RESUMEN
Using simple and cheap low-field (1)H NMR methods such as the magic sandwich echo technique and FID component analysis, we determine jump rates for 180° chain flips in poly(ethylene) (PE) crystallites, which are comparable to literature data obtained from advanced, complex, and time-consuming (13)C-based NMR methods. In the investigated temperature range, we find similar jump rates for the local chain flip process in a melt-crystallized sample containing lamellar crystals with disordered fold surface and in reactor powder samples having a rather adjacent-reentry-like structure. Previous NMR studies of Yao et al. revealed different chain diffusion coefficients for the resulting long-range chain diffusion between amorphous and crystalline regions in melt- vs solution-crystallized (adjacent reentry) samples. From our results, we conclude that, in the investigated temperature range, the fold surface, which presumably influences the effective chain transport, does not have a strong effect on the time scale of the local chain flip process. We confirm an Arrhenius temperature dependence of the jump rate for the local flip process and calculate activation energies which show a slight trend toward smaller values for the reactor powders (~76 kJ/mol) in comparison to the melt-crystallized sample (~103 kJ/mol).
RESUMEN
The 4.4 MeV photon reference field described in ISO 4037 is produced by the (12)C(p,p')(12)C (E(x) = 4.4389 MeV) reaction using a thick elemental carbon target and a proton beam with an energy of 5.7 MeV. The relative abundance of the isotope (13)C in elemental carbon is 1.10%. Therefore, the 4.4 MeV photon field is contaminated by neutrons produced by the (13)C(p,n) (13)N reaction (Q = -3.003 MeV). The ambient dose equivalent H*(10) produced by these neutrons is of the same order of magnitude as the ambient dose equivalent produced by the 4.4 MeV photons. For the calibration of dosemeters, especially those also sensitive to neutrons, the spectral fluence distribution of these neutrons has to be known in detail. On the other hand, a mixed photon/neutron field is very useful for the calibration of tissue-equivalent proportional counters (TEPC), if this field combines a high-linear energy transfer (LET) component produced by low-energy neutrons and a low-LET component resulting from photons with about the same ambient dose equivalent and energies up to 7 MeV. Such a mixed field was produced at the PTB accelerator facility using a thin CaF(2) + (nat)C target and a 5.7 MeV proton beam.