Misura di contaminazioni residue in argon con uno spe2rometro di massa

Transcription

1 Misura di contaminazioni residue in argon con uno spe2rometro di massa E2ore Segreto Laboratori Nazionali del Gran Sasso Per la Collaborazione ICARUS

2 Outline We performed a direct es)ma)on of the argon contaminants in the two cryostats of the ICARUS T600 detector. We developed a set- up based on the use of a commercial mass spectrometer Pfeiffer QMG 220 that can reveal concentrajons of contaminants in argon at the level of 1 ppm. The O 2 (and other electronegajve contaminants) concentrajon is known to be extremely low (tens of ppt!!!) from electron lifejme measurement. On the contrary the concentra5on of N 2 and in general of other contaminants that have negligible effects on electron a?achment, but could be important in scin5lla5on light quenching is not precisely known (and are not filtered out by Oxysorb/Hydrosorb cartridges). We put upper limits on contaminants concentrajon by comparing the spectrometer s signals form the gas sampled from the cryostats with those from a sample of 6.0 argon, with a cer)fied total amount of contaminants below 1 ppm (< 0.5 ppm N 2 ; < 0.2 ppm O 2 ; < 0.5 ppm water; < 0.1 ppm for other contaminants)

3 Layout of the installed set- up

4 Test chamber Mass spectrometer Leak valve Pressure gauge Turbo pump T1 2l bo2le E2ore Segreto - SIF L'Aquila 2011

5 The mass spectrometer The stainless steel test chamber (few tens of cubic cenjmetres) is baked and evacuated down to mbar to reduce background; The background is dominated by water and by the components of air; The water sensi5vity of the instrument is limited due to the high background. Ion current (Ampere) He water N 2 O 2 Ar CO 2 According to the nominal performance of the spectrometer 1 ppm of N 2 should correspond to a current x Ampere (@ 5x10-6 mbar working pressure). Background spectrum Mass (amu)

6 Analysis procedure Three samples of argon gas have been analyzed: ü A sample of GAr taken from a 6.0 argon bo;le from Rivoira with cerjfied total amount of contaminants below 1 ppm (< 0.5 ppm N 2 ; < 0.2 ppm O 2 ; < 0.5 ppm water; < 0.1 ppm for other contaminants) - > zero level of the spectrometer; ü A sample of GAr spilled from the old cryostat, before the filter of the gaseous purificagon system; ü A sample of GAr spilled from the new cryostat, before the filter of the gaseous purificagon system. The gas samples are spilled with the ultra- clean 2l boale (with electro- polished internal surfaces and pracjcally no outgassing) that is then connected to the test- chamber by means of the leak valve. The connec5on lines are baked and evacuated down to 10-7 mbar. The gas is fluxed inside the test chamber by opening the leak valve. The aperture of the valve is regulated in such a way that an equilibrium pressure of 5x10-6 mbar is established with the pumping speed of the turbo pump. Aeer that the system reaches a stable condijon (1 hour) the gas is analyzed for about 30 min.

7 Gaseous recirculajon filter Spill valve

8 40 Ar + 40 Ar ++ Ion current (Ampere) He + H 2 O + N Ar + 36 Ar + O + 2 CO Argon Mass (amu) Ion current (Ampere) Old cryostat Ion current (Ampere) Mass (amu) New cryostat

9 Mass scans (II) The complete mass scan (up to 100 amu the limit of the spectrometer) gives a qualita)ve idea of the sample gas composi)on. The rela5ve intensity of the peaks is not directly linked to the rela5ve abundances of the scanned masses, because of the different ioniza5on probabili5es of the different atoms/molecules and because the most abundant elements (argon in this case) typically saturate the gain of the spectrometer amplifica5on system (very similar to the dynodic chain of a photomuljplier). The mass spectra of the gas samples spilled from the two cryostats looks iden)cal to 6.0 argon spectrum. This means that if some contaminants is present it should be searched among the main air components: N 2, O 2 and CO 2 (water can t be analyzed with this kind of spectrometer because of the high background). We don t find contaminants with mass M > 44 amu. We point out that in this region the spectrometer has the highest sensi5vity because there is no background (except somejmes a M = 58 amu). In order to have a more quan)ta)ve idea of the abundance of nitrogen, oxygen and carbon dioxide we selected the mass 28 for N 2, 32 for O 2 and 44 for CO 2, measured the associated ionic current and compared it with the analogous values for our zero sample (argon 6.0)

10 Mass scans (III) Uncertain5es of the order of 10 % The gas samples spilled from the two cryostats seem to be at least as clean as 6.0 Argon. This means that the Nitrogen concentrajon inside the cryostats should be below 0.5 ppm, consistent with the value measured during the detector filling (0.2 ppm). As expected no Oxygen signal has been found. We verified the capability of our set up in detec5ng Nitrogen contamina5ons of the order of few ppm Aby analyzing a sample of not perfectly clean argon. The sample was spilled from the WArP prototype 2.3l in the Hall di Montaggio of LNGS from the gas phase. Since we measured with the four photomuljplier installed a decay 5me of the slow scin5lla5on component around nsec (1300 nsec for clean argon) we can esjmate a N 2 concentra5on of the order of 2 ppm in the liquid phase, that corresponds to 6 ppm in the gas phase (factor three higher). We measured a signal for M = 28 of 1.1x10-10 Ampere with our set- up (clearly above 6.0 signal). The Oxygen content was found to be consistent with 6.0 Argon (the gas was filtered during the filling of the detector with an Oxysorb/Hydrosorb cartridge).

11 Conclusions We put upper limits on the concentrajon of contaminants in argon samples spilled from the two ICARUS cryostats. Nitrogen content seem to be consistent with the values measured during the detector filling (0.2 ppm). This means that no significant N 2 pollu5on occurred in the last months. No contaminant with mass M > 44 amu was found. Other techniques that allow to push sensijvity down to ppb level are under development.