARMs were considered after JLAB-TN-95-026 was written, but using the same methodology, it was determined that the shielding was required to be 90” of pea gravel. This amount of gravel was required because the radiation from the lower energy electrons are not so forward-peaked as those from the higher energy electrons, and thus more radiation is aimed directly toward the tunnel ceiling. Once again, the limiting factor was normal losses rather than catastrophic loss. Feedback from the (then) Injector Group Leader4 indicated that losses were:

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Loss Region Normal Losses Complete Losses 100 keV 0.1 mA 15 mA 500 keV 0.05 mA 2.5 mA 5 MeV 0.02 mA 2.5 mA 25 MeV 0.01 mA 2.5 mA However, the injector is now completely new, and a review of loss terms provided by the Source Group Leader5 yields the following: Loss Region Normal Losses Complete Losses 100 keV 50 µA 1 mA 500 keV 10 µA 1 mA 5 MeV 5 µA 1 mA 60 MeV 2 µA 1 mA It is reasonable to say that for the assumptions presented, the reduced loss terms for the injector means that the shielding is functioning more than adequately in the area. The Lab could, in the future, reduce shielding in the areas to match the new dose equivalent rates, so a decision to remove the CARMs should be based on the original calculations for shielding. Again, using the MPS BLA system limits of 2.5 µA and 25000µA-µs, we now look at the spreader routine loss dose rates to quantify that loss which would be detected and stopped by the MPS:

µrems

hskWkWhµrem 052.0

3600115.0

161.0/200

=⋅−⋅

and in the arcs and recombiners:

µrems

hskWkWhµrem 27.0

3600115.0

030.0/200

=⋅−⋅

and finally in the injector:

µrems

hskWkW

hµrem033.0

3600115.0

25.0/200

=⋅−⋅

Again, these are de minmus doses. The main reason for this low dose is the fact that the shielding is more than adequate for a single high loss of beam, and the limiting factor was shielding against routine losses.

JLAB-TN-04-012 27 April 2004

Measurements Initial measurements were made2 after prototype shielding was placed to verify the effectiveness of the shielding against both photon and neutron radiations. While the sources were isotopic rather than produced by the machine, the test verified the utility of the calculations. Subsequent radiation measurements above the penetrations have not revealed any detectable radiation.

CARM Data There have been no indications to date of a CARM legitimately detecting loss events in the tunnel in these areas.

TLD Data There have never been any positive doses recorded on the TLDs that have been placed in these areas. The lower reporting limit for the quarterly dosimetry changeout is 10 mrem for photons.

Optics Notes As identified previously, the Source Group Leader provided the loss terms as reasonable assumptions concerning losses due to injector optics.

Conclusions regarding Steered Beam CARMs While the Steered Beam CARMs were the only reliable devices for turning off beam in case of radiation exposure in occupiable areas in the early days of the CEBAF machine, they no longer are required. The CARMs are intended to shut off beam in the event of an unusual loss of beam, rather than either monitoring or shutting off beam during normal operations. It turns out for these higher loss regions that the shielding is dominated by the routine loss terms since the ratio of routine loss current to full loss current is higher than the ratio of 200 µrem/hr to 15 rem/hr. The CARMs were not intended to respond to routine beam loss, and the dose rates associated with these routine beam losses are so small that the CARMs would have no possibility of being able to measure them. Because of the 3 probe capabilities of ADM-616 unit CARMs and the relatively high occupancy of the area, it is recommended that one CARM unit with a neutron probe at each end of the LINACs and the middle, directly over penetrations, be installed in each LINAC service building. For the North LINAC service building this would be in the region of RM 06, RM 09 and RM 11. For the South LINAC, this would be in the region of RM 17, RM 20 and RM 22. See Figures 7 and 8 for a rough approximation of the proposal.

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Exit Stair CARMs (RM08, RM13, RM14, RM19, RM25, RM26) The Exit Stair CARMs may be found at the top of each of the exit stairs on the accelerator site. They are on the outside of the building, while the inside of the building is an Exclusion Area just like the tunnel. The following dimensions are the dimensions of all of the exit stairs (with the exception of the BSY exit stair) based on physical measurements of each portion from the beam line through each section of the stairs: Width Height Length 5’4” 10’ 25’6” 4’ 11’6” 28’ 4’ 11’6” 5’ 4’ 11’6” 16’6” 4’ 11’6” 5’ 4’ 11’6” 16’6”

Calculations Initial calculations may be found in JLAB-TN-87-00611, regarding radiation streaming up labyrinths, specifically, exit stairs. Appendix 3.7 Tech Paper No. 4 indicates that the dose rates at the bottom of three flights of stairs should be approximately 10.8 mrem per hour for a nominal 10 W electron point loss. [This number has been corrected from the original note to account for an increased width of access way (4 ft. to 5.4 ft.) required after the note was written, as well as to correct for errors in reading “Universal curves” for labyrinths]. The following is a breakdown of the calculations, which are derived from applying the labyrinth leg distance over the square root of the area of the labyrinth (i.e., width times height) to the applicable Universal dose transmission curve (Fig. 4.43 and 4.44 of NCRP Report 144 “Radiation Protection for Particle Accelerator Facilities”): Leg 1:

nattenuatioxA

d 21085.3 −⇒≅ from universal neutron dose transmission curve for

first leg Leg 2:

nattenuatioxA

d 2105.112.4 −⇒≅ from universal neutron dose transmission curve for

second and succeeding legs Leg 3:

nattenuatioxA

d 11027.0 −⇒≅ from universal neutron dose transmission curve for

second and succeeding legs

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Leg 4: nattenuatiox

Ad 21044.2 −⇒≅ from universal neutron dose transmission curve for

second and succeeding legs Leg 5:

nattenuatioxA

d 11027.0 −⇒≅ from universal neutron dose transmission curve for

second and succeeding legs Leg 6:

nattenuatioxA

d 21044.2 −⇒≅ from universal neutron dose transmission curve for

second and succeeding legs So for a 10W point loss, this results in: ( )( )( )( )( )( )( )212122214 104102104102105.11081025.2 −−−−−−− ⋅⋅ xxxxxxmhrmremx =

31073.1 −x mrem/hr As one can see, the dose equivalent rate at the top of the stairs, using the same methodology as Appendix 3.7 Tech Paper No. 4 and including all of the labyrinth legs of the stairs, is approximately 1.73E-3 mrem/h. These dose equivalent rates do not take into account additional shielding provided by the exterior concrete wall between the top of the stairs and the CARM probes in their present locations. For a maximum beam loss within the BLA system of 2.5 µA (15 kW for 6 GeV) that is exactly in the worst possible spot, however, the resultant dose equivalent rate would be on the order of 2.6 mrem/h for a total integrated dose of 0.7 microrem. For a full beam loss in the machine of 800 kW, the dose equivalent rate is approximately 138 mrem/hour (with a 0.04 mrem integral dose for a one-second loss). A more reasonable loss assumption may be based on the CEBAF Final Safety Assessment Document6 which states that “a highly pessimistic loss assumption of 1 kW (is) lost throughout one arc region.” This assumption, even when condensed to a point loss directly in front of the tunnel entrance to the exit stairs results in a dose rate on the order of 0.173 mrem/hr or an integrated dose of 0.05 microrem. Again, in all of these scenarios, a point source loss would need to occur at the absolute worst spot, while a theoretical person would need to be at the CARM unit at the top of the exit stair. A slightly cynical view leads to the most likely scenario that a person would be in that area to investigate a CARM unit that has been performing erratically due to power fluctuations. If the CARM unit is removed, the probability of a person being in that area rapidly approaches zero.

CARM Data There have been no indications to date of a CARM legitimately detecting loss events in the tunnel in these areas. RCG personnel recollections indicate there may have been one

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legitimate CARM trip during the construction phase of the accelerator, and beam was terminated on purpose outside of the entrance to one of the exit stairs.

TLD Data There have never been any positive doses recorded on the TLDs that have been placed in these areas. The lower reporting limit for the quarterly dosimetry changeout was 10 mrem for photons. Previous monitoring sensitivity for neutrons was on the order of 20 mrem per quarter.

Conclusions regarding Exit Stair CARMs These Exit Stair CARMs are not necessary as credible accident scenarios result in dose below the normal detection level for TLDs. Additionally, service to the CARMs puts people unnecessarily in an area where maximum dose could be received in the event of an incredible beam loss. See Figures 7 and 8 for a rough approximation of the proposal.

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BSY Exit Stair CARM (RM28) The following dimensions are the dimensions of the BSY exit stairs based on physical measurements of each portion from the beam line through each section of the stairs: Width Height Length 5’4” 10’ 26’ 5’4” 10’ 25’ 5’4” 10’ 8’ 8’ 18’ 27’6”

Calculations Initial calculations may be found in JLAB-TN-87-00611, regarding radiation streaming up labyrinths, specifically, exit stairs. Appendix 3.7 Tech Paper No. 4 indicates that the dose rates at the bottom of three flights of stairs should be approximately 10.8 mrem per hour for a nominal 10 W electron point loss. [This number has been corrected from the original note to account for an increased width of access way (4 ft. to 5.4 ft.) required after the note was written, as well as to correct for errors in reading “Universal curves” for labyrinths]. The following is a breakdown of the calculations, which are derived from applying the labyrinth leg distance over the square root of the area of the labyrinth (i.e., width times height) to the applicable Universal dose transmission curve (Fig. 4.43 and 4.44 of NCRP Report 144 “Radiation Protection for Particle Accelerator Facilities”): Leg 1:

nattenuatioxA

d 21085.3 −⇒≅ from universal neutron dose transmission curve for

first leg Leg 2:

nattenuatioxA

d 31085.3 −⇒≅ from universal neutron dose transmission curve for

second and succeeding legs Leg 3:

nattenuatioxA

d 21081.1 −⇒≅ from universal neutron dose transmission curve for

second and succeeding legs Leg 4:

nattenuatioxA

d 2105.25.3 −⇒≅ from universal neutron dose transmission curve for

second and succeeding legs

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So for a 10W point loss, this results in: ( )( )( )( )232214 1081081081025.2 −−−− ⋅⋅ xxxmhrmremx ( )2105.2 −x = 21088.2 −x mrem/hr As one can see, the dose equivalent rate at the top of the stairs, using the same methodology as Appendix 3.7 Tech Paper No. 4 and including all of the labyrinth legs of the stairs, is approximately 2.9E-2 mrem/h. These dose equivalent rates do not take into account additional shielding provided by the exterior concrete wall between the top of the stairs and the CARM probes in their present locations. For a maximum beam loss within the BLA system of 2.5 µA (15 kW for 6 GeV) that is exactly in the worst possible spot, however, the resultant dose equivalent rate would be on the order of 43.2 mrem/h for a total integrated dose of 12 microrem. For a full beam loss in the machine of 800 kW, the dose equivalent rate is approximately 2.2 rem/hour (with a 0.64 mrem integral dose for a one-second loss). A more reasonable loss assumption may be based on the CEBAF Final Safety Assessment Document6 which states that “a highly pessimistic loss assumption of 1 kW (is) lost throughout one arc region.” This assumption, even when condensed to a point loss directly in front of the tunnel entrance to the exit stairs results in a dose rate on the order of 2.9 mrem/hr or an integrated dose of 0.8 microrem. Again, in all of these scenarios, a point source loss would need to occur at the absolute worst spot, while a theoretical person would need to be at the CARM unit at the top of the exit stair.

CARM Data There have been no indications to date of a CARM legitimately detecting loss events in the tunnel in this area.

TLD Data Since 1998, there has been a total of 31 mrem of gamma exposure on the TLD attached to the BSY CARM.

Conclusions regarding Exit Stair CARMs Although this CARM has only slightly elevated potential dose rates during potential accident scenarios, based on the noted TLD cumulative dose received on the environmental TLDs, and the location of the stairs close to the BSY, it is recommended that this CARM unit remain in place. See Figure 8 for a rough approximation of the proposal.

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Inter-Hall Protection CARMs (RM40, RM42, RM44, RM45) The Inter-Hall Protection CARMs are found in each of the end stations, and are designed to protect occupants of the end stations from radiation produced by experiments in the adjacent halls.

Calculations No calculations have been identified regarding the utility of these CARMs.

Measurements Two main experiments have been identified herein to support the removal of these CARMs from the PSS. First, a test plan titled, “Hall C Shielding Validation Measurements” was performed on July 27, 1994 in Hall C. The target was truly a “thick target,” being a slug of copper 3.8 cm (1.5 inches) in radius and 20 cm (8 inches) in length. (Note that this is a “1400% radiator” since the radiation length for copper is 1.44 cm). Among all the other tests performed, the CARM response in Hall B was checked. With a power loss in the Hall C target of 260 W and the full opportunity for electromagnetic shower production, there was no response whatsoever on the Hall B CARM. It should be noted that in the End Station Radiation Control Workshop7, a very thick target of 5% radiation length, under 0.5 GeV beam and full 200 µA current would generate a source term of 250 W, so the source term is very similar. However, the target used in the experiment was so thick as to fully allow for electromagnetic shower production, therefore overestimating the yield. For this reason, it is believed to be a very conservative approximation of the radiation produced during the worst-case real target scenario. It was again noted in the 1989 APARS Review8 that a 100W thick target equivalent yield was a reasonable level to consider to be a maximum loss. More recently, on June 28-29, 2002, there was a thick lead target test in Hall A. The dose rates were higher than all previous prompt dose rate records for Hall A (~10 times nominal):

• Inside the hall – up to 2 rem per hour (back of the hall) • Top of the roof – up to 50 millirem per hour • Site boundary – up to 10 microrem per hour

During this same time period, there was no response from any CARM in Hall B. See Figure 5 and Figure 6.

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Figure 5: Dose rates in Hall A

Figure 6: Dose rates in Hall B

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CARM Data There have been no indications of the Inter-Hall Protection CARMs responding to radiation from an adjacent hall, despite extremely high source terms in each. Halls A and C have both been tested, and Hall B cannot produce enough radiation from its typically less than 1µA of current to be of any consequence.

Conclusions regarding Inter-Hall Protection CARMs It is clear that the Inter-Hall Protection CARMs are unnecessary, as there is not enough radiation production in the adjacent halls to have an effect. See Figure 9 for a rough approximation of the proposal.

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Redundant CARMs (RM04, RM15) The Redundant CARMs are in the North and South Access buildings. There are two sets of CARMs in these buildings, one near the top of the elevator shaft, and the other near the rollup door near the drop hatch. Radiation measurements under bad conditions (i.e., that which produced undesirable radiation levels in the North and South Access Buildings) showed that the largest amount of scattering from tunnel radiation ended up at the far side of the building, near the probes for CARMs RM05 and RM16. Little radiation was seen at the positions of RM04 and RM15. These positions are shadowed by the elevator structural enclosure.

Calculations Initial calculations on the radiation produced from full power beam into the 45 MeV dump in the North Linac9 showed that the maximum dose rates above the drop hatch to be approximately 0.7 mrem/h if the target is thick and unshielded. When the target is thick and shielded (as in the case of the 45 MeV dump), dose rates may approach 0.04 mrem/h.

Measurements Measurements performed after the calculations and included in the tech note showed that the calculations were accurate to within about 10%. The physical position of the highest dose rates was above the drop hatch, toward the rollup doors.

CARM Data After shielding the dumps, and since only tuneup beam is put into the dumps, the CARMs have had no documented response to real radiation events within the tunnel.

TLD Data There have been no positive TLD readings from the area of these CARMs.

Conclusions regarding Redundant CARMs The CARMs already in the building are adequate and positioned so that radiation in the buildings will be detected appropriately. It is recommended that the redundant CARMs be removed, as they are indeed redundant. The probes for the remaining CARMs should be situated in such a position so as to maximize their effectiveness and not impede materials access through the hatches, perhaps mounted on the external wall of the building. See Figures 7 and 8 for rough approximations of the proposal.

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PROPOSED CARM SET-UP FIGURES Figure 7

Figure 8

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Figure 9

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Figure 10

JLAB-TN-04-012 27 April 2004

Overall Conclusions Concerning Removal of CARMs from the PSS System/Upgrade to ADM-616s The aforementioned recommendations allow for an initial upgrade from ADM-610s to ADM-616s. The ADM-616s have the capability of 3 probes instead of 2, and are not subject to a non-failsafe mode to power fluctuations. A total of 33 ADM-610 CARM units would be removed from the system initially, and 6 ADM-616s would be added. See Figures 7, 8, 9, and 10 for a rough approximation of the proposal. The cost of the ADM-616s would be on the order of $42,000. Twelve ADM-610 units and 2 ADM-600 units (on Hall A and C domes) would remain installed, to be replaced as funding is provided. Six ADM-610s would remain as spares on hand (currently there are no spares.) The 27 other ADM-610 units could be sold to another laboratory in order to cover the costs of ADM-616 units. A lab has been identified, and the money provided could be enough to cover the purchase of 3 ADM-616 units (but this has not been verified at this date.) The remaining 18 ADM-610 CARM units and 2 ADM-600 units would need to be retrofitted with the “watchdog alteration” in order to remove the non-failsafe power fluctuation condition. This would cost approximately $15,000, for a total of $57,000 for the initial modifications. Additionally, cabling for the removed CARMs will remain in place in order to facilitate rapid replacement of the units if desired. Work area TLD monitoring will continue (and in some cases be added) to the former CARM locations. In the event of annual doses exceeding 400 mrem per year (based on a 100 mrem dose to the general public limit, and a 25% occupancy factor) for any controlled area, or any doses indicating a significant beam missteering not terminated by the BLA system, or any other anomalously high personnel TLD readings traced to people working in a sevice building area, the RCM will immediately notify the JRRP, and take action to introduce CARM monitoring into the PSS for the affected area(s). 1 Radiation Control Review, TN-0061, June 16-18, 1987. 2 Schwahn, S. O., “Determination of Shielding Requirements for Service Building/Tunnel Penetrations,” JLAB-TN-95-026, April 28, 1995. 3 Ursic, P., Mahoney, K., Hovater, C., Hutton, A., and Sinclair, C. “CEBAF Beam Loss Accounting,” JLAB-TN-95-034, May 24, 1995. 4 Private email from Dr. Charles Sinclair to Scott Schwahn. 5 Private email from Dr. Matthew Poelker to Scott Schwahn, 13 Sep 2002. 6 CEBAF Final Safety Assessment Document (Revision 4) of April 5, 1994 7 End Station Radiation Control Workshop, TN-0095, September 26-28, 1988. 8 Advisory Panel on Accelerator Radiation Safety, December 8, 1989. 9 Schwahn, S.O., Dose Rate at North Access Hatch Due to 45 MeV Dump,” CEBAF-TN-93-034, 1993.