MODULAr: concept design of internal detector mechanics

1. MODULAr: concept design of internal detector mechanics A. Menegolli, University and INFN Pavia

2. Basic Considerations • The basic requirements of a large LAr detector like MODULAr are: • precision in wires positioning (~100 µm); • uniform purification of the full Argon volume. • The ICARUS-T600 mechanics successfully fulfills the above requirements if evacuated before filling. • Scaling the T600 detector to significant larger sizes requires a relatively small revision of the design: the main new task is to preserve the LAr purity even in case of a filling procedure without previous dewar evacuation. Cryodet 2 - 15/06/07 - A. Menegolli

3. MODULAr structure The structure of the MODULAr detector has been considerably streamlined in order to reduce the number of components, their cost and increase the reliability of the system. The modular structure allows to repeat the initial engineering design of the prototype to a series of several subsequent units, reducing progressively their cost and their construction time: the basic unit is a 10 kton sensitive LAr volume. The main aim of MODULAr detector is the one of filling and maintaining over many years a very large amount of ultra-pure LAr in stable conditions inside a dedicated underground cave, within very rigid safety conditions. Under such conditions, a 20 kton sensitive volume modular detector might be operative in 4 years, and additional clone units of 10 kton each could be built during the years depending on the physics requirements. Cryodet 2 - 15/06/07 - A. Menegolli

4. Geometry of an ICARUS-T600 half-module (T300) “cloned” into a larger detector scaled by a factor 8/3 = 2.66: the cross sectional area of the planes is 8 x 8 m2 rather than 3 x 3 m2. The length of such a detector is 50 meters. 8m 3 m 8m 3 m Cryodet 2 - 15/06/07 - A. Menegolli

5. ICARUS-T600: mechanics review LAr containers: solution with Aluminum honeycomb panels 150 mm thick: the total internal volume of a cryostat is (3.9 x 3.6 x 19.6 = 275) m3. Perforated 142 mm aluminum honeycomb Cryodet 2 - 15/06/07 - A. Menegolli

6. Each of the two T300 half-modules hosts a stainless-steel mechanical structure that sustains the different inner detector subsystems, in particular way the TPC wire planes. Inner detector Horizontal beams Stainless-steel structure (beams and pillars) Peek supports Mechanical precision achieved for all the inner detector parts ~100 mm Event images are not distorted by misalignment of wire planes. Cryodet 2 - 15/06/07 - A. Menegolli

7. MODULAr mechanical structure MODULAr mechanical structure has been studied in order to be highly streamlined, taking as a reference the work done for the T600. It is essentially made of only three main mechanical components: • an external insulating vessel; • a linear supporting and holding structure; • the liquid Argon and Nitrogen supply and refrigeration. Cryodet 2 - 15/06/07 - A. Menegolli

8. The external insulating vessel is made of two metallic concentric volumes with the inter-space filled with perlite as a thermal insulation. Perlite insulation Low conductivity foam glass light bricks for the bottom support layer Perlite: mineral vastly used industrially; easily available at low cost. Cryodet 2 - 15/06/07 - A. Menegolli

9. A 1.5 m thickness of perlite corresponds to a specific heat loss of 3.86 W/m2, significantly smaller than the specific heat loss of ICARUS-T600 (~7 W/m2). Taking into account the dimensions of the vessel, the total heat loss is 8.28 kW: three units of 4 kW (cold) power should be adequate to ensure cooling of the walls of the vessel during normal operation (the plant of ICARUS-T600 consists of 10 units). Evacuation of the perlite is unnecessary Cryodet 2 - 15/06/07 - A. Menegolli

10. Like ICARUS-T600, MODULAr wire frame design is based on the concept of the variable geometry design: beams of the wire frames are movable. Frame beams are connected by a set of calibrated springs compensating for tension increase on wires possibly occurring during cooling and filling phases. T600 tensioning system (springs and movable frame) The wire mechanical tension is kept constant Cryodet 2 - 15/06/07 - A. Menegolli

11. HV system The HV system produces a stable and uniform field over the 4 m maximum drift length. The cathode is biased with negative voltage (-200 kV for a nominal drift field of 0.5 kV/cm); the field is kept uniform by means of a series of equally spaced field shaping electrodes (race tracks) surrounding the active volume; voltage on the race tracks is distributed from the cathode by means of a resistive divider chain. To reduce the noise induced by the power supply ripple, in ICARUS-T600 a highly efficient rejection two-stage RC filter has been designed and implemented. Nominal HV for a T300 half-module: 75 kV for 1.5 m drift (E = 0.5 kV/cm). HV system operated in steady conditions up to 150 kV. HV feedthrough Cryodet 2 - 15/06/07 - A. Menegolli

12. The refrigeration with N2 circulation, provides cooling of the Argon volume ensuring that: the Argon properties (drift velocity) are uniform all over the active volume; the whole liquid is moving orderly inside the vessel volume; also the free electrons lifetime is kept uniform. Cryodet 2 - 15/06/07 - A. Menegolli

13. +60° 0° -60° MODULAr wire arrangement Like ICARUS-T600, a MODULAr LAr TPC is composed of two read-out wire planes (three orders of wire planes at ±60° and 0° each) with a HV cathode in between. The MODULAr read-out wires are also scaled out by a factor 8/3 = 2.66 with respect to the existing ICARUS-T600 wires. Three orders wire planes Spacers Cryodet 2 - 15/06/07 - A. Menegolli

14. Longer MODULAr wires have a higher capacitance and the S/N ratio is significantly decreased (wires ~ 10 pF/m; cables ~ 50 pF/m). This factor is compensated widening the pitch from the 3 mm of ICARUS-T600 to the 6 mm of MODULAr, doubling the dE/dx signals: therefore we expect S/N ratios similar to the ones of the T600 (~ 10/1). The average LAr mass observed by each read-out wire is about 200 Kg/channel: a 20 kton sensitive volume will then require a number of channels of the order 100,000. Cryodet 2 - 15/06/07 - A. Menegolli

15. MODULAr wire features • For each 10 kton modular unit, about 50,000 stainless-steel wires of several lengths will be prepared: • 10 m for ± 60º wires; • - 25 m for 0º wires (2 coplanar, adjacent sets per wire plane); • 130÷1014 cm for corner wires. • Because of the ~50 m length of the MODULAr detector, the horizontal (0º wires) read-out plane will be subdivided in two sub-planes hosting 25 m wires. Cryodet 2 - 15/06/07 - A. Menegolli

16. MODULAr wire production, holding, tensioning and storing make use of the technologies already developed and tested for the ICARUS-T600 wires. ICARUS-T600 9 meter wire Peek comb Slipknots at both ends Golden stainless-steel ferrule Peek shell The wires are held in place by winding over themselves (like guitar chords) after having passed them around a golden stainless steel sleeve. The wires are fixed in position by inserting the sleeves into the pins of 32 wire holding combs. Stainless steel comb holder The 32 wire modules are fixed on the high precision supporting and holding frames. Cryodet 2 - 15/06/07 - A. Menegolli

17. Wire tension: T = 1169 ± 4 g Wire holdings T (g) Breaking tension: B = 2923 ± 98 g T (g) Cryodet 2 - 15/06/07 - A. Menegolli

18. Wiring devices Wire module rolling around a PVC spool Tensioning group to fix and tension the wires Wiring table Wiring could begin at any time and the wire modules can be cleaned and stored until installation. PVC spools after washing and during drying Wire production speed for T600 wires: 1 hourfor the assembly of a 32 wires module Cryodet 2 - 15/06/07 - A. Menegolli

19. PMTs for light collection As for ICARUS-T600 a number of photomultipliers located behind the read-out wire planes are used to provide a t = 0 trigger. This is important for the cosmic rays and proton decay events (no starting signal provided), but could as well very useful in order to tag events coming from CNGS beam. The technique consists in glass 8” PMTs with a thin deposit of wavelength shifter in order to record the prompt (~μs) VUV scintillation light from the LAr. New 8” PMTs are being developed with about twice the Quantum Efficiency of the PMTs originally used in ICARUS-T600, that was around 10%. The minimal requirement for MODULAr PMT trigger is the detection of 10 MeV signals (SuperNova neutrinos): in such case about 100 PMTs for read-out plane are needed (minimum signal ~ 8 photo-electrons). Cryodet 2 - 15/06/07 - A. Menegolli

20. Summary (1) • The mechanical structure of the MODULAr detector will use as a starting point the ICARUS-T600 geometry, which is taken as a reference design. • The structure of the detector will be considerably streamlined in order to reduce the number of components, the cost and the construction time: a modular detector of 20 kton sensitive volume could be operational in 4 years. • The geometry will be a “clone” of an ICARUS-T600 half-module into a larger detector with linear dimension scaled by a suitable factor. Cryodet 2 - 15/06/07 - A. Menegolli

21. Summary (2) • The basic module is a 10 kton sensitive volume ICARUS-T600-like detector (2 x 5 kton) insulated by a vessel made of two metallic concentric volumes with the inter-space filled with perlite as a thermal insulation. • The detector clone modules will be located in a new experimental area, now under consideration, to be realized about 10 Km off-axis from LNGS, tailored specifically to the MODULAr experiment. • Further 10 kton modular units can be added in the course of the years, provided experimental halls are realized, depending on the physics requirements: the engineering of the modular units would strongly reduce the cost and the time for the realization of a LAr detector of several tens of kton. Cryodet 2 - 15/06/07 - A. Menegolli