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PoS(FAIRness2022)003
Solutions for humidity and temperature monitoring in the
Silicon Tracking System of the Compressed Baryonic
Matter experiment: sensors, and testing
Marcel Bajdel,𝑎,𝑏 ,∗Kshitij Agarwal,𝑐Ulrich Frankenfeld,𝑎Johann Heuser,𝑎Shaifali
Mehta,𝑐Hans Rudolf Schmidt𝑎,𝑐 and Peter Zumbruch𝑎
𝑎GSI Helmholtzzentrum für Schwerionenforschung,
Darmstadt
𝑏Goethe University,
Frankfurt am Main
𝑐Eberhard Karls Universität,
Tübingen
E-mail: m.bajdel@gsi.de
The Compressed Baryonic Matter (CBM) is one of the leading scientific programs of the future
Facility for Anti-proton and Ion Research (FAIR), Darmstadt, Germany. The Silicon Tracking
System (STS) will be the core detector system of CBM for charged-particle reconstruction and
momentum measurement. It will be placed inside a 1T·m magnet and kept at an ambient temper-
ature of about −10 ◦C to mitigate radiation-induced bulk current in the silicon sensors. Due to the
conditions inside the STS reliable monitoring and control of humidity and temperature is required
to avoid icing or water condensation on the electronics or silicon sensors. Fiber Optic Sensors
(FOS) based on Fiber Bragg Grating (FBG) have proven to be suitable environmental monitoring
sensors due to their resilience to the magnetic field, ionizing radiation, and miniature size. In
this contribution, we introduce two different approaches to implement relative humidity (RH)
and temperature FBG FOS. The first approach is based on inscribing both RH and temperature
FBG into one fiber and the second one features two separate FBGs arrays. In both cases, the
RH-sensitive FBGs are coated with polyimide.
FAIR next generation scientists - 7th Edition Workshop (FAIRness2022)
23-27 May 2022
Paralia (Pieria, Greece)
∗Speaker
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Attribution-NonCommercial-NoDerivatives 4.0 International License (CC BY-NC-ND 4.0). https://pos.sissa.it/
PoS(FAIRness2022)003
Solutions for RH and T monitoring in the STS of the CBM experiment: sensors, and testing Marcel Bajdel
1. The Silicon Tracking System - temperature and humidity monitoring
The design of the Silicon Tracking System (STS) [1] defines the requirements for ambient
sensors. Apart from resilience to the radiation and magnetic field, the dew point and ambient
temperature define the sensor’s working range. The minimum temperature inside the STS is defined
by the coolant (3M NOVEC 649) temperature of −40 ◦Cused for cooling the Front-End Electronics
(FEE), whereas the targeted ambient temperature is −10 ◦C. To avoid icing or condensation and any
potential damage to the detector, the ambient parameters like Relative Humidity (RH) or temperature
need to be monitored. In order to mitigate the risk, the dew point will be kept at levels below −45 ◦C.
Three different sensor technologies will be measuring the ambient parameters: Fiber Bragg
Grating (FBG) sensors, commercially available capacitive sensors (SHT series), and a sniffing
system based on trace humidity sensors. Several sniffing points inside the detector enclosure will
measure trace humidity and serve as a reference for the two other measurement technologies.
Moreover, the sniffing system is going to be the base for the interlock system, which will take action
in case of hazardous humidity levels. In this contribution, we summarize efforts to choose, design,
and characterize RH FBG sensors.
2. Fiber Bragg Grating sensors
Fiber Bragg Grating is a selective filter that reflects the light signal at a certain wavelength
named Bragg wavelength (see Figure 1). A bare FBG, inscribed into the fiber core, is sensitive to
temperature, and strain.
Figure 1: FBG-based RH sensor (left side) and wavelength shift induced by relative humidity change (20%
to 70%)
By applying a hygroscopic material (for example polyimide) over the cladding, we can measure
RH instead of strain. In this case, the Bragg wavelength shift becomes a superposition of temperature
and humidity effects (see equation 1) [2].
Δ𝜆𝐵
𝜆𝐵
= Δ𝑇 𝑆𝑇+Δ𝑅𝐻 𝑆𝑅𝐻 (1)
where 𝜆𝐵is the Bragg wavelength, 𝑆𝑇is the temperature sensitivity and 𝑆𝑅 𝐻 is the relative humidity
sensitivity.
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PoS(FAIRness2022)003
Solutions for RH and T monitoring in the STS of the CBM experiment: sensors, and testing Marcel Bajdel
In order to measure RH, it is crucial to have precise temperature readouts in the vicinity of
the coated FBG. Otherwise, the actual RH readout may be dominated by uncertainty or just the
inaccurate temperature measurement.
3. Results
Two different designs of FOS were tested – 5 sensors in an array (a multiplexed version was
produced by Technica Optical Components) and a single RH sensor combined with a temperature
sensor (hygrometer was produced by AOS Electronics). The polyimide coating was 15 𝜇m thick
for the hygrometer and 4x5 𝜇m for the array. Both kinds of sensors were characterized in terms of
sensitivity to temperature (−20 ◦C to 30 ◦C, see Figure 2) and humidity (10% to 80%, see Figure
3), time response, hysteresis, and repeatability.
Figure 2: Sensors temperature sensitivity STFigure 3: Sensors humidity sensitivity SRH
The average temperature sensitivity 𝑆Tvalue for the array is 10.25 ±0.02 pm
◦Cand for the
hygrometer 10.87 ±0.02 pm
◦C(see Figure 2). On the other hand, the thicker layer of polyimide coating
results in higher RH sensitivity 𝑆RH for sensors 1–5, enabling more precise RH measurements
with the array. The average 𝑆RH value for the array is 2.77 ±0.03 pm
%RH and for the hygrometer
2.09 ±0.02 pm
%RH (see Figure 3).
To evaluate the time response all sensors were subjected to the humidity change from 10% to
80%. The array sensors have a longer response time (described as a time to reach 63% of the final
RH 𝜏63 =10 min) than the hygrometer (𝜏63 =6 min), which is also related to the coating thickness.
A thicker coating of more than 20 𝜇m leads to increasing time response and humidity sensitivity
[3]. The hygrometer’s uncertainty is around 1%, whereas for the array on average around 2.5%.
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PoS(FAIRness2022)003
Solutions for RH and T monitoring in the STS of the CBM experiment: sensors, and testing Marcel Bajdel
4. Conclusions
The hygrometer shows generally a better performance (time response, hysteresis, repeatability)
than the sensors in the array. Dew points down to −70 ◦Cwill be further tested with the thermal
demonstrator, which simulates final STS ambient conditions. Due to the sensor’s characteristic, it
will not measure accurately below 1% RH. Nevertheless, the main features that make the sensor
a viable solution are radiation hardness and repeatability. It is going to provide a valuable insight
into the detector’s environment conditions throughout its lifetime. In addition, sensors’ readouts
will ensure safe operation of the detector.
References
[1] J. Heuser, W. Müller, V. Pugatch, P. Senger, C.J. Schmidt, C. Sturm et al., eds., [GSI Report
2013-4] Technical Design Report for the CBM Silicon Tracking System (STS), GSI, Darmstadt
(2013).
[2] P. Kronenberg, P.K. Rastogi, P. Giaccari and H.G. Limberger, Relative humidity sensor with
optical fiber bragg gratings,Opt. Lett. 27 (2002) 1385.
[3] T. Yeo, T. Sun, K. Grattan, D. Parry, R. Lade and B. Powell, Characterisation of a
polymer-coated fibre bragg grating sensor for relative humidity sensing,Sensors and
Actuators B: Chemical 110 (2005) 148.
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