Why is this causing my program to crash?

I have created the following user defined function which generates random variables using the Acceptance-Rejection Method. However, whenever it is called, my program goes on and eventually crashes or I have to force quit. I have gone through it several times. Any ideas as to what could be causing this? Thank you!

P.S. I am aware that this may not be the best way to have written this (Yesterday was the first time I have used R) so any extra tips are a bonus! Thanks!

acceptReject <- function(){
  Z <- 0
  Y <- c(0,0)
  while(Y[2] < .5*(Y[1]-1)**2){
    U <- runif(2,0,1)
    Y <- log(U)
  }
  Z <- Y[1]
  U <- runif(1,0,1)
  if(U <= .5){
    Z <- abs(Z)
  }
  else{
    Z <- -abs(Z)
  }
  Z
}