What is this part of an aircraft (looks like a long thick pole sticking out of the back)?
That is called a Survey Tail Boom and includes sensitive instruments that need to be mounted away from the aircraft body for clean measurements.
You can see the name of the manufacturer LCAS printed on the boom. In this case, it seems to be a tail boom for magnetic field measurements installed on a Douglas DC-3, which is used for Survey according to Borek Air's fleet information. The LCAS website has a detailed description of the tail boom:

LCAS’ fully certified (Supplemental Type Certificate) tail-mounted transition and boom assemblies carry the magnetometer sensor and requisite electronics. The aft part of the boom system contains the provisions for the magnetometer sensor, the mid section houses the amplifier and flux gate and the transition assembly can house optional electronics such as pressure/temp sensors. The transitions or forward boom assemblies can also be modified for installation of optional video cameras.  The orientation of the sensor is adjustable, to provide optimum coupling with the earth’s magnetic field. This innovation eliminates the traditional towed bird system and offers significant advantages in data quality, pilot work load reductions, survey flight efficiencies, and safety in all types of terrain.
The mod kit includes all the necessary doublers, support assemblies and attaching hardware, as well as new ‘end cap’ with Nav. light provisions, this allowing the aircraft to be flown without the tail boom and the OEM tail cone assembly to remain un-altered.


(lakecentral.com)