  Modern management of research is increasingly based on quantitative
bibliometric indices. Nowadays, the h-index is a major measure of research
output that has supplanted all other citation-based indices. In this context,
indicators that complement the h-index by evaluating different facets of
research achievement are compelling. As an additional bibliometric source that
can be easily extracted from available databases, we propose to use the number
of distinct journals Nj in which an individual's papers were published. We show
that Nj is independent of citation counts, and argue that it is a relevant
indicator of research scope, since it quantifies readership extent and
scientific multidisciplinarity. Combining the h-index and Nj, we define a
two-dimensional index (H,M) that measures both the output (through H) and the
outreach (through M) of individual's research. In order to probe the relevance
of this two dimensional index, we have analysed the scientific production of a
panel of physicists belonging to the same Department but with different
research themes. The analysis of bibliometric data confirms that the two
indices are uncorrelated and shows that while H reliably ranks the impact of
researchers, M accurately sorts multidisciplinary or readership aspects. We
conclude that the two indices together offer a more complete picture of
research performance and can be applied to either individuals, research groups
or institutions.
