EUROPEAN LABORATORY FOR PARTICLE PHYSICS
CERN/TIS–RP/IR/95–19.1
Summary of Design Values, Dose Limits, Interaction Rates etc. for use in estimating Radiological Quantities associated with LHC Operation.
M. H ¨ofert, K. Potter and G. R. Stevenson
Abstract:
This note contains:
• Dose and dose-rate limits for different classes of areas
• Relevant machine parameters
• A generic LHC annual schedule
• Inelastic interaction intensities for high-luminosity experimental areas
• Proton source intensities for dump regions
• Proton losses at the betatron scrapers
• Proton losses around the main ring
CERN, Geneva, Switzerland Last update: 10 July 1995
1. Dose and dose-rate limits
Reference: H ¨ofert and Stevenson [Hof95a].
Specification of shielding parameters resulting from the full loss of LHC beams (which is assumed to occur much less frequently than once per year) should be based on an ambient dose equivalent per event of 50 mSv at the shield surface leading to a Controlled Radiation Area. It is considered to be most unlikely that the effective dose could exceed 20 mSv in such an incident.
The design limit appropriate to a Surveyed Radiation Area for such an event should be an ambient dose of 2.5 mSv, assuming the same expected relationship between ambient dose equivalent and effective dose.
The limit for a “Public” area should be an ambient dose of 0.3 mSv, which is numeri- cally equal to the limit of effective dose to the public given in the CERN Radiation Safety Manual.
Specification of shielding parameters resulting from exposure to continuous loss pro- cesses should be based on time-averaged ambient dose equivalent rates of:
Controlled area 10µSv/h Surveyed area 1µSv/h Public area 0.1µSv/h
If measured instantaneous dose rates exceed three times the design level, operators and personnel must be warned.
If measured dose rates exceed ten times the design level, the offending operation must be stopped or the area classification changed to the appropriate stricter category.
2. Relevant machine parameters
Reference: The “White Book”, Baconnier et al. [Bac93].
• Circumference: 26658.87 m
• Revolution frequency (β = 1): 11.24551 kHz
• Revolution period: 88.924 µs
• Design energy: 7.0 TeV
• Maximum energy: 7.5 TeV
• Injection energy: 450 GeV
The White Book gives 2835 proton bunches in the machine, each containing1011pro- tons, leading to a circulating current per beam of approximately 536 mA. The present maximum current considered is 850 mA, which gives maximum number of protons in one ring of 4.7×1014.
The White Book luminosity is 1034cm−2s−1, but the ultimate luminosity given by the beam-beam limit is quoted to be 2.5× 1034cm−2s−1.
One can consider then four different machines:
The Design Machinewith an initial number of protons of 2.84× 1014and a luminosity of 1034cm−2s−1.
The Pessimistic Machinewhich supposes that one has to go to the maximum current of stored protons in order to reach the Design Luminosity.
The Optimistic Machine which assumes that one can reach the maximum luminosity with design value of the proton current
The Maximum Machine with the maximum values for both the proton current and lu- minosity.
3. A Generic LHC Annual Schedule
Reference: Current “best guesses” of the authors.
• Number of fills per day: It is suggested that two scenarios are considered: ei- ther one of 20 hours duration or two of 8 hours duration, always with a 4 hour gap between fills (the former is considered to be more probable)
• Days of operation per year: 180 days of high-luminosity proton operation split into 3 periods of 60 days with two 10-day shut-downs between the periods. This leads to a 10-week cycle.
• Heavy ions: After proton operation one can assume a 17-day shut-down and then a heavy-ion run of 6 weeks.
• Annual shut-down: This then leaves 15 weeks for the annual shut-down.
• LHC lifetime: The canonical duration of LHC operation at design luminosity is 10 years. It can be assumed that in the first year intensities will be one-tenth of the design figures, in the second year one-third, in the third year two-thirds and that design intensities will be reached in the fourth year and extend for ten years, or 13 years in total.
4. Inelastic Interaction Intensities for High-Luminosity Experimental Areas
Reference: Potter and Stevenson [Pot95a].
The interaction rates averaged over 24 hours of operation for the two fill periods are given in the following Table:
Table 1: Average inelastic interaction rates in one high-luminosity area for selected fill- times (4 hours between fills).
Machine Peak Interaction Average Interaction Rate (s−1) Interaction Rate (s−1) 8 hour Fill 20 hour Fill Design 7.00× 108 3.71× 108 3.53× 108 Pessimistic 7.00× 108 3.99× 108 4.09× 108 Optimistic 1.75× 109 7.31× 108 5.82× 108 Maximum 1.75× 109 8.50× 108 7.50× 108
From Table 1 it will be seen that a suitable design figure for fixed shielding specifica- tion is thus
109interactions per second.
This is the same rate that should be used to predict annual releases of radioactive gases and fluids and radioactivity in rock and ground-water and experimental components that destined for eventual disposal. The annual number of interactions is then
109ips× 180 days/year × 24 h/day × 3600 s/h
which is 1.56× 1016interactions/year. It is proposed to round this to the value of 1.6× 1016interactions/year
The design figure for the estimation of induced radioactivity and radiation damage in the experimental area and the concentration of radioactive gases in the area during operation is
3.5× 108interactions per second.
The annual number of inelastic interactions is then 5.5× 1015.
5. Proton Source Intensities for Dump Regions
Reference: Potter and Stevenson [Pot95b].
Table 2 gives the number of protons dumped per fill for the Maximum and Design ma- chines for the two fill durations chosen as representative of LHC operation. The number
two fills per day of operation for the 8 hour fill duration and one per day for the 20 hour fill duration with 180 operating days per year,
Table 2: Number of protons dumped from one LHC beam for various fill-times.
Fills Fill Duration Fill Period Number of protons Fills per Number of protons per day (hours) (hours) dumped per fill year dumped per year Maximum Machine
1 20 24 2.5× 1014 180 4.6× 1016
2 8 12 3.5× 1014 360 1.3× 1017
Design Machine
1 20 24 1.7× 1014 180 3.1× 1016
2 8 12 2.3× 1014 360 8.3× 1016
For radiological assessments concerning the environment it is recommended to use 1017 as a realistic maximum number of protons reaching each dump block in one year, on the basis of one fill on each of the 180 days of LHC operation, i.e. each fill gives 5.6× 1014protons.
For the assessment of radiation damage and induced radioactivity in the dump and the cavern containing the dump, it would be more appropriate to use 5× 1016 for the number of protons reaching each dump block in one year on the basis of one fill on each of the 180 days of LHC operation, i.e. each fill gives 2.8× 1014protons..
6. Proton Losses at the Betatron Scrapers
Reference: Potter and Stevenson [Pot95c].
The analysis described in Reference [Pot95c] shows that interaction rates in the be- tatron scrapers are mainly functions of the scraper efficiencies, but in fact they are not strong functions of either the scraper efficiency or of the beam-gas lifetime. The loss rates are also independent (at the 20% level) of the fill time. These facts allow one to deduce values of the loss at the scrapers which will be valid for most machine conditions.
The highest value averaged over 24 hours for the Maximum Machine is 2.5× 109 in- tercepted protons per second. It is recommended that this value should be used for ra- diological assessments concerning the environment. On the assumption of 180 days of operation per year this leads to an annual number of intercepted protons per beam of 4.0× 1016.
For the assessment of radiation damage and induced radioactivity in the betatron scraper region, it is more appropriate to use a value based on the expected parameters for the Design Machine which lies close to 1.0× 109 intercepted protons per second. The annual number of intercepted protons per beam is then 1.6× 1016.
7. Proton Losses around the Main Ring
Reference: Potter and Stevenson [Pot95c].
For a given scraper efficiency, the loss rate was found to be approximately inversely proportional to the beam gas lifetime. However in [Pot95c] it was proposed that loss estimates should be based on a beam-gas lifetime of 250 hours.
It was also proposed that the radiological estimates should be based on the rates for a 97.5% scraper efficiency. The Maximum Machine parameters then lead to a 24-hour aver- age loss around the ring of 2.2× 108 protons per second which on the assumption of 180 days of operation per year leads to annual numbers of lost protons per beam of 3.4× 1015. It is recommended that these values should be used for radiological assessments concern- ing the environment.
For the assessment of radiation damage and induced radioactivity in the main ring, the Design Machine parameters should be used i.e. a 24-hour average loss of 1.4× 108 protons per second and an annual number of lost protons per beam of 2.2× 1015.
8. Summary
It should be noted that the figures in the Table refer to only one circulating proton beam or one high-luminosity interaction point. There are in fact two beams and two high- luminosity interaction points. The inelastic pp-cross-section is assumed to be 70 mbarns.
References
[Bac93] Y. Baconnier, G. Brianti, Ph. Lebrun, A. Mathewson and R. Perin (eds.), LHC: The Large Hadron Collider Project, CERN/AC/93–03(LHC) (1993).
[Hof95a] M. H ¨ofert and G. R. Stevenson, Design limits for doses and dose rates from beam operation at the LHC, CERN Internal Report TIS–RP/IR/95–04 (1995), LHC Note 309.
[Pot95a] K. Potter and G. R. Stevenson, Average interaction rates for shielding specification in High-Luminosity LHC Experiments, CERN Internal Report TIS–RP/IR/95–05 (1995),
Table 3: Proton Intensitiess for the Radiological Design of the LHC
Loss Mechanism Environmental Assessments Internal Assessments
Number Unit Number Unit
Accelerated protons 4.7× 1014 fill−1 2.8× 1014 fill−1 8.5× 1016 y−1 5.1× 1016 y−1 Maximum Luminosity 2.5× 1034 cm−2s−1 1.0× 1034 cm−2s−1 Average Luminosity 1.3× 1033 cm−2s−1 5.0× 1033 cm−2s−1 2.0× 1041 cm−2y−1 8.0× 1040 cm−2y−1 Inelastic Interactions 1.0× 109 s−1 3.5× 109 s−1
1.6× 1016 y−1 5.5× 1015 y−1
Dumped 5.6× 1014 day−1 2.8× 1014 day−1
1.0× 1017 y−1 5.0× 1016 y−1 Lost at Betatron Scrapers 2.5× 109 s−1 1.0× 109 s−1 4.0× 1016 y−1 1.6× 1016 y−1 Lost around the Main Ring 2.2× 108 s−1 1.4× 108 s−1 3.4× 1015 y−1 2.2× 1015 y−1
[Pot95b] K. Potter and G. R. Stevenson, Proton source intensities for use in the radiological assessment of LHC dump regions, CERN Internal Report TIS–RP/IR/95–11 (1995).
[Pot95c] K. Potter and G. R. Stevenson, Source intensities for use in the radiological assessment of the effect of proton losses at the scrapers and around the main ring of the LHC, CERN Internal Report TIS–RP/IR/95–16 (1995), CERN AC/95–04(DI), LHC Note 322.
