Arduino chrony - interrupts not fast enough?

so, want build device measure speed of airsoft bbs (they fly around 100-150 m/s). so, have tube, in 1 wall there ir led , on other side, there few photodiodes. bb flies through tube, crosses beam of light, casts shadow on 1 of photodiodes , sends signal arduino. there 3 photodiodes in series, 1 end connected +5v , other pin 2 on arduino nano + pull down resistor. device works fine when throw bbs through tube, registers every single 1 of them. when shoot them @ full speed, device not register anything. bbs flying fast , creating short of pulse arduino detect? how can make thing work?

this code:

code: [select]

int x = 0;
void setup() {
  pinmode(2, input);
  serial.begin(9600);
  attachinterrupt(0, interrupt, falling); 
}

void loop() {
}
void interrupt() {   
  x = x+1;
  serial.println(x);
}

you should not using serial prints in isr.

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