Electromagnetic shower profile measurements in iron with electrons
Introduction
There is a pressing need for radiation damage and activation analyses studies for the detectors planned at the Large Hadron Collider (LHC), particularly because some of them will experience several hundreds of megarads during their useful lifetime. The forward calorimeters especially will face unprecedented particle fluxes. In the Compact Muon Solenoid (CMS) experiment, for example, at η=5 in ∼10 years of LHC operation, the forward calorimeters are expected to experience of dose [1].
The CMS forward calorimeter consists of multi-mode synthetic silica-core optical fibers embedded in an iron absorber. The details of this type of a calorimeter can be found in [1], [2]. In order to study the response of the forward calorimeter under these intense radiation conditions, we first performed dose and activation measurements in iron absorbers after having them irradiated with a electron beam at the LEP pre-injector (LPI) facility at CERN. We made Monte Carlo calculations to compare them with the data in an attempt to generalize and to use these results for the radiation damage studies of the prototypes irradiated at the same facility.
The details of the lateral and longitudinal dose profiles in the calorimeter absorber determine the magnitude and the extent of the radiation damage in quartz fibers. The radiation dose degrades the optical transmission in quartz fibers and worsens the response of the detector. In the 400– range, for example, the irradiation induced transmission loss reaches for for most synthetic silica-core and fluorine-doped silica-clad fibers.
The CMS forward calorimeter absorber material(s) will get activated under intense LHC radiation fields. After 2 months of running at an average luminosity () and a day of cool-down period, the activation level is estimated to be several thousand μS/h at the higher rapidity region. In addition to safety and maintanence concerns, this background introduces noise into the calorimeter system. In the case of the iron absorber (Fe I), as this study shows, of dose due to electrons results in of total activity at the shower maximum after about an hour of cool down.
In Section 2, we describe the two iron absorber structures that we constructed. The LPI beam properties are listed in Section 3. The dosimetry and activation analyses are discussed in 4 Dosimetry, 5 Activation analyses, respectively. The EGS4 Monte Carlo results are presented together with the dosimetry and activation analyses. We summarize the results in the last section.
Section snippets
First iron matrix (Fe I)
Fig. 1 shows the structure of the first iron matrix. The total length of the absorber is , deep enough to fully contain 500-MeV electron showers. The iron plates are positioned normal to the beam direction; the first four plates are and the last eight are thick with respect to the beam direction. They measure on a side. Between each plate, a 0.5-mm thick () Fe plate and a radiation sensitive (RISO)2
LPI beam
There is a 10-year time scale planned for the LHC experiments and the expected radiation doses are measured in many Mrads. The existing facilities that are able to provide comparable integarated doses in practicable times are low-energy but intense particle beams. One such facility is the LIL Experimental Area (LEA) at CERN [4], which can provide up to electrons in a dedicated facility of the LPI, downstream of the linac. The samples are placed on a remotely controlled table,
Dosimetry
RISO sheets were placed in the absorber structures (Fig. 1, Fig. 2) and were analysed in square grids. The typical precision of dose measurements using this technique is ±20%. The glass RPL dosimeters were placed in small holes in the absorber, as mentioned in Section 2, and later analyzed. We assume ±20% uncertainty in these measurements. The RPL dosimeters are well-suited and reliable at lower doses. We depended on RISO paper measurements for doses that exceed .
Fig. 3 shows the
Activation analyses
The reasons for performing activation studies were two-fold: to measure the activation levels of the iron absorber subjected to an intense electron beam and to attempt to measure the longitudinal shower profile using these data. In doing so, we have identified the active isotopes and their levels of activation and also observed that the activation profile along the depth of the detector is a good measure of average gamma profiles and energies within the absorber. We also attempted to determine
Results and discussions
We draw the following conclusions at present:(1) The CMS forward calorimeter will experience integrated doses up to a Grad or more at the highest rapidities. The activation of the iron absorber will reach , if we use a simple conversion of without being concerned with the short half-life isotopes and activation/deactivation cycles during runs and assume that all of the dose is due to photonuclear reactions. This clearly would be an underestimate. (2) The EGS4 simulation reproduces
Acknowledgements
We thank Marc Tavlet for illuminating discussions in dosimetry, and the PLI operation team for running the LEA facility with efficiency. This work was supported in part by the US Department of Energy and the Scientific and Technical Research Council of Turkey, TUBITAK.
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Now at Department of Physics, Texas Tech University, Lubbock, TX 79409, USA.