PRESENT AND FUTURE OF RESONANT DETECTORS

Transcription

1 RENCONTRES DE MORIOND 2003 PRESENT AND FUTURE OF RESONANT DETECTORS or Bars and Spheres : The hardware side MASSIMO BASSAN Università di Roma Tor Vergata and INFN - Sezione Roma2 For the ROG Collaboration AND the MINIGRAIL group

2 ABSTRACT some elementary considerations on: BAR DETECTORS: SOME TECHNICAL TERMS Crucial components that make an antenna work Sensitivity: h, S h (f), T eff, f and all that MORIOND 2003 SENSITIVITY and BANDWIDTH: WHERE WE STAND and where can we go? Handles to improve present performances : New amplifiers and/or transducers Colder bars New resonators The SPHERICAL WORLD: News from MINIGRAIL

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4 The peak sensitivity depends on T/MQ h ( f ) The bandwidth depends on the transducer (β) and amplifier (T n ) Calibration peak h min h ( f a ) τ g f We Need to broaden AND deepen the dips in this curve: => More peak sensitivity => AND more bandwidth E min k B T τ f

5 Therefore, to improve sensitivity: We need to improve the peak spectral sensitivity - Increase M : large and/or multimode detectors - Reduce T/Q : ultracryogenics. New materials h ( f a ) We also need to increase the bandwidth f - Increase β : transducer w/ tighter coupling - Reduce T n : better amplifier (double SQUIDs)

6 A DICTIONARY OF ANTENNA TERMS Thermal noise S F = MkTω r /Q V p R p Amplifier noise V n ; I n T n = V n2 I n2 /k Antenna Cd M L 0 L i The mechanical oscillator Mass M Speed of sound v s Temperature T Quality factor Q Res. frequency f a The transducer Efficiency β The amplifier Noise temperature T n

7 NOISE TEMPERATURE, WAVE AMPLITUDE AND SPECTRAL SENSITIVITY E min k B T eff = 2k B T wideband noise thermal noise f Minimum detectable energy change Bandwidth f = 4 f Q T Teff A low effective temperature makes the sensitivity higher and the bandwidth larger strain sensitivity h o = 1 τ g S h (f o ) 2 π f = L 2v s 2 τ g k B T eff M

8 RESONANT TRANSDUCER (I) : A resonant transducer with a mass m=µm allows us to gain a factor µ -1 in β. But it also introduce a bandwidth limitation f< f a µ (transducer motion noise grows intolerably outside f ) < x th 2 >= kt mω 2

9 TWO MODE DETECTOR (2) So, coupling β improves with lighter transducer mass m t But bandwidth and thermal noise improve with heavier m t An optimum does exist for m t : As amplifiers improve, transducers can be made more massive Beats in Explorer -Aug 2002

10 IMPROVING β : Better Transducers As the coupling grows, the resonant transducer mass can be made larger : The largest you can imagine it, it is as massive as the antenna itself: two large masses coupled by a readout (looks familiar?) the detector becomes very wide band R. Drever et al. Nature, circa 1971 This idea has been recently reexamined by the Auriga group (J.P. Zendri in a few minutes).

11 IMPROVING TRANSDUCER TECHNOLOGY: MORIOND 2003 The rosette capacitive transducer; gap=9µm

12 EXPLORER has been on the air since May 2000 with: -new, 10 µm gap transducer -New, high coupling SQUID The noise temperature is < 5 mk for 84% of the time. Bandwidth: the detector has a sensitivity better than Hz -1/2 on a band larger than 40 Hz WIDENING THE BAND IN EXPLORERER before 1999 after 2000

13 GW spectral amplitude (h/rt(hz)) EXPLORER PERFORMANCES Calibration peak E= V/m h = E= V/m h = frequency (Hz)

14 IMPROVING Τ n : Better Amplifiers A SQUID is so good an amplifier that noise from the second stage is usually dominant. The only suitable second stage is another d.c. SQUID. However the two devices tend to disturb each other!!! Trento (2 stage) Several efforts underway to produce a reliable amplifier for antenna readouts (see P. Falferi in about 40 )

15 Dream noise spectrum of Roma double SQUID 10-5 Φ n (Φ 0 Hz) T=4.2 K ε =28 h 10-8 T= 0.9 K = 5.5 h ε frequency (Hz) Carelli et al. 98

16 IMPROVING T/Q : (I) New, powerful Dilution Refrigerators MINIGRAIL was cooled (Jan 2003) to 80 mk Cooling below 30 mk appears possible T min probably limited by ortho-para H conversion.

17 IMPROVING THE ANTENNA CROSS SECTION (II): SPHERES (Warren, are you there?) Need a larger mass (larger cross section, or lower thermal noise). This can be achieved with One single huge resonator Distributing the mass over many small detectors Besides, the resonator mass can be better exploited by monitoring all the modes that are sensitive to g.w. => use the 5 quadrupole modes of a sphere.

18 A NEW KID ON THE BLOCK : MINIGRAIL The MINIGRAIL Team: G.Frossati, A.de Waald, L.Gottardi (+ one intruder/visitor ) Kammerling -Onnes Laboratories of Leiden Univ. (NL)

19 A vibrating sphere has two classes of normal modes: ü ü Toroidal (no radial displacement) r ψ lm = cψ l (kr )( r r Υ lm ) ψ l (x) = 1 x d dx Spheroidal (radial and transverse displacement) l sin x x r ψ lm = [ a l (r) n r + b l (r)r r ]Υ lm a(r) and b(r) are dimensionless radial eigenfunctions.

20 Extracting the information from the 5 quadrupole modes Five degenerate quadrupole modes (described using the basis of the five spherical harmonics Y 2m ; the same basis can be used to express h ij ) x n B mn h ij Using The response a metric of theory a sphere of gravity, to a GW such can be as measured General Relativity, by resonant the direction motion sensors and polarization strategically of placed the wave on the can surface be inferred of the sphere. from the A measured linear combination components of the (and outputs also a possible of these motion spin 0 amplitude sensors can of be the wave, made to due detemine to a scalar all field the Brans and components Dicke 1961) of a GW. h x h + H δ h s

21 Absorbed energy: Sphere σ s = F n G c 3 Mv s 2 omnidirectional s c 2R = L Same fundamental frequency σ s 18σ c max σ s 70σ c

22 Cross section sphere bar Mode order σ 2c 0 σ 1c σ 3c 9 σ 1s σ 2 s 2.6 a single spherical detector constitutes a xylophone in its own

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24 Exploiting the resonantmass detector technique: the spherical detector MINIGRAIL Leiden (Netherlands) MARIO SHENBERG Sao Paulo (Brasil) We might eventually have an array of small spherical resonators! TIGA, PRL 1993 Hollow sphere, PRD 1998 Dual sphere, PRL 2001 SFERA Frascati (Italy) CuAl(6%) sphere Dia= 65 cm Frequency = 3 khz Mass = 1 ton

25 SPHERES AROUND THE WORLD MORIOND 2003

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29 COOLING MINIGRAIL (II)

30 COOLING MINIGRAIL (III) Nautilus

31 IMPROVING THE ANTENNA CROSS SECTION (I) : New materials MORIOND 2003

32 That s all folks!

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34 EXPLORER PRESENT AND NEAR FUTURE Typical condition h = decreasing electronic noise increasing Q and decreasing electronic noise h ~ GW spectral amplitude (h/rt(hz)) Calibration peak frequency (Hz)

35 BANDWIDTH IN A RESONANT DETECTOR Why are we sensitive only around resonance? Why can we be sensitive in a region f >>f/q?

36 BD GR Maximum deformation at two times differing by half a period induced by the six polarization states of a metric wave incoming along the vertical axis: a) Φ 22 ; b) Ψ 2 ; c) Re Ψ 4 ; d) Im Ψ 4 ; e) Re Ψ 3 ; f) Im Ψ 3

37 Absorbed energy: Cylindrical bar E = dωφ(ω )σ (ω ) The cross section depends on the wave propagation direction and polarization ω GW flux MORIOND 2003 Cross section σ c = 8 G π c 3 Mv 2 [ s sin 4 (θ ) cos 2 (2ϕ )]