To answer this question, I would need more specific information about the role of the resistor R1 within your circuit. Depending on the function of the resistor, its required value can change quite drastically.

But, let's consider the resistor R1 is to limit the current through the SCR Thrysistor (D1). In that case, we will use Ohm's law to find the value of the resistor.

Given: Voltage, V = 9 Volts The gate trigger current Igt for the C106B1 SCR is typically around 200 μA (0.0002 A).

Ohm's law: Voltage (V) = Current (I) x Resistance (R)

Solving for Resistance (R):

R = V / I

= 9V / 0.0002A

= 45000 Ω

So, a resistor value around 45kΩ should work, but be aware this is a rough estimation. The actual value might vary depending upon your particular application, the specific characteristics of your components, and safety margin considerations.

Note: Remember to consider the power rating of the resistor as well. Power (P) is given by P = IV, and should be within the power rating of the chosen resistor.