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SHIELDING and GROUNDING in LARGE DETECTORS* Veljko Radeka Brookhaven National Laboratory, Upton, NY 11973-5000 ([email protected])

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SHIELDING and GROUNDING in LARGE DETECTORS* Veljko Radeka Brookhaven National Laboratory, Upton, NY 11973-5000 (Radeka@Bnl.Gov) SHIELDING AND GROUNDING IN LARGE DETECTORS* Veljko Radeka Brookhaven National Laboratory, Upton, NY 11973-5000 ([email protected]) Abstract Coupler: Shield (“Faraday cage”) Shielding effectiveness as a function of shield thick- ness and conductivity vs the type and frequency of the interference field is described. Noise induced in trans- mission lines by ground loop driven currents in the shield is evaluated and the importance of low shield resistance is emphasized. Some measures for preven- Cable shield connected to detector shield at the penetration: tion of ground loops and isolation of detector-readout systems are discussed. Detector Counting Area Signal, Low V, ferrite 1. INTRODUCTION High V, control lines core Prevention of electromagnetic interference (EMI), or “noise pickup”, is an important design aspect in large detectors in accelerator environments. It is of particu- ~ 5 50 m lar concern in detector subsystems where signals have insulate ac a large dynamic range or where high accuracy position electrically! power “1” interpolation is performed. Calorimeters are very sen- vacuum pumps, cooling water sitive to coherent noise induced in groups of readout cryo lines, mechanical supports channels where energy sums are formed, covering a “Solid ground” ( “earth” ) large dynamic range. There are several potential noise sources and means of transmission: ac power “2” 1) Noise from digital circuits generated locally on a major ground loops (global) single front end read-out board on the detector; (low impedance) 2) electromagnetic radiation in the space around the Figure 1. An illustration of shielding and ground loop detector generated by other detector subsystems, control concepts. power supplies, silicon-controlled rectifiers, ma- chinery, etc.; Figure 1 illustrates some of the basics of shielding 3) noise induced by penetrations into the detector en- and ground loop control. No leads of any kind should closure (e.g., cryostat) and the front end readout enter a Faraday cage without their shield being con- electronics located on the detector; nected to the detector enclosure at the penetration, oth- 4) currents coming through ground loops, of which the erwise a lead connected to the enclosure would inject detector enclosure and the front end electronics are noise (it presents a coupler) into detector electrodes and a part, caused by any apparatus and machinery out- front end electronics. If the detector enclosure (i.e., its side the detector. front end electronics) and the readout electronics in the Problem 1) of internally generated noise is being counting area have to be connected electrically and not addressed by a careful layout and filtering on the only by optical links, then the cable shield should have board(s), shielding of preamplifiers, and by minimiz- a low resistance and a high inductance to minimize the ing digital operations on the board. noise at the receiving end due to the ground loop volt- The effects of externally generated noise in the form age. Ground loop currents in the enclosure walls and of EM radiation are best reduced by a well designed the transmission lines will be minimized by isolating Faraday shield (“cage”). The effects of noise currents the detector enclosure from the surrounding. flowing through the shields, due to ground loops, are In Section 2, some basics of shielding against exter- also reduced by the Faraday cage. nal EM fields are reviewed. In Section 3, noise pickup in Ideally, ground loops should be avoided entirely. transmission lines from ground loop voltages and currents In practice, preventing formation of ground loops means is discussed. In Section 4, potential ground loops in a large to increase a ground loop impedance over most of the detector subsystem are illustrated in the example of the frequency range as much as possible. ATLAS liquid argon calorimeter. Some practical steps for *This work is supported by the U.S. Department of Energy: isolation are described in Section 5, and in Section 6, the Contract No. DE-AC02-98CH10886. question of the safety ground is addressed. 2. SHIELDING EFFECTIVENESS A copper shield 0.5 mm thick provides a far field AGAINST EM RADIATION attenuation at 100 kHz of only about 21 dB by absorp- tion, and nearly 120 dB by reflection. In this case, ab- 2.1 Thick Shields (t ≥ δ) sorption becomes dominant only above ~ 5 MHz. The reflection attenuation increases with the angle of inci- Shielding properties of enclosures are analyzed in dence. detail in Ref. 1. Only some main points are emphasized here. A hermetic detector or an electronics enclosure 2.2 Very Thin Shields (t<<δ) of highly conductive material, such as copper or alumi- num, provides very high attenuation against external EM For very thin shields, multiple reflections of the mag- fields in the frequency range from a few kHz up to very netic field component within the shield reduce shield- high frequencies. The shielding effect is obtained by ing effectiveness. Total attenuation, including multiple reflection and absorption of the EM wave. Attenuation reflections, is then given by[1], by reflection of an external plane EM wave at normal Z A = 20 logw + 20 log(2 sinh t ) (5) incidence to the shield, with wave impedance ZW is: δ 4Zs Zw δ ≈ δ Ar = 20 log [dB] (1) For very thin shields sinh (t/ ) t/ , and by substi- 4Zs σ tuting Eq. (3) for ZS, and 1/ t for shield dc resistivity per square ρ , where ZS is characteristic impedance of the shield ¨ material, Z σt Z ≈ w w (6) Z = (ωµ/σ)½ = 3.7 × 10-7 f ½ [Ω] (2) Ar 20 log = 20 log . S 2√2 2√2 ρ (for copper) Thus, for very thin shields, attenuation due to re- ω = 2π µ where f is the frequency, is the permeabil- flection is determined simply by the ratio of the wave σ ity, and is the conductivity of the shielding material. impedance and the sheet resistivity of the shield. Low Z can be expressed in terms of the skin depth δ = 2/ S mass shields, such as aluminized Mylar windows on gas ωµσ ½ ( ) as, proportional chambers, can still provide very useful √2 shielding. For example, 1000 Å (0.1 µm) of aluminum, Z = s δσ . (3) which is about 1/800th of the skin depth at 1 MHz, gives ρ ≈ Ω ≈ 1/δσ is simply dc sheet resistivity per square of the ¨ 0.25 /square and Ar 533 = 55 dB. shield layer, one skin depth thick. Characteristic im- A closer inspection shows that Eq. (1) for thick ≈ Ω shields is approximately valid down to t/δ = 0.2, with pedance of the shield is very low, ZS 1m at 10 MHz; 0.1mΩ at 100 kHz. an error of 9.6 dB. The wave impedance ZW (the ratio of the electric field and the magnetic field) depends on the nature of the source 2.3 The Role of Apertures (Gaps) in the Shield (electric or magnetic antenna) and the distance from the source. In the far field, i.e., distance greater than λ/2π, it Any gaps in the shield interrupt the flow of currents approaches the impedance of the free space (and air), which are essential for field attenuation provided by the 377Ω. At f = 10 MHz, λ = 30 m, so that for frequencies less shield. Attenuation by an aperture in the shield is given than 10 MHz, most detectors will be in near field condi- by[2], tions. For an “electric antenna” (high voltage and low cur- λ Aap = 20 log [dB] (7) rent), the wave impedance varies as 1/r, and for a “mag- 2L netic antenna” (high current and low voltage), it varies as where λ is the wavelength and L is the longest dimen- λ π r in the near field r < /2 . Taking the above values for ZS sion of the aperture, regardless of its shape. This indicates Ω and for ZW = 377 , the shielding effectiveness in the far significant field penetration, which increases with fre- field, due to reflection, is: quency. For example, for L = 10 cm, at 10 MHz, Aap is × 4 Ar = 9.4 10 = 99.5 dB at 10 MHz barely above 40 dB. The attenuation reduces to zero dB at = 9.4 × 105 = 119.4 dB at 100 kHz. the wave guide cutoff frequency, λ/2L = 1. The attenua- The attenuation is higher in the near field for an tion can be increased if the openings form a wave guide of electric source, and much lower for a magnetic source some length (e.g., a honeycomb structure). at low frequencies[3]. At lower frequencies (1 kHz–1 MHz), it is possible Shielding attenuation by absorption due to skin ef- to achieve very high shielding attenuation. At high fre- fect is, quencies (>10 MHz) shielding effectiveness will be ap- ωµσ ½ δ erture-limited. The importance of electrical continuity Aa = 6.2 t ( ) = 8.7 t/ [dB] (4) (for copper) of any shield cannot be overemphasized. This is also important for ground loop-driven currents (section 3). where t is thickness of the shield. a) Single ended PA C12 Ω Z Z i 0 0 Z0, Ls, rs 0 n Z0, Ls, rs Z0 Z0 iext in or vext Cb ∆ ω * v in the shield = iext ( j Ls + rs) for rs << Z0 vext Cb >> C12 * ∆v in center lead = iext · jωLs with a shield * emf in the receiver = iext · rs v r 1 i = ext s i r n ω common mode * noise current = n = s for current in the shield Z0 /2 rs + j Ls cmr iext 2Z0 rejection ratio v r * noise current i = ext s for voltage in the loop Figure 3. Balanced transmission line with high rejection n ω 2Z0 rs + j Ls of ground loop noise.
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