In population studies, in which patients and controls are of comparable size, bone mineral area density (BMD) gives reliable results for mean bone mass data although, with sequential data, BMD may under-estimate the degree of change in bone mass. In children BMD data should be reliable, provided that patients and controls, matched for age and sex, are also of the same size. With disease children may be small for their age so that low bone mass by BMD may be due to small body size and not necessarily to osteopoenia. In these situations the bone mineral content (BMC) index may be more reliable than BMD. To assess bone mass status in individuals, BMC index, as well as BMD, should be used, particularly with adults at the extremes of body size (the very small or very tall).

Using the corpus of data on the (gamma,p), (gamma, n), (gamma, 2n), and (gamma, n0) cross sections of nine light nuclei (C-13, C-14, N-15, O-17, O-18, Mg-25, Mg-26, Si-29, and Si-30), the T> and T< isospin components of the giant dipole resonance have been separated. The relative strengths of these components have been extracted, together with the energy differences between the centroids of the components. The ratio of the T> energy-weighted integrated cross section to the total cross section is somewhat better represented by the simple geometric factor 1/(T0 + 1), where T0 is the isospin of the ground state of the excited nucleus, than by a more complete expression which takes into account dynamical effects. If the energy difference between the centroids is represented by U*(T0 + 1)/A, the average value of U* for the three p shell nuclei is found to be 57 MeV, while that for the six s-d shell nuclei is 93 MeV.

The giant dipole resonance (GDR) in $^{17}\mathrm{O}$ has been studied with the reaction $^{17}\mathrm{O}$(\ensuremath{\gamma},p${)}^{16}$N from ${\mathit{E}}_{\ensuremath{\gamma}}$=13.50 to 43.15 MeV using quasimonoenergetic photons. The measured cross section shows major peaks at 15.1, 18.1, 19.3, 20.3, 22.2, 23.1, 24.4, and \ensuremath{\sim}26.5 MeV. The intermediate structure in the main GDR region is remarkably similar to that observed in $^{16}\mathrm{O}$, indicating that the valence neutron outside the doubly magic $^{16}\mathrm{O}$ core perturbs the core-excited states minimally, in support of the weak-coupling hypothesis. We correlate the trends in GDR structure of $^{16,17,18}\mathrm{O}$ with changes in ground-state properties related to static deformation. The (\ensuremath{\gamma},p) reaction selects strength predominantly from two-particle--one-hole configurations formed via E1 transitions from the 1${\mathit{p}}_{1/2}$ subshell; comparison with other reactions (photoneutron and radiative capture) provides information on the microscopic structure of E1 states. The peak observed near threshold at 15.1 MeV is remarkably strong; we infer that it originates from photoexcitation of a few narrow T=3/2 states and that M1 transitions contribute to the measured strength. The total absorption cross section is approximated by summing the (\ensuremath{\gamma},p) cross section and the previously published photoneutron cross section; comparison with particle-hole shell-model calculations shows that the main cross-section features, including isospin distribution, are well predicted. Evidence is found for isospin splitting in $^{17}\mathrm{O}$. Systematics of the integrated cross sections for the carbon, nitrogen, and oxygen isotopes are delineated.

The C-14(gamma,p)B-13 reaction cross section has been measured from threshold to 29.1 MeV using bremsstrahlung photons. A contribution from the C-14(gamma,pn + d)B-12 reactions is included, and is significant only at the highest energies measured here. The main features of the cross section are a weak resonance at almost-equal-to 22.5 MeV and a dominant, broad resonance at almost-equal-to 25.6 MeV. The integrated cross section up to 29.1 MeV is 17.9 +/- 3.2 MeV mb. We deduce that essentially the entire cross section results from decay of T > dipole states. In combination with the previously reported photoneutron cross section an estimate of the total photoabsorption cross section for C-14 is obtained. The T < and T > components of the photoabsorption cross section (up to 30 MeV) are estimated to carry strengths of 88 +/- 12 MeV mb and 37 +/- 8 MeV mb, respectively. An isospin splitting of the giant dipole resonance of 8.4 +/- 0.5 MeV is obtained. Comparisons of several shell-model calculations are made with the data, and general agreement is found. A comparison of photoabsorption cross sections for C-12,C-13,C-14 and O-16,O-17,O-18 shows dramatic redistribution of dipole strength as neutrons are added to the core nuclei.

The $^{14}\mathrm{C}$(\ensuremath{\gamma},p${)}^{13}$B reaction cross section has been measured from threshold to 29.1 MeV using bremsstrahlung photons. A contribution from the $^{14}\mathrm{C}$(\ensuremath{\gamma},pn+d${)}^{12}$B reactions is included, and is significant only at the highest energies measured here. The main features of the cross section are a weak resonance at \ensuremath{\approxeq}22.5 MeV and a dominant, broad resonance at \ensuremath{\approxeq}25.6 MeV. The integrated cross section up to 29.1 MeV is 17.9\ifmmode\pm\else\textpm\fi{}3.2 MeV mb. We deduce that essentially the entire cross section results from decay of ${\mathit{T}}_{>}$ dipole states. In combination with the reported photoneutron cross section an estimate of the total photoabsorption cross section for $^{14}\mathrm{C}$ is obtained. The ${\mathit{T}}_{<}$ and ${\mathit{T}}_{>}$ components of the photoabsorption cross section (up to 30 MeV) are estimated to carry strengths of 92\ifmmode\pm\else\textpm\fi{}14 MeV mb and ${31}_{\mathrm{\ensuremath{-}}5}^{+9}$ MeV mb, respectively. An isospin splitting of the giant dipole resonance of 8.1\ifmmode\pm\else\textpm\fi{}0.2 MeV is obtained. Comparisons of several shell-model calculations are made with the data, and general agreement is found. A comparison of photoabsorption cross sections for $^{12,13,14}\mathrm{C}$ and $^{16,17,18}\mathrm{O}$ shows dramatic redistribution of dipole strength as neutrons are added to the core nuclei.

Nuclear magnetic resonance techniques can measure the fluoride levels in bone of the finger after a patient has ingested F in the treatment of osteoporosis, but does uptake in cortical bone reflect uptake in the critical trabecular bone? Investigation has been made of the relative uptake of fluoride from drinking water into trabecular and cortical bone of the rat. For fine detail of the uptake of F into the femur and vertebra, microprobe techniques were used with a spatial resolution of 10 microns; for broader studies, treating the femur as representative of cortical bone and the vertebra as typical of trabecular bone, chemical techniques using spectroscopy and ion-selective electrodes were employed. The conclusion is that in the rat uptake of F by cortical bone is indicative of uptake by trabecular bone, and that therefore as a working hypothesis NMR measurement of F in the finger may be taken as reflecting uptake of F by trabecular bone.

Two new facilities for in vivo activation analysis of patients have been designed, developed, and constructed at Toronto General Hospital One of these is for the determination of body calcium for the diagnosis of osteoporosis and other diseases associated with bone loss. The other is for the measurement of total body nitrogen for the determination of protein status. These facilities replace old university facilities and take into account the comfort and management of patients. In addition, in the case of the calcium facility, the precision of the measurements has been improved because of larger detector volume and increased neutron source strength. Both the facilities are now in routine hospital clinical use.

The design and construction of a hospital clinical facility for in vivo prompt gamma neutron activation analysis for total body nitrogen (TBN) measurement is described. The use of 252Cf neutron sources gives a better signal-to-background ratio compared with 238Pu-Be sources of equal strength, thus yielding better reproducibility of measurements. By measuring the hydrogen and nitrogen signals separately using appropriate gating circuits, signal-to-background ratio is further improved. Measurements using a urea phantom (5.63 kg nitrogen as urea in 34.53 kg of water) show that 2 x 6 micrograms 252Cf sources gives a nitrogen signal-to-background ratio of 5.6 (compared with 3.4 in the case of a 2 x 10 Ci 238Pu-Be source) and a reproducibility for nitrogen signal of +/- 1.1% (CV) and for hydrogen signal (internal standard) of +/- 2.33% (CV). Approximately 30 minutes of patient's time is required for each TBN measurement with an estimated reproducibility of +/- 3.8% (CV). The radiation dose to the patient is about 0.2 mSv (effective dose equivalent; QF = 10) per 20 min measurement. A report for the clinician is produced within a few minutes after the measurement by a dedicated IBM-PC computer. The entire facility is clean, comfortable and the electronics and computer processing are simple and economical.

This Brief Report presents a reanalysis of the energy scale of published measurements by Johnson {ital et} {ital al}. and Jury {ital et} {ital al}. of the {sup 17}O({gamma},{ital n}{sub 0}) cross section which brings the two sets into good agreement. This leads directly to new ({ital T}{sub {lt}}) isospin assignments for levels previously reported by Ajzenberg-Selove at energies of 14.4, 15.2, and 15.6 MeV.

A major problem in the measurement of nitrogen in the body by in vivo neutron activation analysis is the size of the background. Investigations show that random summing of gamma rays in the range 4-7 MeV is a major contributor. By direct comparison, 252Cf is shown to be a better neutron source than Pu-Be in this regard. Data are presented on the contribution to the background of water and chloride in the body.

Two independent measurements of the $^{15}\mathrm{N}$(\ensuremath{\gamma},n) reaction cross section have been made using both an enriched-gas target, and one of ${(}^{15}$${\mathrm{NH}}_{4}$${)}_{2}$${\mathrm{SO}}_{4}$. The results are self-consistent, but reveal discrepancies with an earlier measurement of this cross section, particularly in the giant dipole resonance region, and with a measurement of the $^{15}\mathrm{N}$(\ensuremath{\gamma},${n}_{0}$) cross section. These discrepancies are discussed and in general resolved. The results are used in conjunction with other isospin-selective reactions data on $^{15}\mathrm{N}$ to elucidate the isospin distribution of E1 states in $^{15}\mathrm{N}$.