CDECK  ID>, HWREXT.

*CMZ :-        -26/04/91  11.11.55  by  Bryan Webber

*-- Author :    David Ward, modified by Bryan Webber

C-----------------------------------------------------------------------

      FUNCTION HWREXT(AM0,B)

C-----------------------------------------------------------------------

C     Random number from dN/d(x**2)=exp(-B*TM) distribution, where

C     TM = SQRT(X**2+AM0**2).  Uses Newton's method to solve F-R=0

C-----------------------------------------------------------------------

      DOUBLE PRECISION HWREXT,HWR,AM0,B,R,A,F,DF,DAM,AM

      INTEGER NIT

      EXTERNAL HWR

      R=HWR()

C --- Starting value

      AM=AM0-LOG(R)/B

      DO 1 NIT=1,20

      A=EXP(-B*(AM-AM0))/(1.+B*AM0)

      F=(1.+B*AM)*A-R

      DF=-B**2*AM*A

      DAM=-F/DF

      AM=AM+DAM

      IF(AM.LT.AM0) AM=AM0+.001

      IF(ABS(DAM).LT..001) GOTO 2

   1  CONTINUE

      CALL HWWARN('HWREXT',1,*2)

   2  HWREXT=AM

      END