  A stack of thin Nb foils was irradiated with the 100 MeV proton beam at Los
Alamos National Laboratory's Isotope Production Facility, to investigate the
$^{93}$Nb(p,4n)$^{90}$Mo nuclear reaction as a monitor for intermediate energy
proton experiments and to benchmark state-of-the-art reaction model codes. A
set of 38 measured cross sections for $^{\text{nat}}$Nb(p,x) and
$^{\text{nat}}$Cu(p,x) reactions between 40-90 MeV, as well as 5 independent
measurements of isomer branching ratios, are reported. These are useful in
medical and basic science radionuclide productions at intermediate energies.
The $^{\text{nat}}$Cu(p,x)$^{56}$Co, $^{\text{nat}}$Cu(p,x)$^{62}$Zn, and
$^{\text{nat}}$Cu(p,x)$^{65}$Zn reactions were used to determine proton
fluence, and all activities were quantified using HPGe spectrometry. Variance
minimization techniques were employed to reduce systematic uncertainties in
proton energy and fluence, improving the reliability of these measurements. The
measured cross sections are shown to be in excellent agreement with literature
values, and have been measured with improved precision compared with previous
measurements. This work also reports the first measurement of the
$^{\text{nat}}$Nb(p,x)$^{82\text{m}}$Rb reaction, and of the independent cross
sections for $^{\text{nat}}$Cu(p,x)$^{52\text{g}}$Mn and
$^{\text{nat}}$Nb(p,x)$^{85\text{g}}$Y in the 40-90 MeV region. The effects of
$^{\text{nat}}$Si(p,x)$^{22,24}$Na contamination, arising from silicone
adhesive in the Kapton tape used to encapsulate the aluminum monitor foils, is
also discussed as a cautionary note to future stacked-target cross section
measurements. \emph{A priori} predictions of the reaction modeling codes CoH,
EMPIRE, and TALYS are compared with experimentally measured values and used to
explore the differences between codes for the $^{\text{nat}}$Nb(p,x) and
$^{\text{nat}}$Cu(p,x) reactions.
