Prevent capacitor discharge after it was charged using a microcontroller

Question

I want to use a micro-controller (like an Arduino) to:

• Charge an external capacitor
• Measure voltage across the capacitor
• Capacitor discharge rate as low as possible.
• Capacitor discharge rate while using ADC to measure voltage as low as possible.

The microcontroller has

• DAC (to charge capacitor)
• ADC (to measure capacitance)
• Multiplexer to attach DAC/ADC to whatever pin you want
• I use a multimeter to measure resitance between pin 0 and pin 1 = 1Mohm (Q1 is pin0-1 resistance called chip resitance? What is the name?)

I can attach a capacitor between pins 0 and 1 however it will discharge as 1Mohm resitance isn't very high.

^{simulate this circuit – Schematic created using CircuitLab}

Question:

1. Is the pin 0 to 1 resistance called chip resistance? What is the name?
2. How can I reduce the capacitor discharge after I charge it with the DAC?
3. How can I reduce the discharge while measuring voltage with ADC?

My actual chip is the NHS3152. I am looking for any solution, even one with other components to be added.

EDIT EXTRA CONTEXT INFORMATION

I was thinking of using the capacitor as an X-ray detector:

1. X-rays would interact with the dielectric to create e-h pairs
2. The electric field present on the charged capacitor would separate the charges
3. This would affect voltage across capacitance.
4. Measure the voltage with NHS3152 and have a working device.

If the capacitor discharges over time, then the field within the dielectric would change, separating the e-h pairs less, so low-discharge is important.