lhcc questions and answers - moedal experiment...moedal - lhcc referee’s questions & answers...

September 15th 2009 MoEDAL/LHCC/01-17-2009/1.0 J.L.P

MoEDAL - LHCC Referee’s Questions & Answers

Question 0 I guess my main question remains wanting a better justification for having only 3 layers of CR39, even though it is somewhat more expensive. Since one layer of CR39 has to be sacrificed to the strong etching having only two remaining layers of CR39 seems to me to be at least one two few! Just e.g., Is position resolution different for strong and weak etching? If so, with 3 soft-etched layers of CR39 one could use "triplets" to measure the spatial resolution actually achieved in the detectors exposed for a year in the LHCb cavern (which might not be the same as in the test beam?). How much extra would it cost to replace one of the lower-tech layers with an additional layer of CR39? Given the overall cost of the project in $ and people, my gut feeling is the incremental cost would be worth it.

The stack design with CR39 layers is easily adequate for the detection of relativistic magnetic monopoles (with 6-9 layers sensitive). Remember that with only 2 layers of CR39 that are soft etched we still have four “soft” etch pits on our track, plus the 6 hits in the MAKROFOL and (depending on the velocity) the 3 Lexan hits. But, there is no doubt a 4th layer of CR39 would add some valuable redundancy for the physics case when we turn our interest to heavy stable conventionally charged particles or to slow monopoles where we essentially lose one layer of CR39 create attempting to create the map for relativistic monopoles. In the case the higher threshold layers do not help us. In our design for the MoEDAL detector stack housings we did leave extra space in case we needed to add extra plastic layers. We can indeed add an extra layer of CR39 without sacrificing a layer of MAKROFOL. The MoEDAL collaboration has agreed that we would add the extra layer of CR39.


Question 1 I would like to have some more explanation about the maximum dose tolerable by the CR39 and MAKROFOL/Lexan and the radiation level in the VELO cavern. There is some limit at 2Mrad and 200Mrad respectively, but it is not clear which are the corresponding radiation level near the detector. Which are in one year the radiation dose expected in MOEDAL? The radiation dose levels are discussed in section 2.5 of the TDR. As you will have read the dose level 1 m from the LHCb Interaction Point (IP) has been estimated by the LHCb collaboration to be ~ 1kRad/LHC year. As nearly all of the MoEDAL array is further from the IP we expect the does level to be smaller than ~1 kRad/year at the nominal LHCb luminosity (1032 cm-2 s-1. There is one small region of the MoEDAL detector that comes quite close to the beam and that is mentioned in subsection 1.4.3 and shown in Figure 21. This region is where MoEDAL detectors are deployed on the shielding wall of the VELO cavern, near where the beam pipe penetrates (see circular ``cut-out'' region in Figure 21). This region will be treated specially. Even so we do not expect background levels to be a problem even here. The maximum dose of 2 Mrads for CR39 was determined by Kay Kinoshita (one of the MoEDAL Collaborators) and Buford Price from measurements made at the Tevatron [1] this is the dose at which there are two many etch pits created for a signal to be discerned, corresponding to > 108 etch pits/cm2 As you will notice the radiation tolerance of CR39 is 2Mrad compared with the expected dose rate of less than ~1krad per year.

Figure 1 Flux of charge hadrons in the VELO region, with KE> 20 MeV.


Another way to look at this is to take the total hadron flux with sufficient energy to give a recoil proton of sufficient energy to potentially register in the CR39. Using the LHCB calculated fluences for such hadrons, shown in Figure 1 (not shown in the TDR) below, we see that our canonical distance of 1m the number of hadrons (with KE >20 MeV) per collision per cm2 is ~10-4. As there are 1.6 x 1014 collisions per year we would get a flux of ~1.6 x 1010 hadrons (KE>20MeV) per cm2 per year. From Price’s and Kinoshita’s measurements, that each hadron gives 10-4 etch pits, we obtain ~106 etch pits per cm2 per year, a factor of 100 less than the critical level (of 108 etch pits/cm2). This is a conservative estimate as: a) all MoEDAL NTD detectors are more than 1m away from the IP; b) the hadrons near the 20 MeV threshold will have low efficiency for creating a spallation product – that would in turn create an etch pit. The 200 Mrad level refers to the radiation tolerance of MAKROFOL which is about 100 times less sensitive than CR39. Hence it can stand a lot more radiation than can conceivably delivered to the MoEDAL region in a few years of LHCb running. According to Kinsohita & Price, in MAKROFOL we have 10-6 etch pits/hadron created and thus the background from spallation products is a factor of at least 10,000 (1.6 x 1014 x 10-4 x 10-6) lower than the critical level (of 108 etch pits/cm2).


Question 2 It would be interesting to see simulation of the signal and background to see that the selection of the events is such that for monopole candidates the background for MOEDAL is substantially zero. First let us say that we are in the situation where we are near to, at, or exceeding the limiting etch pit density reliable measurements are not possible and no attempt would be made to analyze plastic under these conditions. However, we expect to be at least a factor of 100 away from this critical situation (see the answer to Question 1) Thus, the background situation we consider here corresponds to a scenario where radiation levels are low enough that ``standard particles’’ can, in some way – unambiguously - mimic the behaviour of a magnetic monopole. This background arises from a radiation environment in the LHCb region dominated by charged hadrons and neutrons, which can interact in surrounding materials via the nuclear interaction to produce Target Fragmentation Events (TFEs) also known as spallation events. The experimental understanding of TFEs, and how they can contribute to the background, is discussed in more detail in Appendix 1. Of the products of the target fragmentation event, only alpha particles and recoil nuclei can locally deposit enough energy to reach the CR39 threshold. The simulation of the background from TFEs is difficult as we are dealing here with nuclear physics and would have to consider the extreme tails of generated distributions. Thus, instead of simulation we have used existing measurements of TFE events, along with physics arguments based on the properties of the monopole, in order to assess the background. We summarize here, in point form, why backgrounds from target fragmentation are expected to be essentially nonexistent in the MoEDAL data (please see Appendix 1 for more details):

1) Recoil nuclei from TFEs typically have a typical range ~1 cm in air, ~1 micron in aluminium and ~10 microns in CR39, independent of primary incident particle energy. This range has an exponential tail of low probability that can extend for a few hundred microns. The typical kinetic energy (KE) of such a fragment is small – of the order of 10MeV, again, roughly independent of primary incident particle energy. Thus, any background must be generated in the detector or in the material immediately in front of the detector (the aluminium faceplate).

2) A recoil nuclei from a TFE would typically not be able to penetrate a single layer of the detector – the gold plated signal for a magnetic monopole requires any background particle to penetrate all detectors layers with total thickness of 4.1 mm (with the extra layer of CR39) that is hundreds of times the typical range of the recoil nuclei.

3) As the KE and range of the TFE recoil nuclei is small it will be slowing down rapidly with the dE/dx due to ionization increasing rapidly according to the Beth-Bloch formula and typically not even penetrate one NTD sheet. The signature of this background would be – at the very most - a few etch pits of increasing radius. However, a relativistic monopole would only slow down slightly as it crosses all the MoEDAL stack – giving a gold plated signature of 18 aligned, contiguous, etch pits of constant etch pit radius.

4) Even if a fragment traveled hundreds of times its own range to penetrate all layers of the stack it

would still have a dE/dx signature opposite to that of a relativistic monopole. Since, the ionization signature of a relativistic monopole is proportional to the velocity – opposite to that of a slowing and stopping conventionally charged particle. The fragment would also be stopping with etch pits increasing in size whilst a relativistic magnetic monopole would be passing though with little change in velocity.


5) As the incident particle energy increases the angle of the recoil fragment from the TFE scatters less and less into the forward direction – as defined by the parent particle. For protons incident on aluminium of energy more than ~300 MeV the distribution of recoils is very roughly isotropic. Assume a normally incident proton creates a TFE recoil product in a detector 1m from the IP that penetrates far enough for its angle to be measured. Then for a recoil angle greater than ~1 degree, the aligned etch pits would not point back to the beam axis within ~2 cm. If we adopt this as the pointing criteria for sour signal and assume that the distribution of recoil angles is flat (which is a reasonable assumption in the case of protons incident on aluminium above ~300 MeV) then only roughly one in several thousand recoils would pass the pointing criteria required for monopoles.

To summarize, the background is suppressed by the small range (~10 microns in CR39 with an exponential tail, of low probability, extending for a few hundred microns) and low energy (ofo 10MeV) of the recoil nuclei from the TFE (which is independent of incident particle energy) resulting in a typical recoil nucleus being absorbed in only one layer of the MoEDAL detector and giving rise to typically one etch pit. On the other hand a relativistic monopole in a MoEDAL stack would give rise to a set of 18 etch pits of the same radius because the extremely high ionizing power of a relativistic monopole is effectively constant across the stack. Note that the ionizing power of a background spallation product rapidly increases as it slows and stops in the NTD stack with etch pits increasing in size as they penetrate the stack. Also, the energy loss of a relativistic monopole falls with falling velocity compared to that of a conventionally charged stopping particle where the dE/dx rises rapidly with falling velocity. Thus, the energy loss signatures of the monopole and background are totally different. An additional source of background reduction is the non-pointing nature of a TFE recoil, where the recoil angle becomes roughly isotropic above ~300 MeV (for protons on Aluminium), resulting in most recoil tracks pointing away from the IP. This background characteristic is only useful when the spallation product range is enough to penetrate one or more sheets, in order that pointing may be performed, this would be a very rare event. We believe that the fact that no background particle, or signal candidate, has ever been observed in an experiment employing plastic NTDs, is due to these very strong differences between signal and background discussed above. However, in the interests of doing our “due diligence”, we have planned a test of roughly 1 m2 of the MoEDAL array, corresponding to 9m2 of plastic for 2010, as we discussed in the TDR. We will carefully analyze this plastic in order to assess experimentally all potential sources of background in the new energy regime of the LHC.


Question 3 How etching conditions are taken under control and with which precision? It should be very important for the measurement of the REL after a calibration. What are standard conditions? and again which control is possible on these "standard"? The temperature of the etching bath is kept constant to within ± 0.1 °C The concentration (normality) of the etching solution is measured by titration with an accuracy of 0.05 N. Such are the accuracies for calibration and analysis methods applied in the past for the MACRO and SLIM experiment. See ref. [99] in the TDR ( NIM B254, 254-258 (2007) By the phrase “standard manner” mentioned page 49 of the TDR, we mean etching the detectors either in “soft” or “strong” etching solution, using the same set-up described on page 48, sec.3.1. The corresponding calibration data ( for “soft” and “strong” conditions) are reported in fig. 48. Question 4 The deployment of 1m2 of NTD was foreseen for end of July (correct?), it has been possible to proceed according to the LHCb operations or it has been delayed? The plan was (see section 5.3 of the TDR) that we would assemble ~1 m2 of plastic stacks in June-July for initial deployment if July-August. We would also install the MoEDAL framework at that time. Work is now underway and we will have drilled all the holes for the complete MoEDAL framework and installed the holders for the initial deployment of plastic detectors. This work was finished on September 16th. The Bologna group is now preparing the 1m2 of MoEDAL test stacks. We are on track to install these test stacks before the start up in November. We have also manufactured the MoEDAL housings + framework which are now ready for deployment. The small delays arose from the necessity to deploy the scaffolding to finish hole drilling in the ceiling of the VELO cavern, when it would not interfere with LHCb work. A small delay was also incurred due to delivery time for CR39 plastic.


Question 5 I imagine that the schedule for the complete installation reported in the TDR will be modified with the new LHC schedule, do you have a new plan already? And also what about the requested luminosity and the energy? It is relatively easy to make flexible plans since we have a passive detector that requires minimal resources and manpower. Here is the plan we have at present:

• Deploy the test 1m2 stack in 2009 for data taking during 2010;

• Take out a small amount for testing during 2010 running (in a short shutdown) if possible;

• In the next long shutdown 2011(?) we will deploy the full array and run for as long as possible at the beam energy available (3.5 TeV?) or until we have reached an integrated luminosity of at least 6 fb-1, whichever comes first;

• At each new major beam energy increase we would need to change the plastic detectors and then run

again for as long as possible at that new beam energy or until we have reached an integrated luminosity of at least 6 fb-1, whichever come first (we envisage it will take 3 complete working days to change the plastic);

• The experiment would end when we have run for as long as possible, or until we have reached an

integrated luminosity of at least 6 fb-1, whichever come first, at the highest beam energy that the LHC can achieve (14 TeV?),

To Summarize: we would like to run at each main centre-of-mass energy point that LHC reaches (7/10/14 TeV?). Since, each new energy frontier reached opens up the possibility of a discovery. We would like to take a 6 fb-1 at each point to enable us to be competitive with other searches at that same energy. However, we realize that for the intermediate energy steps (7/10 TeV?) this may not be possible at the LHCb intersection point. In that case we would like to take as much data as possible at that centre-of-mass energy. This program only requires us to change the plastic at each major shutdown.


Question 6 It would be possible to see a distribution of the bulk-etch velocity and of the track-etch velocity as function of the irradiation dose? We haven’t performed recently a systematic test. On this matter we have relied , for the time being, on the results obtained by some of us in the past (table 2 in the TDR is quoted from ref. [79]) and available also in more recent literature (See Appendix 2). Question 7 Which is the required accuracy on the mechanics, the holes and the pins used for the alignment of the stack in order to get the necessary precision in the measurement, and how it is achieved? The MoEDAL NTD stacks are assembled then clamped. Four holes with a diameter that is of a precise size to fit a precision dowel pin (available off the shelf machined to a precision of ~2 mm) are drilled one in each corner, defining a precise square, and a dowel pin of the requisite diameter is “press fit” in each hole. The precision with which the distance between the holes can be determined by machining is ~10 mm These dowel pins define and maintain the position of the sheets with respect to each other to an accuracy of better than 50 mm (See Section 2, page 27 of the TDR). The use of dowels in this way is standard engineering practice and the machine tools we have at our disposal can easily achieve this accuracy. The stacks are placed within the housing to an accuracy of ~1mm. The housings are attached to the rails using screws. The rails are attached to the wall and ceilings of the cavern. In order to fit together the holes in the housings with the threaded holes in the rails the hole sizes and placements must be machined to an accuracy of better than 0.5 mm, where we will allow a small slot in the housing hole to take up any small mismatch between the rail hole position and the corresponding housing hole position. This level of accuracy in construction of these components has already been demonstrated and is perfectly within the range of ordinary engineering expectations. Surveyors “bull’s eyes” will be attached to each housing in order that the position of each of the MoEDAL stacks, contained within the housing, can be measured to an accuracy (using the CERN surveyors) of ~1mm in x and y. Also, the angle that the plane of the housing makes with the horizontal and vertical will be determined by the surveyors to an accuracy of better than 1mm in the width (50 cm) or length (75 cm) of the housing


Question 8 It is essential for eliminating any background of monopoles tracks to be able to point back at the production with an accuracy of 1 cm (page 15) for incident angles ranging easily from -pi/4 to +pi/4. This accuracy has not been validated in the calibration setup, at least no results are shown for the angle reconstruction. In addition it looks like that in the calibration setup the layers are largely separated while in the proposed NTD modules the layers are contiguous. The calibration stack results reported in the TDR are from previous experiments (IMARCRO AND SLIM). The purpose of the these experiments was to determine the energy calibration of CR39 from SLIM and MACRO . In these cases, no attempt was made to “track” the individual calibration particles though the calibration stack in order to determine pointing accuracy. The measurement you mention is certainly one we will with a high energy heavy-ion facility make with a MoEDAL stack. However, the mechanism of etch-pit formation with heavy-ions has been well studied on MACRO and SLIM and elsewhere. All a monopole track is, is an aligned set of 18 such contiguous etch pits in a stack of plastic mechanically held in place by precision dowel pins. So based on our past experience with MACRO and SLIM we are confident that our estimate is realistic The pointing resolution of ~1-2 degrees, corresponding to a ~1cm resolution in the determination of the IP (for a track normally incident on a detector stack 1m from the IP) is a conservative estimate based on how well we can mechanically align the NTD sheets with respect to each other in the stack. The fact that monopole track points back to the IP is a required added confirmation of the signal-like properties of the candidate monopole i.e. that the track is consistent with pointing towards the IP. It is not a primary means of background reduction. We maintain that the background is rendered negligible by the key gold plated signature of a singly charged relativistic magnetic monopole i.e. 20 (with the extra layer of CR39) contiguous, aligned, etch pits with equal size and with dE/dx corresponding to a particle with ionization power ~4000-5000 (or more) that of a minimum ionizing particle with standard charge. We maintain that the background to this signal is negligible (see the answer to question 2).


Question 9 Data analysis requires to have good recordings of the fabrication parameters, installation positions, procedures followed, and many other parameters for each NTD module. Are there any database and associated applications being developed for this? The CR39 for MoEDAL will be produced by the manufacturer according to our specifications concerning the chemical composition of the mixture, the polymerizing curing cycle, the storage and laser cutting procedures. For the MACRO experiment about 3000 m2 of CR39 were produced in this way from 1989 to 1992; more recently for the SLIM experiment more than 1000 m2 of CR39 were produced in about 30 batches. The reproducibility of the production procedure was tested by measuring, for each production batch, the average thickness of un-etched material, and on etched samples, by measuring the bulk etching rate (soft conditions) and the density of “fake” tracks (from residual impurities in the mixture). Each detector stack was assigned a unique number and we recorded:

• For CR39 foils, the batch number; for Lexan/ Makrofol foils a reference number provided by the seller;

• The date of stack assembling; • The position in the shipping box; • The exposure starting date at the experimental site; • The date of its retrieval from the experimental site.

For each etched foil we recorded:

• The stack number; • The etching date; • The etching condition and length; • The initial thickness; • The thickness after etching.

In addition we would insert a number of parameters regarding the treatment of each NTD sheet with regards to the regulations concerning the handling and disposal of material from a radiation zone. The Windows Microsoft Access database framework was used to develop a data base that was utilized successfully for the SLIM experiment. The computation of flux upper limits was made using software programs accessing directly the DB data.


Question 10 What is the role of the MAKROFOL layers for the signal extraction? (page 51) MAKROFOL has a much higher threshold (Z/β~50) than CR39 (Z/β ~5). Thus, it is only sensitive to relativistic monopoles. Lexan has a slightly higher threshold of Z/β ~65. The higher threshold of MAKROFOL and Lexan allows them to operate in much higher radiation environments ( up to 200 Mrad) since they have a factor of ~100 smaller sensitivity to the spallation background than CR39 (see Question 2). Also, these different thresholds give us information on the properties of the detected particle. Question 11 With respect to the physics productivity how many theses are envisaged for this experiment? The thesis topics we envisage are (if no discoveries are made):

1) Search for the Magnetic Monopole in LHC Data at √s = 7TeV/10 TeV/14 TeV (~3 theses)

2) Search for heavy stable SUSY particles in LHC Data at √s = 7TeV/10 TeV/14 TeV (~3 theses)

3) Search for Heavy, Long Lived, Doubly Charged Higgs Production in LHC Data at √s = 7TeV/10 TeV/14 TeV (~3 theses)

4) Search for Heavy Stable/Pseudostable Particles in LHC data at √s = 7TeV/10 TeV/14 TeV (~3 theses)

5) Search for black hole remnants at √s = 7TeV/10 TeV/14 TeV (~3 theses) 6) Development of GEANT4 Code for the Simulation of Magnetic Monopoles (1 Thesis) 7) Simulation of Backgrounds in the MoEDAL NTDs for Target Fragmentation Events

(1 Thesis) 8) Design, Construction and Calibration of the MoEDAL Detector (1 thesis) 9) Study of Backgrounds in the VELO Cavern Using the MoEDAL NTDs and MEDIPIX Real Time

Radiation Monitors (1 thesis)

Thus ~20 theses are possible in the event of no discovery. If a discovery is made, say the discovery of the magnetic monopole, then I think the number of theses would increase to around 25 theses, as we would: a) need to involve the LHCb experiment in joint analyses; b) study in much greater detail the calibrations, background studies, etc. We expect that we will be limited by the number of students rather than the number of possible thesis topics. For each of the thesis topics we expect to produce a paper in a refereed journal.


Question 12 Exactly how was the acceptance for magnetic monopoles calculated? Was this done analytically or with a Monte Carlo? We made the acceptance calculation using the GEANT4 simulation we developed for the purpose. This is reported on subsection 1.3.1 on page 18. Question 13 Has there been any attempt to simulate backgrounds? Please see the answer to Question 2. Question 14 At some level the CR39 layers are used as offline “triggers”. Please describe the use of the Markofol and Lexan foils thereafter in more detail? Monopole Search Only one CR39 layer is used as a “trigger” in the monopole analysis. It is strongly etched and scanned under low power in order to reveal any correspondences between etch pits on either side of the CR39 layer. All candidates are analyzed under strong magnification in order to make a “map” which is used to identify “regions of interest” (RoI). The third/fourth layer is etched under soft etching conditions and the corresponding region of interest is examined under high magnification. If a coincidence between the first and third/fourth sheets is found a second and third sheet would be examined. This technique is reported in section 3.3 (page 51) of the MoEDAL TDR If a coincidence is seen between all three/four sheets of the CR39 then the MAKROFOL would be examined. The same procedure is used for three/four MAKROFOL except that the RoI defined by the CR39 is utilized and all MAKROFOL sheets are soft etched. If all MAKROFOL sheets show a coincidence then the Lexan sheets would be analyzed. Search For Conventionally Charged Heavy Stable Particles The above procedure is also applicable for the search for heavy stable charged particles with a Z/β ≥ 14. However, the search for heavy stable charged particles with 7 ≤ Z/β ≤14 we can use the two/three CR39 sheets which must be soft etched to preserve the low threshold. We could use the most upstream layer of soft etched CR39 as the “map” to define the RoI for the other one/two sheets. If coincidences are seen between the two/three soft etched sheets then the MAKRFOFOL would be scanned. We could reduce the threshold of CR39 using “super-soft” etching to 5 although this is more time consuming.


Question 15 How will the radiation damage of the plastic layers be monitored? The radiation monitoring of the MoEDAL detector is described in Subsection 2.5.3 (page 41) of the MoEDAL TDR. We will use standard CR39 dosimeters that can be extracted and replaced within minutes whenever we can access the VELO/MoEDAL cavern. In addition, we plan to deploy MEDIPIX silicon pixel radiation monitors that are read out onto the web. In this way we can follow and record the radiation conditions in real time. Last, but not least, the MoEDAL NTDs essentially act like a gigantic dosimeter. The analysis of the plastic will also reveal the effects of the radiation dose integrated over the exposure of the NTDs. Question 16 What are the track/charge measurement resolutions? What is needed for rejection & monopole ID? The charge resolution of CR39 (whose threshold is Z/β > 5) measured using heavy ions is 0.2 e/√n (n is the number of independent measurements of a particle track, i.e. the etch-pit height or surface area on different foils of the same stack). Thus our charge resolution is much better than a single charge. Such charge resolution is more than adequate to identify fast Magnetic Monopoles of any magnetic charge (even a charge resolution of ±1e would be). MAKROFOL has a similar charge resolution and a higher threshold (Z/beta > 50), which makes it as effective as CR39 for fast MM detection. For further information see Ref [2]. We point out that the most critical parameter for Magnetic Monopole ID in our search, is the equality of etch-pit sizes on both faces of each detector foil, given the constant energy loss along the MM track. The experimentally measured difference (using heavy fast ions such as, for example, lead at the SPS) on front and back sides is not larger than 20%.


Question 17 A yearly swap-out of the scintillator (J.L.P - I assume you mean NTDs here) seems to be driven by the accelerator schedule. Is there a different, more natural optimum from the point of view of the experiment? The proposal suggests three runs, was this chosen to be in line with the first phase of LHC operations? What happens to the sensitivity if only two runs are approved or achievable? According to the radiation level calculations made by the LHCb collaboration, at the Point 8 interaction region we should be able to deploy one set of plastic for several years with no problem from radiation damage. We summarize here the reasons we have chosen to change the plastic every year, if possible, because:

1) The backgrounds in the plastic would be reduced making the scanning easier; 2) Changing the plastic more frequently would not improve the physics result and only increase the cost

of the experiment;

3) Frequent accesses to change plastic would be disruptive to the LHCb operation;

4) We need ~3 days to change the plastic. In a short shut down it is likely that the LHCb collaboration would need unfettered access to their detectors and thus it might not be possible for MoEDAL physicists to gain access to the VELO region. However, in a long shut down that would be roughly every year we could negotiate the time required to replace the plastic NTDs. So a change in a long shutdown (yearly?) is a practical option.

So bottom line, we can do the physics we want to do, whilst minimizing the costs and the disruption to the LHCb experiment, by only changing plastic in the long shutdowns (yearly?). More frequent changes would not enhance the physics result. However, these statements are all predicated on the LHCb assessments of radiation levels in the LHCb cavern not being wildly wrong. We wish to run at each main LHC energy “point”. We envisage these points will be 7 TeV/ 10 TeV/ and eventually 14 TeV. Each new energy frontier breached gives us the chance to make a discovery or allow us to push limits up to to higher energies. The 6 fb-1 luminosity will also allow us to make limits competitive with existing experiments in the energy range of those experiments.


Question 18 What establishes the position and number of radiation monitors? We need to assess the radiation levels at the position of the MoEDAL detectors across the cavern. A number of points will be measured and used to “calibrate” the simulation. Thus, the passive radiation monitors will be placed in the centre of the various group of MoEDAL detectors on the left wall (1) the right wall (1) to the left (1) and right (1) on the beam shielding wall and on the left(1) and right (1) ceiling mounted detectors, plus one near to the beam-pipe on the shielding wall (1). It is envisaged that the 4 MEDIPIX detectors will be placed at 4 points - distributed as far as possible uniformly around the VELO cavern – on MoEDAL detectors that are closest to the IP i.e. ceiling, left wall, right wall, shielding wall. Question 19 Who designs the fixtures and who has responsibility for them on site? How are the designs reviewed and accepted by LHCb and the LHC? The MoEDAL detector array is mounted onto the walls and ceiling of the VELO cavern using aluminium rails that are screwed to the walls. The flat aluminium housings that hold the NTD stacks are then screwed to these rails. This system of support was designed by Richard Soluk and James Pinfold of the University of Alberta. A prototype of this arrangement has been deployed on the VELO cavern walls for several months (see the photograph – Figure 24 – on page 30). All designs and installation procedures were approved by Werner Witzeling the Technical Coordinator of the LHCb experiment. The LHCb has allowed us to drill all of the holes required for the mounting of the rails - this work has now been done. The person who has the day to day responsibility for all MoEDAL fixtures in the cavern is Daniel Lacarrere a CERN employee and a member of the LHCb collaboration and the MoEDAL collaboration. Daniel is the experimental area manager for the LHCb experimental area. Both Daniel and the MoEDAL spokesman (Jim Pinfold) form the liaison with the LHCb experiment and the LHC. We meet with Werner Witzeling in order to obtain final approval from the LHCb for any MoEDAL experimental infrastructure we wish to deploy.


Question 20 The calibration procedure is somewhat vague, is there a calibration run for each batch of plastic and for each etching run? A step-by-step description of the calibration procedure is necessary. We made in the past (MACRO and SLIM) several calibrations using fast and slow heavy ions. Assuming the availability of ion runs at BNL, PS and SPS, we plan to make a batch-by-batch calibration before running at LHC. The calibration will be repeated to check the stability of the plastic response ( which, according to our experience, is stable over a period of many years, with no “fading” effects ). The calibration procedure is as follows:

• Stack preparation: for each production batch, 10 CR39 sheets + fragmentation target;

• Exposure to about 2000 ions/cm2;

• Etching in “soft” and “strong” conditions;

• Analysis using the automatic image analyzer system;

• Determination of p(=vT/vB) vs REL. With our etching apparatus we are able to reproduce the etching conditions from one run to the other, so that the bulk etching rate doesn’t vary by more than 3% and the determination of the charge as derived from the initial calibration is altered by a systematic error of about 0.1e. NB If the CERN or BNL high-energy heavy-ion beams are not accessible we can go to a number of other available sites, although less convenient. These sites are given in the table below:

The medium energy and low energy heavy‐ion accelerator sites are numerous, although we plan to use the Montreal machine for the low energy calibration as the University of Montreal is a member of our collaboration.


Question 21 Tracking of the sheets is essential and crucial, how will this be done, with bar-codes and trailers In MACRO and SLIM, each module unit was uniquely identified externally during the exposure according to its position with respect to the detector. During construction of the MACRO/SLIM stacks and before etching, a code was engraved at one corner of the foil. This etched label could not be erased in the etching process. As the Bologna group has experience with this system thought their work with MACRO & SLIM we will also utilize this labeling scheme for the MoEDAL plastic. NB: Recently the manufacturer has introduced laser technology to inscribe a bar code on the foil, following customer requests. We plan to “test drive” this laser system and utilize it if these test show that it is clearly superior to the standard labeling method employed by MACRO & SLIM described above Question 22 Are there any plans to remove a small number of detectors a few months into the 2009/10 run? Just to check for any unexpected degradation? - Maybe install an "extra" few layers of detector somewhere for this purpose? A m2 of MoEDAL NTD stacks corresponds to around 3 panels of detectors. We could remove one panel in a short shutdown very easily. We hadn't planned that because we had been told there would be no shutdowns over the winter of 2010. But we can plan that. We do have a person in place (Daniel Lacarerre who is the manager for the LHCb experimental area). So we could do it easily if the chance came up. We have now incorporated the potential for the removal, if possible, of one panel in a, as yet unplanned, shut down during running in 2010. Presumably, this removal should take place after at least a few month of running. Question 23 CR39 Z/beta thresholds: 14 for strong etching and 7 for soft etching If do strong etching to identify ROI then doesn't this limit the Z/beta reach to 14? In practical terms how does one achieve the more aggressive Z/beta thresholds discussed in the TDR. You are correct the strong etching does limit the INITIAL search for the relativistic monopole to Z/β > 14. However, the remaining sheets are soft etched - giving the lower threshold. These can be scanned in phase-II of the experiment when we are looking for signals from conventionally charged heavt stable particle and slow monopoles. In this case we would have 3 sheets (adopting Terry Wyatt’s suggestion of 4 sheets of CR39) of soft etched plastic left in which we would search for lower threshold signatures. Again, we would scan the first sheet of soft etched plastic (wrt the IP) and use this as a map for the other CRT39 detectors. In order to get down to the lowest threshold of CRD I,e Z/b > 5 we would need to “super-soft” etch. This is more time consuming but we could do it if we need to. Our present plans have us working at the threshold of Z/β > 7


Question 24 What are the principle issues for LHCb concerning the possibility of HI collisions at pt 8? Radiation? Access? Are LHCb concerned about this? They were very concerned about the suggestion of a heavy-ion running scenario. There worries were primarily radiation damage to electronics, and detector elements.


APPENDIX 1 Target Fragmentation (Spallation) Events

Figure 2 Schematic depiction of a typical “target fragmentation” event

The particles knocked knocked-out from the target fragmentation have very short range as is demonstrated by the example from a study of the recoil properties of nuclei produced in proton interactions on aluminium [2] where the recoil fragment with the longest range traveled only ~1cm in air, ~1µm in aluminium, and ~10 µm in CR39, as can be found from the last column in Table 1. However, these ranges have exponential tails, of low probability out the a few hundred microns. The range of the alpha particles produced is typically only centimeters in air. Of all the products of the Target Fragmentation Events (TFEs), or spallation events, only alpha particles and recoil nuclei produced by the interaction can locally deposit enough energy to reach the CR39 threshold. Note that the ranges of the recoil heavy fragments are small and do not change with increasing energy of the proton. We can draw the following conclusions:

1. One is only concerned with target fragmentation events that take place in the detector or in the aluminium faceplate of the MoEDAL detector housing (fragments formed in the VELO detector region do not reach MoEDAL);

2. Considering p-Al collisions the longest range fragments (Be) travel only ~10 µm in CR39 - penetrating only ~1/50 the way through 1 sheet of CR39. However, the range distribution has an exponential tail, of low probability that, rarely, allows the recoil nuclei to penetrate a complete foil.


Table 1 Table showing the range of target fragments from proton interaction on aluminium

Table 2 Showing average kinetic energy of recoil nuclei for various incident particle energies.

The alpha particles and recoil nuclei are typically produced with low energy, with average value ~10 MeV, as is shown in Table 2 [3] where a number of combinations of target nucleus and recoil nucleus are shown for incident proton energies from 1 to 400 GeV. Note, that the average kinetic energy of the recoil nuclei is essentially independent of incident proton energy.


The angular distribution of heavy recoil fragments provides another way of distinguishing these backgrounds from the signal. Measurements have shown that as the energy of the incident particle increases the angle of the recoil fragment scatters less and less into the forward direction. Remember that the fragment will need to scatter into the forward direction in order to mimic a through going magnetic monopole. Examples of measurements are shown in Figure 3 [4] s which shows angular distributions of 47Sc fragments in p-U reactions for four different energies. The forward to backward ratio (F/B) initially increases with bombarding energy, peaks at about 3 GeV, and then decreases to a value close to unity at 300 GeV. Figure 5 shows some lower energy measurements from protons on aluminium. The angular distribution for the recoiling nuclei for 50 MeV incident protons is very forward. For 200 MeV protons, compound nucleus formation is rare and the recoil is pushed backwards (180 degrees) flattening the angular distribution of the recoil particles as shown in Figure 5. If in some extremely unlikely event a recoil product could penetrate more than one layer of CR39 any recoil angle greater than ~1 degree the aligned etch pits resulting from etching would not point back to the beam axis within ~2 cm. If we assume that the distribution of recoil angles is flat, which is a conservative assumption above ~10 GeV, only roughly 1 in 10,000 recoils would pass the pointing criteria.

Figure 3. Angular distributions of 47Sc emitted in interactions of with high energy protons


Figure 4 Angular distribution of fragments for 50 and 200 MeV incident protons. The recoils are more isotropic for the 200 MeV incident protons incident on Aluminium


APPENDIX 2 DOSE EFFECTS ON ETCHING, AN EXAMPLE


1. P. B Price, J. Giuru, and K. Kinoshita, Phys. Rev. Lett., 65, 149 (1990). 2. ∗ S. Cecchini et al. (Fragmentation cross‐sections and search for nuclear fragments with

fractional charge in relativistic heavy ion collisions) Astropart. Phys. 1 (1993) 369; G. Giacomelli et al. (Extended calibration of a CR39 nuclear track detector with 158‐A‐GeV Pb‐207 ions) NIM A411 (1998) 41; S. Balestra et al.(Bulk etch rate measurements and calibrations of plastic nuclear track detectors) NIM B254 (2007) 254

3. L. Winsberg et al., “ Recoil Properties of Nuclei Produced in the Interaction of Protons with Al”, Phys. Rev. C22, No. 5, p2108, (1980).

4. L.Winsberg, Systematics of Particle Nucleus Reactions: II Nuclear Recoil in Non-Fission Reactions Induced by 1 GeV to 400 GeV Protons, Phys. Rev. C22, No. 5, p2123, (1980).

5. I. Otterlund, Nuclear Physics A335, p507, (1980) 6.

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