Q-Values in File 3


 The definitions for the two Q-values given in sections of File 3 are as follows:
QM
is the mass-difference Q value (eV), defined as the mass of the target and projectile minus the mass of the residual nucleus in the ground state and the masses of all other reaction products; that is, for

a+A->b+c+...+B,
QM=[(ma+mA) -(mb+mc+...+mB)] x U,

where the mass unit U is 9.3150146e8 eV for masses given in AU, the standard C12-based mass units, or 9.395728e8 eV for masses given in neutron masses (AWR).

QI
is the reaction Q value for the (lowest energy) state defined by the given MT value in a simple two-body reactions or a breakup reaction. It is defined as QM for the ground sate of the residual nucleus (or intermediate system before breakup) minus the energy of the excited level in this system. Use QI=QM for reactions with no intermediate states in the residual nucleus and without complex breakup (LR=0).
See ENDF102, Section 3.2.

Q Values


 Accurate Q values shnould be given for all reactions, if possible. If QI is not well defined (as for a range of levels in MT=91, 649, 699, 749, 799, or 849), use the value of QI that corresponds to the threshold of the reaction. Similarly, if the value of QM is not well defined (as in elements or for summation reactions like MT=5), use the value of QM that gives the threshold. If there is no threshold, use the most positive Q value of the component reactions. Note that these ill-defined values of QM cannot be relied on for energy-release calculations.

As an example to clarify the use of QM and QI, consider the reaction &alapha;+9Be->n+X. After the neutron has been emitted, the compound system is 12C with QM=5.702 MeV and energy levels (EX>) at 0.0, 4.439, 7.654, and 9.641 MeV. The ground state is stable against particle breakup, the first level decays by photon emission, and the higher levels decay with hight probability by breaking up into three alpha particles (7.275 MeV is required). This pattern can be represented as follows:

Reaction QM QI EX MT
9Be(α,n0)12C 5.702 5.702 0. 50
9Be(α,n1)12C 5.702 1.263 4.439 51
9Be(α,n2)12C(3α) -1.573 -1.952 7.654 52
9Be(α,n3)12C(3α) -1.573 -3.939 9.641 53
9Be(α,nc)12C(3α) -1.573 -1.573 none 91

The gamma for the second reaction is not written explicitly in this notation. The last reaction includes the contributions of all the levels above 9.641 MeV, any missed levels, and any direct four-body breakup; therefore, the threshold for MT=91 may be lower than implied by the fourth level of 12C. Note the value used for QI.

See ENDF102, Section 3.3.2