Post-1980 corrections to the Climax,
Colorado
Neutron Monitor Counting Rate
John A. Simpson
University of Chicago
933 E. 56th St.,
Chicago, IL 60637, USA
K. Roger Pyle
Bartol Research Institute
University of Delaware
Newark, DE 19716, USA
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INTRODUCTION
The Climax, Colorado neutron monitor was established in 1950 (Simpson,
1955), and is the oldest continuously-operating neutron monitor,
covering more than four 11-year solar modulation cycles, or more than
two 22-year solar magnetic cycles. This neutron monitor, at 11,400 foot
elevation, is an IGY-design monitor, with two sections of six tubes
each, and has a vertical cutoff rigidity of ~3 GV (see Pyle, 1993 for a
complete description of the University of Chicago neutron monitors).
Over the 47-year history of its operation, great care has been taken to maintain the stability of atmospheric pressure measurements and the configuration and response of the monitor itself. Recently, a precise comparison (Ahluwahlia, 1995) of yearly averages of the counting rates from the Climax and Deep River (1956-1995) monitors indicated that, if the two monitors were normalized at the 1965 solar minimum modulation period, a small difference appeared at the 1987 solar minimum, and has persisted into the current approach to solar minimum modulation.
Since the Climax monitor has been and is used extensively as a measure of the level of galactic cosmic ray flux over nearly a half-century, it is important that we maintain its long-term accuracy to much better than one percent. Accordingly, we have performed a careful analysis of the Climax data in order to identify possible causes for this change relative to Deep River. We believe that we have found the cause of this change, and can measure the effect quite precisely.
From time to time, especially during our annual station calibrations, testing of individual tubes results in a decision to replace certain tubes with spares. We have carefully examined these time periods, and looked at changes in the ratio of the Climax counting rate to seven other neutron monitors. From these comparisons we have found that twice, when a large number of tubes was changed, all of the ratios of Climax to other monitors changed by ~-1.1 percent (September 1980) and ~+2.3percent (August 1981), resulting in a net correction for all data after August 1981 of ~+1.2 percent.
We will present detailed comparisons of the revised Climax counting rates with those of other neutron monitors. The revised data is being made available immediately to the scientific community, and has recently been submitted to World Data Center A (Boulder).
DATA ANALYSIS
Over the long history of the Climax Monitor, individual tubes have many times been replaced because of a) outright failure or, b) a developing noise problem. In general, care has been taken to ensure that the ratio of the counting rates from the two sections remained unchanged when tubes were replaced or exchanged. Following, in Table 1, is a complete record of the BF3 tubes in use at Climax, from 1953 until the present. Eleven times over the 45-year period tubes were changed; usually during our annual station service visits. All of these tube changes occurred over the period 1976-1988.
TABLE 1: BF3 Tube History, Climax Neutron Monitor
YY-YY A section B section Date Changed ##
----- ------------------------------ ----------------------------- ----------- -- --
53-76 1414 1443 1417 1423 1427 1412 D4 1419 1420 1446 1442 1444 26 Sep 1953 SS*
76-77 1412 1414 1417 1423 1427 1443 D56 1419 1420 1442 1444 1446 03 Aug 1976 SS 7
77-80 1412 1414 1417 1423 1427 1443 D24 1419 1420 1442 1444 1446 02 Aug 1977 SS 1
80-81 D24 1419 1420 D22 D31 D26 1412 1414 1417 1423 1427 1443 09 Sep 1980 SS 12
81-83 G413 G414 G460 1442 1444 1446 D24 1417 1420 G461 G462 G463 28 Aug 1981 SS 12
83-84 G413 G414 G460 1442 1444 1446 D24 1417 1444 G461 G462 G463 09 Oct 1983 SS 1
84-85 G413 G414 G460 1419 D26 D46 D24 1417 1444 G461 G462 G463 29 Aug 1984 3
85-85 G413 G414 G460 D26 1442 1419 G461 G462 G463 1414 1417 1444 15 Mar 1985 SS 9
85-86 G413 G414 G460 1419 1420 D31 G461 G462 G463 1414 1417 1444 20 Jul 1985 3
86-88 G413 G414 G460 G102 G107 G099 G461 G462 G463 1414 1417 1444 18 Oct 1985 SS 3
88-Now G413 G414 G460 G107 G102 D32 G461 G462 G463 1417 D46 D27 10 Oct 1988 SS 4
## = Number of tubes changed or moved SS = Station Service
G413,414,460 461,462,463 are G36- series; G102,107,099 are G39- series.
The analysis method we chose was to compare the counting-rate ratios of the Climax monitor to a set of well-established monitors, which cover the period 1976-1988.
Daily-average counting rates from the following seven monitors were obtained from World Data Center C2 (Japan), courtesy of Takashi Watanabe at Ibaraki University:
Deep River, Kiel, Jungfraujoch, Thule, Calgary, Mt. Washington, and Durham. The daily average count rates for these eight monitors are plotted in Figure_1.