=Paper=
{{Paper
|id=Vol-2023/318-323-paper-51
|storemode=property
|title=First results of the radiation monitoring of the GEM muon detectors at CMS
|pdfUrl=https://ceur-ws.org/Vol-2023/318-323-paper-51.pdf
|volume=Vol-2023
|authors=L. Dimitrov,P. Iaydjiev,A. Marinov,G. Mitev,F. Ravotti,I. Vankov
}}
==First results of the radiation monitoring of the GEM muon detectors at CMS==
https://ceur-ws.org/Vol-2023/318-323-paper-51.pdf
Proceedings of the XXVI International Symposium on Nuclear Electronics & Computing (NEC’2017)
Becici, Budva, Montenegro, September 25 - 29, 2017
FIRST RESULTS OF THE RADIATION MONITORING OF
THE GEM MUON DETECTORS AT CMS
L. Dimitrov1, P. Iaydjiev1, a, A. Marinov1, G. Mitev1,
F. Ravotti2, b, I. Vankov1, c
1
Institute for Nuclear Research and Nuclear Energy, BAS, Blv. Tzarigradsko Shosse 72, Sofia, 1784
2
CERN, CH-1211 Geneva 23, Switzerland
E-mail: a plamen.stoianov.iaydjiev@cern.ch, b Federico.Ravotti@cern.ch, c Ivan.Vankov@cern.ch
The higher energy and luminosity of future HL-LHC imposed the development and testing of a new
type high-rate detector known as GEM (Gas Electron Multiplier). A monitoring system designed to
measure the radiation dose and particle fluence absorbed by the GEM detectors has been produced
and installed at the CMS detector of LHC. It consists of a readout-control module, to which there can
be connected up to 12 radiation monitors (RADMON). There are in each unit two types of sensors:
RadFETs, measuring the total radiations dose and p-i-n diodes – for particle fluence.
For the first test, a group of 6 GEM chambers was placed at the inner CMS endcap in March this year
with one RADMON controlling the dose. After about 4 months of operation, the first results are
analyzed. They show that for the integral luminosity 46 fm-1 the dose and the particle fluence are low
(about 0,5 Gy and 3.1010 cm-2 1 MeVneq fluence) and only two more sensitive sensors are giving
measurements and providing data that can be analyzed. Nevertheless, the experimental results
confirm the dose and fluence values simulated by FLUKA. This is an important result for the
radiation hardness test plans of the GEM part of the CMS muon detector.
Keywords: gas electron multiplier (GEM) detector, radiation dose, particle fluence,
monitoring
© 2017 L. Dimitrov, P. Iaydjiev, A. Marinov, G. Mitev, F. Ravotti, I. Vankov
318
Proceedings of the XXVI International Symposium on Nuclear Electronics & Computing (NEC’2017)
Becici, Budva, Montenegro, September 25 - 29, 2017
1. Introduction
The increase of the energy and luminosity during the coming upgrades of the CERN LHC creates
a more hostile environment for the detector used. In the muon system of the CMS the heaviest conditions
will be created in the region with pseudorapidity 1,6 < η < 2,2, (fig. 1), where for the high- luminosity
phase of the LHC, Monte Carlo simulation gives particle rates of several kHz/cm2. This imposes severe
restriction on the technology that can be used.
Figure 1. Transverse section of the CMS detector showing the present muon system including RPCs, DTs
and CSCs. The test GEM chambers are installed at the free place in inner endcap marked as GE 1/1
To solve the problem it was decided [1] to investigate the possibilities of the so-called GEM
detectors. They are [2] Micro-Pattern Gaseous Detectors that feature 50-100 microns spatial resolution, 4-
5 ns time resolution, high detection efficiency, as well as proven high-rate capability and resilience against
aging effects. The very high time and spatial resolution enables their simultaneously application for
triggering and tracking information.
After numerous tests [3,4] three GEM detector prototypes were produced and tested at the end of
2016. Each prototype is composed by two chambers, which are mounted face-to-face and called GEM
super-chamber. In correspondence with the CMS plans [5], in December 2016 they were installed in the
first vacant insertion slots at inner endcap stations (labeled GE1/1, fig. 1). For the first test, only one
radiation monitor (RADMON) of the created GEM dosimetric system [6] was installed at the center of one
of the GEMs super-chambers.
2. Sensors and Readout system.
The installed RADMON (fig. 2) is a little different from the described in [6]. The RadFET LAAS
-3
16, which has a relatively small working dose range (10 ÷10 Gy [7]), was replaced by another type –
REM 130, with a dose range up to 200 kGy. The reason is that the same RADMON will be used at the
GEM chambers, which will be installed nearer to the beam – in the insertion slot ME0 (2,1 < η < 2,5, see
fig. 1). There a much higher dose value is expected.
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Proceedings of the XXVI International Symposium on Nuclear Electronics & Computing (NEC’2017)
Becici, Budva, Montenegro, September 25 - 29, 2017
Figure 2. RADMON PCB. The four radiation sensors are installed at its right side
Table 1. Information of the used radiation sensors
The basic data of the radiation sensors used are shown in Table 1. The relation between the
RadFET’s gate threshold voltage shift Vth and the radiation dose D is strongly dependent on the electric
field in the oxide during irradiation and on the process parameters, especially the thickness of the gate
oxide. It is nonlinear and best approximated by Vth = a×D , (resp. D = (Vth) /a), where the coefficients
b 1/b
a and b depend on the RadFET type as well as on the measured dose range. For this purpose, the entire
operating dose range is divided into zones for each of which a and b have different values [8,9]. As can be
seen from the table 1 the new RadFET REM 130 has about six times lower sensitivity than the REM 250
for the initial dose range.
Figure 3. RADMON control and read-out
system 320
Proceedings of the XXVI International Symposium on Nuclear Electronics & Computing (NEC’2017)
Becici, Budva, Montenegro, September 25 - 29, 2017
The shift of the p-i-n diodes forward voltage VF is proportional to the 1-MeV neutron equivalent
particle fluence Φ [cm-2]. The relation is generally linear – Φ = cVF, where c depend of the diode type.
Table 1 shows that LBSD Si-1 is about 50 times more sensitive than BPW34S.
The readout system measures the voltages on the sensors using current pulses of different
amplitudes and durations prescribed by the producer [6]. Each read analog voltage is fed to a common 12-
bit ADC whose least output voltage step is 8 mV and defines the resolution of the dose/fluence
measurements. The system has a modular structure and one module (fig. 3) can read and control up to 12
RADMONS.
4. Experimental results
Here we shall try to analyze the first data, read during the period of 15.05 to 1.11.2017 (the last
data are received after the NEC-2017 symposium, during the preparation of this talk for publication
and
Figure 4. CMS integrated luminosity 2017
were very important to precise the analyses). First of all it has to be mentioned, that the CMS integrated
luminosity delivered by LHC during this period (about 46 fb-1, fig. 4 [10]) is relatively low,
RADMON RADMON
POSITION POSITION
Figure 5. FLUKA simulation of the dose and 1-MeVneq fluence around the
RADMON position in CMS
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Proceedings of the XXVI International Symposium on Nuclear Electronics & Computing (NEC’2017)
Becici, Budva, Montenegro, September 25 - 29, 2017
for the operating ranges of the sensor used. This can be seen from the FLUKA simulations of the
dose and fluence distribution in CMS and cavern (fig. 5, [11]): around the RADMON position a dose of
about 0,5 Gy and a 1MeVneq fluence of about 1010 cm-2 can be estimated.
These results immediately show that there will be no useful data from the sensors REM 130 and
BPW34S, whose operating ranges begin from a few Gy and 2.1012 cm-2 respectively. Nevertheless, we
found, that the voltage values received from them are not zero, which means that these sensors are
operating and the control system read their data normally.
For our investigation, we took the experimental data from 5 different days during the period of
LHC operation. In Table 2 for each of these days there are shown: the LHC integral luminosity until this
moment; the total radiation dose – simulated by FLUKA and measured by the RadFET REM250; the
1MeVneq-fluence – simulated and measured by the p-i-n diode LBSD Si-1. These results are illustrated
and linearly approximated in fig. 6. The errors are relatively high because of the low values of the dose
and fluence – the maximal measured voltage shifts are only about several tens of millivolts.
Table 2. Dose and fluence results
Date of Integral DOSE FLUENCE
measurements luminosity FLUKA REM250 FLUKA LBSD Si-1
fb-1 Gy Gy cm -2
cm-2
7.8.2017 15 0,15 0,13 4,50E+09 4,63E+09
15.8.2017 17 0,15 0,13 4,50E+09 4,42E+09
5.9.2017 22 0,20 0,22 6,50E+09 6,95E+09
18.10.2017 38 0,35 0,36 1,05E+10 1,13E+10
1.11.2017 46 0,5 0,45 1,35E+10 1,43E+10
D [Gy] REM250 - Absorbed dose
0,6
FLUKA
y = 0.011x - 0.0336
0,4 Measured
y = 0,0104x - 0,0285 Linear
0,2 (FLUKA)
Linear
(Measured)
10 20 30 40 50 L [fb-1]
Φ [cm-2]
Si-1 - 1MeVneq fluence
y = 3E+08x - 4E+08 FLUKA
Measured
y = 3E+08x - 9E+07
Linear
(FLUKA)
Linear
(Measured)
0 10 20 30 40 50 L [fb ]
Figure 6. Simulated and experimental results and their linear
approximation
The results show a very small difference between the simulated and measured value especially
taking into account the lower precision of the sensors in their initial operating range. They indicates also,
that the relation between the integrated luminosity delivered by LHC and the absorbed dose (resp. the
fluence) can be consider linear.
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Proceedings of the XXVI International Symposium on Nuclear Electronics & Computing (NEC’2017)
Becici, Budva, Montenegro, September 25 - 29, 2017
5. Conclusions
Due to the relatively low integral luminosity delivered by LHC until now, the measured dose
and fluence values are in the initial operating range of the sensors. For this reason, only the
more sensitive sensors give useful data until now though with lower accuracy.
There is good agreement between the data from FLUKA and the experiment, which is a
confirmation of the reliability of the CMS BRILL dose simulation at the position of the
GE1/1.
All radiation sensors are active and with the increasing of the LHC integral luminosity the
data from all the RADMON sensors can be analyzed and compared with the FLUKA.
6. Acknowledgements
The "Radiation Monitoring of the GEM Muon Detectors at CMS" is part of the "CMS
MUON ENDCAP GEM UPGRADE" project, which is financed by the Bulgarian Scientific Fund at
the Ministry of Education, Youth and Science – grant DCERN 01/2 25.11.2011.
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