RADIATION ISSUES     

For overview of radiation issues see presentation to RPC  (5/12/2003) and to INB Paris (November 2005)

Clearly access has to be prevented to the tunnel within given boundaries whilst beam tests are in progress. Thereafter the we must anticipate the remnant radiation doses and the appropriate precautions that must be taken after the test. These precautions depend on the remenant dose levels, see radiation areas below.

Input courtesy of Graham R. Stevenson, Doris Forkel-Wirth, Stefan Roesler & Helmut Vincke.

INB

• Full LHC Dossier de Surete to be finished by end 2005 (LHC/CNGS/SPS).
• Separate paper to be prepared for sector test. Short paper ready by October 2005. To Paris April 2006.
• Manpower in RP to look specifically at sector test from January 2006.

Open questions

• Proton loss points other than dump - where and how much can we expect?
• Permitted dose rates in LHCb, corresponding number of protons that can be lost.
• Required sensitivity of radiation detectors in LHCb to ensure we don't exceed maximum dose?
• Given time displacement of test - do LHCb still need to keep their cavern clean?
• Check with other equipment groups the implications of irradiation during test.
• (Helium lines, chilled water pipes in IR7 - is this an issue?)

1) How many protons of the LHC beam will be absorbed by the spoiler in LSS4 of SPS (for CNGS: 0.1 - 0.5 % of the beam). This number might determine the dose rate in ECA4 where the shielding wall is not yet finalised in 2006.

Should of course be very low, however, worst case would be occasionally losing a few bunches with kicker problems (timing, strength, missing) so 0.1 % average loss would be reasonable. (CNGS commissioning more of an issue...)

2) How many protons will be send on the TED at the end of TI8

• We have 24 hours at before the start of the test, with all beam on this TED - 24 * 60 * 5 e9 gives 7.2 e12, say 1 e13.
• Almost certainly some periods with beam on the TED during the sector test itself (recovering from LHC access, when LHC is recycling, etc. etc.)
• Therefore 2 e13 absolute maximum onto TED at the end of TI8. Most of this concentrated at the start of the test.

3) Do we have any idea how many protons we will loose at the collimator?

• 7 e12 absolute maximum onto the TDI.   2.5 e12  at the start - the rest potentially towards the end
• 4 e12 onto transfer line collimators (TCDI) towards the end of the test.
  

4) How many protons (in percent of the beam do we loose in LHC-b (for dose rate estimates for counting rooms of LHC-b)

Large aperture  - un-squeezed optics - no crossing angle. Not more than 1% of total (therefor around 3 e11).  Would be very careful about switching on crossing-angle bumps and injecting into different bumps. Could possibly mis-steer beam into LHCb during initial commissioning, so say 5 e11 total.

5) How many protons on the final TED

We estimate 2.7 e13, rounding up to 3 e13  would be fine - this more or  less evenly distributed over a 2 week period

6) distance between TED in Point 7 and entrance to TZ 76

UJ76 is 33.88 m from IP7, spans around 22 m of the tunnel and sits next to the  two TCS upstream (nearer IP7) of Q4.

Approximately 135 metres between dump and start of UJ76

Expected Radiation Levels

RPG have taken case 3 shown in the table below as their maximum for the sector test. This scenario represents an extreme case. At these intensities: the dose rates for 1 day irradiation and 1 day cooling following a proton intensity of 1.5 10¹⁰ ps^-1 (50% efficiency - case 3) are given below at the dump and in the arcs. This dose totals 1.3 10¹⁵ protons in 24 hours.

It is estimated that during the test around 3 x 10¹³ protons will be used over a 2 week period. Thus the figures below can be scaled accordingly (see latest presentation).

TED

C/o Doris

In 2003 we made very conservative first extrapolated predictions for the TED activation for the sector tests:

• 40 uSv/h alongside the TED and 500 uSv/h at the backside of the TED after one day cooling. It is really difficult to read the values from
  the graphics in particular for the backside as the slope of decrease in dose rate is very steep.
• in summer 2004 Helmut repeated the calculations for TED activation once more in the context of the TT40 tests. If we extrapolate these more
  recent calculations to the sector test conditions:
• 50 uSv/h alongside the TED and 110 - 120 uSv/h at the backside of the TED (still suffering from the same problem of extracting data from a
  graphic - see above)
• This can be compared to the measurements done after 2 days of cooling and measured in 10 cm distance. These measurements gave values between
  20 and 60 uSv/h (backside of the TED).

Therefore I suggest that we stick to:

• 40 uSv/h along the TED (prediction for 1 day cooling) to be compared to 20 uSv measured (after 2 days cooling)
• 120 uSv/h at the backside of the TED (prediction for 1 day cooling) to be compared to 60 uSv/h (after 2 days cooling)

• Will need a extra beam stop (Iron/concrete) after the TED

These figures are somewhat higher initially due to the short-lived activation of the concrete walls along side the dump. Effects confirmed by measurements following the TT40 tests (see presentation above).

ARC

Assume beam is lost uniformly along the sector between point 8 and point 7 

• Typical dose rate or 1 day irradiation and 1 day cooling (50% efficiency) between 0.5 and 2 mSv/h scales to negligible.

Assume beam is lost repeatedly in one dipole (unlikely scenario)

• Typical dose rate or 1 day irradiation and 1 day cooling (50% efficiency) between 200 and 500 mSv/h  scales to 4 to 10 mSv/h.

With the un-scaled dose rates:

• The situation would not be disastrous, the injection region, arc and dump region could be classified as a simple controlled radiation area
• Expect anyway [whatever intensities we use] to have to declare a simple controlled radiation area.
• Potential problem areas: 
•     1. Installation after the event near dump location (less of a problem with lower intensity). Prepare as best as possible.
•     2. Installation after the event in the injection region.
•     3. LHCb - see below
• Localised losses before the dump could lift levels by one or two orders of magnitude in the worst possible case. Again wait, thereafter passage should be possible if not working in such a location.
• If we want the dose rates to be insignificant after 1 month - we should foresee at least one order of magnitude less beam that assumed above - which in fact we anticipate nearer 2 orders of magnitude.

Intensities

Other considerations

• Gate required in sector 6-7 at least 800 m. from IP7
• BLMs are required to be operational
• RAMSES (new radiation monitoring system)  planned to be operational.
• LHCb: Essentially no loss permitted. Can not be declared as simple controlled radiation area. Nominal setup in place,  Be vacuum chamber etc. Needs monitoring. BLMs plus additional radiation monitors.
• Important : all TEDs will have to be equipped with an extra beam stop (160x80x80 cm3) of iron surrounded by 80 cm of concrete.

LHCb

Access

Want access to their counting rooms during the test - fair enough. Interlocked Access door through their shielding wall should be in place.

DFW proposes that the first few hour(s) no access into LHCb counting rooms is granted to give RP time to control the prompt radiation levels in the counting room. Perhaps it would just be a "cosmetic" survey as we don't expect any radiation level - but we could check for (very) weak points in the shielding...Of course, we have fixed monitors installed but this survey I would like to be done in a more precise manner with technicians......After this survey and having found no surprise we could grant access immediately....

To my opinion this would be a decent RP approach - of course we have the results of our calculations - but nothing better than experimental, real data - they will be convincing....

Traceability

"Under all circumstances we have to avoid tracing of equipment before we have circulating beams and therefore as well after the sector test. We can not trace all the equipment which will go in and out of the UX cavern during our commissioning phase which will end with the closing of our area in summer 2007. We might have still some temporary equipment which will leave the area after the injection test. INB implications on tracing from a supervised LHC area need to be established."  Rolf Linder

Doris Forkel-Wirth:

About controlling material leaving LHC - I am convinced that after the sector test we will have to perform a radiological control of all material (including conventional) leaving the zone of the sector test.

About tracing ....no idea and I don't have any idea who decides on it - perhaps John Poole?

It might be that we have a major problem:

For people working in the LHC cavern it is only the dose rate at the workplace that counts. Hopefully we can convince IRSN that in case we will stay below one or two μSv/h, people will be permitted to work without dosimeters. However for the radiological classification of material (radioactive or conventional) the limits are much lower. In case Lindner wants to move his material we will have to ensure that e.g. the concentration of 60Co is below 1 Bq/g or better 0.1 Bq/g in his material........According to the Swiss legislation, material has to be considered as radioactive when the dose rate measured in 10 cm distance is above 0.1 uSv/h (after subtraction of the background). You see the problem?

The new French law (31 March 2003) keeps the classification of radiation  workers of B and A - but the only difference is just the yearly dose limit  (6 mSv - 20 mSv). There is no difference foreseen in the law with respect  to medical control. 

Given the removal of the temporary access gates, the TED etc. the main knock-on effect will be the declaration of the zone around the dump position and the injection region as supervised areas. Supervised areas imply:

• Between 1 mSv/year and 6 mSv/year
• Category B radiation worker to pass through
• Registered
• Medical every 2 years
• Personal dosimeter
• Followed the courses

During the test:
Prohibited area stretching from ECA4 to the interlocked gate close to point 6. Monitors will be installed close to the gates in direction of point 6 and point 1. Additional RAMSES monitoring in LHC-b experimental cavern. Ventilation system equipped with radiation monitors. Installation of Beam loss monitors, beam intensity monitors and pick-ups….necessary. Access prevention from Point 7 and Point 8.

After the test :
Supervised area (with exception of LHC-b experimental cavern) stretching from ECA4 to the area where the temporary TED will be installed. Behind the TED fencing of the area such, that on the other site the dose limit of 0.5 mSv/h will be kept. Film badge obligatory in the supervised area, no passage permitted for persons without film badge. It might be that some accelerator components will have to be fenced off for some time immediately after the test.

According to the information acquired I would propose to the French authorities DGSNR the following for the LHC sector tests:

Assuming that the persons are not working in the zone but just trespassing it (temporary stay):

1) Classification of the zones according to dose rates.

2) After the tests we will do the radiation survey and depending on  the outcome we will either proof: the dose rate in the zone are comparable to those in a surveilled zone (< 2.5 microSv/h) -> we will declassify the zone as a surveilled one. The persons intervening into this zone don't need any dosemeter nor medical clearance

3) In case the dose rate will be above 2.5 but below 10 microSv we will be most probably permitted to send "non-classified workers (neither A or B) temporarily" into this zone. However, we have to verify the doses (electronic dosimeter or visitor film badge). And we have to make sure that the dose of such a person stays well below 1 mSv/year. No medial  clearance would be required.

This proposal will have to be checked by DGSNR and hopefully approved - but parts of the French law are still under revision.

Doris Forkel-Wirth