choosing a thyristor

[dexter001]

I am making a laser door alarm in which whenever somebody breaks the laser path, a continuous alarm should be generated. Although the design which i used is working properly, the problem is that buzzer only beeps when somebody breaks the laser beam and stops as soon as the laser beam falls on the phototransistor again.
A thyristor can be a perfect choice for solving the problem. But I don't understand how to choose the correct model of a thyristor for a particular circuit (especially where it is used for latching)
The circuit design is similar to what is given here:
electroschematics.com/4929/laser-based-door-alarm

 

[MikeMl]

C106 is a sensitive gate "logic" SCR.

 

[Mr RB]

Or use any small DPDT relay and use 2 contacts as a "hold" contact to keep the relay turned on.

 

[4pyros]

A thyristor can be a perfect choice for solving the problem

How can it help?