. ▁ ▁Area ▁ ▁Bar n , ▁ou th ouse , ▁shed ▁A ▁bar n ▁is ▁a ▁serious ▁unit ▁of ▁area ▁used ▁by ▁nuclear ▁phys ic ists ▁to ▁quant ify ▁the ▁scatter ing ▁or ▁absor ption ▁cross - section ▁of ▁very ▁small ▁particles , ▁such ▁as ▁atomic ▁nuc lei . ▁One ▁bar n ▁is ▁equal ▁to ▁ 1 . 0   m 2 . ▁ ▁The ▁name ▁der ives ▁from ▁the ▁folk ▁expression ▁" Could n ' t ▁hit ▁the ▁broad ▁side ▁of ▁a ▁bar n ", ▁used ▁by ▁particle ▁acceler ator ▁phys ic ists ▁to ▁refer ▁to ▁the ▁difficulty ▁of ▁ach ieving ▁a ▁collision ▁between ▁particles . ▁The ▁ou th ouse ▁( 1 . 0   bar ns ) ▁and ▁shed ▁( 1 . 0   bar ns ) ▁are ▁derived ▁by ▁anal ogy . ▁ ▁N ano acre ▁The ▁n ano acre ▁is ▁a ▁unit ▁of ▁real ▁estate ▁on ▁a ▁Very ▁Lar ge ▁Sc ale ▁Integr ation ▁( V L SI ) ▁chip ▁equal ▁to ▁ ▁or ▁the ▁area ▁of ▁a ▁square ▁of ▁side ▁length ▁. ▁V L SI ▁n ano ac res ▁have ▁similar ▁total ▁costs ▁to ▁acres ▁in ▁Sil icon ▁Valley . ▁ ▁Volume ▁ ▁Bar n - m eg ap ar sec ▁This ▁unit ▁is ▁similar ▁in ▁concept ▁to ▁the ▁att op ar sec , ▁combining ▁very ▁large ▁and ▁small ▁scales . ▁When ▁a ▁bar n ▁( a ▁very ▁small ▁unit ▁of ▁area ▁used ▁for ▁meas uring ▁the ▁cross ▁section al ▁area ▁of ▁atomic ▁nuc lei ) ▁is