A positron emission tomography detector being configured to identify a depth of a photon gamma interaction, comprising: a crystal array including a plurality of crystal elements, the plurality of crystal elements being arranged along a first direction and a second direction so as to define a plurality of crystal groups along the first direction, each of the plurality of crystal groups including at least two crystal elements of the plurality of crystal elements, each of the plurality of crystal elements including a first end and a second end and extending along a third direction from the first end to the second end; and a photon-sensor array optically coupled with the crystal array, wherein at least one pair of neighboring crystal groups of the plurality of crystal groups include: a first optical separator of a first length located between the crystal groups of the at least one pair of neighboring crystal groups, the first length being along the third direction; and a second optical separator of a second length located between two neighboring crystal elements of a crystal group of the at least one pair of neighboring crystal groups, the second length being along the third direction, wherein the first length of the first optical separator is equal to or greater than the second length of the second optical separator, and the second optical separator extends from the first end of and without reaching the second end of at least one of the two neighboring crystal elements between which the second optical separator is located; characterized in that the PET detector further comprises one or more third optical separators each located between two neighboring crystal elements along the second direction, each third optical separator extending from the second end of and without reaching the first end of at least one of the two neighboring crystal elements between which the third optical separator is located. a crystal array including a plurality of crystal elements, the plurality of crystal elements being arranged along a first direction and a second direction so as to define a plurality of crystal groups along the first direction, each of the plurality of crystal groups including at least two crystal elements of the plurality of crystal elements, each of the plurality of crystal elements including a first end and a second end and extending along a third direction from the first end to the second end; and a photon-sensor array optically coupled with the crystal array, wherein at least one pair of neighboring crystal groups of the plurality of crystal groups include: a first optical separator of a first length located between the crystal groups of the at least one pair of neighboring crystal groups, the first length being along the third direction; and a second optical separator of a second length located between two neighboring crystal elements of a crystal group of the at least one pair of neighboring crystal groups, the second length being along the third direction, wherein the first length of the first optical separator is equal to or greater than the second length of the second optical separator, and the second optical separator extends from the first end of and without reaching the second end of at least one of the two neighboring crystal elements between which a first optical separator of a first length located between the crystal groups of the at least one pair of neighboring crystal groups, the first length being along the third direction; and a second optical separator of a second length located between two neighboring crystal elements of a crystal group of the at least one pair of neighboring crystal groups, the second length being along the third direction, wherein the first length of the first optical separator is equal to or greater than the second length of the second optical separator, and the second optical separator extends from the first end of and without reaching the second end of at least one of the two neighboring crystal elements between which the second optical separator is located; characterized in that the PET detector further comprises one or more third optical separators each located between two neighboring crystal elements along the second direction, each third optical separator extending from the second end of and without reaching the first end of at least one of the two neighboring crystal elements between which the third optical separator is located.