Princeton Applications of Modern Physics in Medicine, By Mark Strikman, Kevork Spartalian, Milton W. Cole

https://books.google.com/books?id=FFgiBQAAQBAJ&pg=PA50&lpg=PA50&dq=twin+paradox+Proper+time:+Time+interval+%CE%94t+%3D+t2+%E2%80%93+t1+between+two+events+measured+&source=bl&ots=593e1JwI7H&sig=Nq4eImAQxz1INhPGFkPSEbMCORY&hl=en&sa=X&ved=0ahUKEwi1sZybvZ_JAhUBG2MKHTvXAFgQ6AEIRTAG#v=onepage&q=twin%20paradox%20Proper%20time%3A%20Time%20interval%20%CE%94t%20%3D%20t2%20%E2%80%93%20t1%20between%20two%20events%20measured&f=false

Applications of Modern Physics in Medicine

 By Mark Strikman, Kevork Spartalian, Milton W. Cole