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wrapped with aluminized mylar and surrounded by a vac- uum cryostat, also made of SS, with typical vacuum in the mTorr range. The cryogenics system was based on a Pulse Tube Refrigerator (PTR) with 100 W of cooling power at 165 K, with a 3.5 KW compressor. This type of cryocooler was developed specifically for Xe liquefaction and re-condensation in the LXe scintillating calorimeter of the MEG experiment [9]. To meet the ultra-high pu- rity requirement for the XENON10 TPC, the cryocooler was placed outside of the detector vessel, mounted on the vacuum cryostat top flange and attached to a copper plate which worked as re-condenser for the evaporated Xe gas. The cooling power was sufficient to cool-down the detector, liquefy the gas and re-condense it. The desired temperature of the LXe was adjusted by a Proportional- Integral-Derivative (PID) controlled heating element on the cold head of the PTR. Due to the narrow temper- ature margin of less than 4 degrees between the liquid and solid phase of Xe, temperature control during liq- uefaction is especially important. The typical operating temperature for XENON10 was -94 $^{◦}$C with a stability better than 0.05 $^{◦}$C. At this temperature the Xe vapor pressure is ∼ 2.1 atm. With this cryogenic system we have been able to achieve the level of long term stability required for a dark matter experiment. Fig. 10 shows the stability of the LXe temperature and corresponding Xe vapor pressure during several months of continuous operation of XENON10. 

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The XENON10 cryogenics system included an inde- pendent LN$_{2}$ system for emergency cooling. A pressure rise in the detector, above a set value, would automat- ically trigger a flow of LN$_{2}$ through a cooling coil next to the PTR. The TPC pressure sensor and the solenoid valve control were powered by a dedicated uninterrupt- ible power supply. The emergency cooling system would keep detector in a safe sate even in the extreme case of power failure and no access to the laboratory for more than 24 hours. We note that the large Xe mass and high thermal capacity stabilizes the temperature and the pres- 

sure against abrupt changes. A rupture disk would finally burst if the absolute pressure in the detector would rise above 3.5 atm, to avoid damage to the PMTs. 

## E. Xe Purification and Recirculation System 

The 15 cm electron drift gap in the XENON10 detec- tor imposes ultra-high purity requirements on the LXe, in order to minimize charge loss by electron attachment to impurities molecules. The overall concentration of elec- tronegative impurities (Oxygen equivalent) must be well below 1 part per billion (ppb). Moreover, stable oper- ation of the detector demands that this high purity be maintained over long periods of time. For detector and gas system ultra high vacuum materials and procedures were used, and for cleaning the Xe gas a single purifier, a high temperature SAES getter (PS4-MT3-R) [35] was used. 

However, the PMTs limited the bake-out temperature of the detector to 70 $^{◦}$C. In addition, LXe is a good sol- vent due to Van der Waals interactions with impurity molecules. Thus, once pure Xe is liquefied in the detec- tor, the purity level can easily deteriorate. We therefore implemented a closed circulation system to continuously clean the gas through the getter, after initial liquefaction. Such a system was successfully developed and tested at Columbia on several smaller scale prototypes, prior to adapting it to XENON10. For gas circulation a double- diaphragm pump (KNF-N143.12E) [36] was used. With a gas flow rate of 2.6 slpm, limited largely by the PTR cooling power, it would take about one month to reach the purity level required for negligible electron attenu- ation. A schematic of the gas purification and recircu- lation system used on XENON10 at LNGS is shown in Fig. 11. 

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