Saturated fluorocarbons (SFCs) of the form CnF(2n+2) are chosen for their optical properties (high transparency in the near UV, adapted refractive index and low chromatic dispersion over wavelength ...ranges of interest) for use as Cherenkov radiators. The COMPASS and LHCb experiments currently use C4F10 and CF4 gas radiators. SFCs have high Global Warming Potentials (GWPs), however (in the range 5000–9000). There is thus an impetus to reduce their use and wastage through leaks in existing installations.
Newer fluids of the form CnF2nO, including the 3M NOVEC® range, can offer similar optical performance to SFCs of the same order, n, but with GWPs equivalent to CO2. These GWPs are, however, geometry-specific: closed molecular rings having an oxygen atom as a link can have GWPs as high as those of SFCs, and should be avoided.
While the optical constraints of RICH detectors can motivate a “special case” argument to retain the use of preferred SFCs, legislation and external market forces will limit their future availability, leaving “holes” in the CnFx spectrum that might not be filled by NOVEC equivalents. This situation might require the blending of low molar concentrations of heritage-stock higher-order SFCs and NOVEC vapours with transparent light gases such as nitrogen, and is the subject of this paper.
While continuous optical measurement of refractive index in dynamically changing gas mixtures is very demanding, the monitoring of sound velocity has been historically shown to provide simple, reliable and continuous real time mixture information. Indeed the speed of sound is, in itself, a monitor of the speed of light and the Cherenkov threshold in a gas radiator.
This ultrasonic (“sonar”) technique was first used for controlling the real-time blending C5F12 with N2 in the SLD CRID at the SLAC linear collider. The technique could become important again in future operation to meet the optical and low GWP constraints of future Cherenkov gas radiators. Some examples are explored in this paper.
Saturated fluorocarbons (SFCs) of form C
n
F
(2
n
+2)
are chosen for their optical properties as Cherenkov radiators, with C
4
F
10
and CF
4
currently used at CERN in the COMPASS and LHCb ring ...imaging Cherenkov detectors. Their non-conductivity, non-flammability and radiation-resistance also make SFCs ideal coolants: C
6
F
14
liquid cooling is used in all LHC experiments, while C
3
F
8
is used for the evaporative cooling of TOTEM and the ATLAS silicon tracker. These fluids, however, have high global warming potentials (5000–10000*GWP
CO2
), and represented around 36% of CERN’s CO
2
-equivalent emissions in 2018. There is thus an impetus to reduce their use, losses in purification and wastage through leaks, through improved monitoring and closed circulation system design. Newer spur-oxygenated fluoro-ketones, for example from the 3 M NOVEC
®
range, with C
n
F
2
n
O structures, can offer similar performance to SFCs with but with very low, or zero GWP. Although these fluids do not yet exist in large quantities over the full C
n
F
2
“matrix” the radiation tolerance and thermal performance of NOVEC 649 (C
6
F
12
O) was sufficiently promising for it to be chosen as a C
6
F
14
replacement for cooling silicon photomultipliers. Additionally, subject to optical testing, NOVEC 5110 (C
5
F
10
O) could (if blended with nitrogen) replace both C
4
F
10
and CF
4
in Cherenkov detectors. Lighter molecules (for example C
2
F
4
O, with similar thermodynamics to C
2
F
6
)—if and when available in industrial quantities—might allow lower temperature operation than evaporative CO
2
in future silicon trackers operated at very high luminosity. Ultrasonic gas mixture analysis is very sensitive to concentration changes of a heavy vapour in a light carrier, and is used—in the only such fluorocarbon coolant leak monitoring system operating at LHC—for real-time monitoring of C
3
F
8
coolant leaks from the ATLAS pixel and SCT silicon trackers into their nitrogen-flushed environmental volumes. A typical C
3
F
8
sensitivity of better than 10
−5
is achieved. Advanced new ultrasonic algorithms allow measurement of the concentrations of a pair of gases of particular interest on top of a varying known baseline of other gases. The technique is thus of considerable value in leak monitoring and could be used to blend fluoro-ketones with nitrogen or argon to reduce the GWP “load” of large volume atmospheric pressure gas Cherenkov radiators without the recourse to higher-pressure noble gas approaches. This paper outlines an approach to GWP reduction with fluoro-ketone fluids and the blending of heritage SFCs or fluoro-ketones with lighter gases using ultrasonic monitoring and control. Possible avenues for the use of fluoro-ketones in liquid phase and evaporative cooling of silicon trackers are discussed.
Despite its success in the SLD CRID at the SLAC Linear Collider, ultrasonic measurement of Cherenkov radiator refractive index has been less fully exploited in more recent Cherenkov detectors ...employing gaseous radiators. This is surprising, since it is ideally suited to monitoring hydrostatic variations in refractive index as well as its evolution during the replacement of a light radiator passivation gas (e.g. N2, CO2) with a heavier fluorocarbon (e.g. C4F10CF4; mol. wt. 18888). The technique exploits the dependence of sound velocity on the molar concentrations of the two components at known temperature and pressure. The SLD barrel CRID used an 87%C5F12/13%N2 blend, mixed before injection into the radiator vessel: blend control based on ultrasonic mixture analysis maintained the β=1 Cherenkov ring angle to a long term variation better than ±0.3%, with refractivity monitored ultrasonically at multiple points within the radiator vessel.
Recent advances using microcontroller-based electronics have led to ultrasonic instruments capable of simultaneously measuring gas flow and binary mixture composition in the fluorocarbon evaporative cooling systems of the ATLAS Inner Detector. Sound transit times are measured with multi-MHz transit time clocks in opposite directions in flowing gas for simultaneous measurement of flow rate and sound velocity. Gas composition is evaluated in real-time by comparison with a sound velocity/composition database.
Such instruments could be incorporated into new and upgraded gas Cherenkov detectors for radiator gas mixture (and corresponding refractive index) measurement to a precision better than 10−3. They have other applications in binary gas analysis - including in Xenon-based anaesthesia. These possibilities are discussed.
The ANTARES neutrino telescope is currently the largest operating water Cherenkov detector and the largest neutrino detector in the Northern hemisphere. It comprises 885 optical modules distributed ...on 12 detection lines anchored at a depth of 2.5 km in the Mediterranean Sea near Toulon, France; at a latitude that accesses a large part of the Galactic Plane, including the Galactic Centre. Its main scientific target is the detection of multi-TeV neutrinos predicted in charged cosmic particle acceleration mechanisms. In addition, ANTARES has developed a range of multi-messenger search strategies to look for correlations with optical counterparts and other cosmic messengers including gamma -rays and charged cosmic rays. Other topics of investigation include the search for neutrinos from dark matter annihilation, searches for exotic particles and the measurement of neutrino oscillations. Details of the telescope are discussed together with examples of recently-conducted searches.
The ANTARES underwater neutrino telescope is in an advanced stage of construction at a depth of 2500
m in the Mediterranean Sea near Toulon. The complete detector will contain 12 anchored detection ...lines, each equipped with 75 photomultipliers housed in glass pressure spheres and arranged in 25
triplets between 100 and 450
m above the sea floor. The photomultipliers are arranged to look down at
45
∘
to be sensitive to Cherenkov light from upward going muon tracks produced by extraterrestrial high energy neutrinos traversing the Earth. Such neutrinos arrive undeviated from a variety of astrophysical sources and might be produced in the possible annihilation of dark matter neutralinos. Data from five detection lines in operation since January 2007—and 10 lines since December 2007—are allowing not only a detailed study of detector performance but also the reconstruction of downgoing cosmic ray muons and the search for the first upward going neutrino-induced muons. Constructional details of the telescope are presented and the current status and operations are discussed.
The status of Cherenkov detectors in astroparticle physics Hallewell, G.D.
Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment,
11/2005, Letnik:
553, Številka:
1
Journal Article, Conference Proceeding
Recenzirano
The Cherenkov detectors used in astroparticle physics are found in the most varied of environments. Used for the detection of charged cosmic rays, γ-rays and neutrinos, these detectors can be found ...on land, suspended beneath high-altitude balloons, orbiting in space, or implanted at great depth under land, in the oceans or southern polar ice. Stereoscopic imaging air Cherenkov telescope arrays are used in the study of high-energy γ-rays on several continents. The largest detector arrays in existence use thousands of photomultiplier tubes to record the arrival of the Cherenkov light from neutrinos or from secondary particles created in extensive air showers. Representatives from this rich catalog of innovation are outlined.
Status of the ANTARES underwater neutrino telescope Hallewell, G.D.
Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment,
04/2003, Letnik:
502, Številka:
1
Journal Article, Conference Proceeding
Recenzirano
Odprti dostop
The ANTARES Collaboration is constructing a deep underwater neutrino detector for operation at −2400m off the French Mediterranean coast near Toulon. The detector, which will begin operation in 2004, ...will have an aperture of ∼0.1km2, and will contain 900 photomultiplier tubes. The photomultiplier axes will be angled 45° downward toward the seabed to observe the Cherenkov emissions of upward-going muons created by the interactions in or near the detector of high energy neutrinos traversing the Earth. These neutrinos arrive undeviated from a variety of galactic and extragalactic sources of astrophysical interest, and might be produced in the possible annihilation of dark matter neutralinos. The design and present status of the detector are summarized. Results from site evaluation and the development of supporting instrumentation are outlined.
•Addition of up to 25% of C2F6 into a C3F8 refrigerant effectively decreases the evaporation temperature.•The heat transfer coefficient somewhat decreased with the progressive addition of ...C2F6.•Various flow regimes in the horizontal evaporator are clearly visible from the obtained data.
We have measured the flow boiling heat transfer coefficient (HTC) of saturated fluorocarbon blends in a horizontal copper-nickel tube with a diameter of 4mm. Direct (Joule) heating of the tube wall was used to obtain heat fluxes from 5 to 13.7kWm−2. Two different injection capillaries were used, permitting measurements at mass fluxes varying between 94 and 164kgm−2s−1. The evaporation pressure was approximately 0.2MPa for the mass fluxes close to 164kgm−2s−1 and 0.15MPa for the lower mass fluxes. The same tube dimensions and material are used in the on-detector cooling channels of the silicon micro-strip charged particle tracker (“SCT”) of the ATLAS experiment at the CERN Large Hadron Collider. The range of heat flux and mass flow studied encompasses the operating conditions of the tracker. When operating in the high radiation environment near to the proton beam collisions radiation tolerant coolants are essential. Saturated fluorocarbons (CnF(2n+2)) are radiation resistant and allow thermodynamic “tailoring” by blending saturated molecules of different orders. Measurements were made with pure C3F8 (R218) and zeotropic blends containing 5, 10, 15, 20 and 25% (molar) C2F6 (R116). This work is a continuation of a previous study which showed that the operating temperature of the ATLAS SCT could be reduced by around 10°C with the admixture of 25% (molar) C2F6, with no changes needed to the existing on-detector and circulatory pipework. The data analysis revealed multiple flow boiling regimes of the two-phase flow that varied as a function of coolant flow rate, heat flux, vapour quality and mixture composition. As expected, the HTC in pure C3F8 was higher than in blends with increasing C2F6 content. Nevertheless, the study confirmed that the ATLAS SCT could be operated at full power dissipation with cooling tube temperatures up to 10°C colder than in pure C3F8 with C3F8/C2F6 blends having relatively modest values of HTCs in the range 1500–4000Wm−2K−1.
A
bstract
A search for pair production of vector-like quarks, both up-type (
T
) and down-type (
B
), as well as for four-top-quark production, is presented. The search is based on
pp
collisions at
s
...=
8
TeV recorded in 2012 with the ATLAS detector at the CERN Large Hadron Collider and corresponding to an integrated luminosity of 20.3 fb
−1
. Data are analysed in the lepton-plus-jets final state, characterised by an isolated electron or muon with high transverse momentum, large missing transverse momentum and multiple jets. Dedicated analyses are performed targeting three cases: a
T
quark with significant branching ratio to a
W
boson and a
b
-quark
T
T
¯
→
Wb
+
X
, and both a
T
quark and a
B
quark with significant branching ratio to a Higgs boson and a third-generation quark (
T
T
¯
→
H
t
+
X
and
B
B
¯
→
H
b
+
X
respectively). No significant excess of events above the Standard Model expectation is observed, and 95% CL lower limits are derived on the masses of the vector-like
T
and
B
quarks under several branching ratio hypotheses assuming contributions from
T
→
Wb
,
Zt
,
Ht
and
B
→
Wt
,
Zb
,
Hb
decays. The 95% CL observed lower limits on the
T
quark mass range between 715 GeV and 950 GeV for all possible values of the branching ratios into the three decay modes, and are the most stringent constraints to date. Additionally, the most restrictive upper bounds on four-top-quark production are set in a number of new physics scenarios.
A
bstract
Charged-particle spectra obtained in Pb+Pb interactions at
s
N
N
=
2.76
TeV and
pp
interactions at
s
N
N
=
2.76
TeV with the ATLAS detector at the LHC are presented, using data with ...integrated luminosities of 0.15 nb
−1
and 4.2 pb
−1
, respectively, in a wide transverse momentum (0
.
5
< p
T
<
150 GeV) and pseudorapidity (|
η
|
<
2) range. For Pb+Pb collisions, the spectra are presented as a function of collision centrality, which is determined by the response of the forward calorimeters located on both sides of the interaction point. The nuclear modification factors
R
AA
and
R
CP
are presented in detail as a function of centrality,
p
T
and
η
. They show a distinct
p
T
-dependence with a pronounced minimum at about 7 GeV. Above 60 GeV,
R
AA
is consistent with a plateau at a centrality-dependent value, within the uncertainties. The value is 0
.
55 ± 0
.
01(stat
.
) ± 0
.
04(syst
.
) in the most central collisions. The
R
AA
distribution is consistent with flat |
η
| dependence over the whole transverse momentum range in all centrality classes.
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