The present invention relates to a pump (15) for pumping a coolant (9) within a Dewar vessel (1) and to a corresponding Dewar vessel (1) for storing samples in a coolant (9). The Dewar vessel (1) comprises a thermally insulated reservoir (3) for the coolant (9) and a sample vessel (11) provided separately and arranged in the thermally insulated reservoir (3). The reservoir (3) is connected to the sample vessel (11) in such a way that the level of coolant (9) is constant in the sample vessel (11). Pump (15) may help in keeping the level of coolant (9) in the sample vessel (11) constant. For this purpose the pump (15) comprises a chamber (17) with an inlet (19) and an outlet (21), a closing element (23) and a pressure increasing device (25). Therein, the inlet (19) is connectable to the reservoir (3) and the outlet (21) is connectable to a sample vessel (11) of the Dewar vessel (1). The chamber (17) is adapted to fill with coolant (9) through the inlet (19) by gravity and the closing element (23) is adapted to automatically close the chamber (17) when it is full of coolant (9). The pressure increasing device (25) is adapted to increase the pressure within the chamber (17), after the chamber (17) is closed, until the coolant (9) is released through the outlet (21).