This research was sponsored by the National Science Foundation Grant NSF PHY-931478

Chowdhuri, Z.

Dewey, M.S.

Gilliam, D.M.

Greene, G.L.

Keith, C.D.

Richardson, J.M.

Snow, W.M.

English

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AN ABSOLUTE MEASUREMENT OF NEUTRON FLUX 
USING CALORIMETRY 
Z. Chowdhuri, C.D. Keith, and W.M. Snow 
Indiana Universi ty ,  Bloomington,  Indiana 4 74 0 8  
M.S. Dewey, D.M. Gilliam, and G.L. Greene 
Nat ional  Ins t i tu te  of Standards and Technology, Gaithersburg, Maryland 20899 
J.M. Richardson 
Science Research Laboratory, Somervi l le ,  Massachuset ts  02149 
We are developing an instrument that can measure the flux (neutrons per second) of 
a low energy neutron beam to an absolute accuracy below 0.1%. The primary motivation 
for the proposed work is to improve the accuracy of the NIST-Indiana measurement of the 
decay rate of the neutron.' The effort in progress at NIST is expected to be limited to 
an accuracy of 0.3% without further progress in the accuracy with which the neutron flux 
can be measured. A calibration of the neutron flux monitor used in the neutron lifetime 
experiment with our proposed instrument would improve the accuracy of the lifetime to 
the 0.1% level, which would make it the most accurate measurement of the neutron lifetime 
to date. 
Our instrument will measure neutron flux by sensing the heat produced by neutron ab- 
sorption in a target which absorbs the entire beam and thermalizes the reaction products.273 
For neutron fluxes in the range of interest (lo6 to lo8 neutrons per second), the total heat 
generated in a totally absorbing target is in the microwatt range. The required sensitivity 
can be attained by cooling the target to temperatures near or below 4 K with a helium 
cryost at. 
Tests of the sensitivity of the device to a 1.25 pW electrical heat input have already 
shown a statistical error of less than 0.2% in 20 minutes and a systematic error of less than 
0.01 %. Initial test runs at IUCF at a temperature of 4.5 K have reproduced the sensitivity 
achieved at NIST. 
We are now constructing a new helium cryostat to reach lower temperatures (5  1.5 K). 
The lower temperature will decrease the heat capacity of the target, which will increase 
the sensitivity of the calorimeter. We will also cool crucial parts of the temperature control 
bridge circuit to liquid helium temperature to reduce room temperature noise. We have 
also developed a new calorimeter target based on a 'Li-?dg alloy that has a higher lithium 
content than the current target, a 6 ~ i P b  compound. 
Ultimately, it would be highly desirable to verify in an independent measurement that 
all of the energy released in the neutron absorption reaction appears as heat in the target. 
For example, defects created in the target will not anneal at low temperatures. We are 
pursuing two possible techniques: measurements with a monoenergetic alpha source of 
known absolute activity and intercomparison with a carefully-designed Faraday cup on a 
monoenergetic proton beam. 
1. M.S. Dewey, G.L. Greene, and W.M. Snow, Proc. of the XI1 Moriond Conference, ed. 
J. Tran Than Van (in press). 
2. R.G.H. Robertson and P.E. Koehler, Nucl. Instrum. & Methods A251, 307 (1986). 
3. J.M. Richardson, T.E. Chupp, J.F. Wilkerson, and R. G.H. Robertson, Nucl. Instrum. 
& Methods A306, 291 (1991); J.M. Richardson, Ph.D. thesis, Harvard University 
(1993). 
4. D.M. Gilliam, G. Lamaze, and G.L. Greene, Nucl. Instrum. & Methods A284, 220 
(1989); W.M. Snow and D.M. Gilliam, Bull. Am. Phys. Soc. 37, 888 (1992). 
DEVELOPMENT OF MICROSTRIP GAS CHAMBERS 
K.P. Browne, N. Jenkins, W.K. Pitts, B.P. Reteneller, and K.M. Walsh 
Univers i ty  of Louisville, Physics Depar tment  and Electrical 
Engineering Depar tment ,  Louisville, K e n t u c k y  40292 
K. Solberg 
Indiana Universi ty  Cyclotron Facility, Bloomington,  Indiana 4 74 0 8  
We report here the first results of a project to develop microstrip gas chambers (MS- 
GCs) for use in nuclear physics experiments. The project is a collaborative effort between 
researchers in the Physics Department and the Electrical Engineering Department at the 
University of Louisville and the Wire Chamber Laboratory at the IUCF. The MSGC is 
a multiwire proportional chamber (MWPC) fabricated using the photolithographic and 
microfabrication techniques developed for the semiconductor industry.' The anode wires 
of the MWPC are replaced by metal traces on a glass substrate in the MSGC. The precise 
control of the size (R 10-pm wide anodes) and spacing (as small as 50-pm anode-cathode 
separation) can result in a detector with position resolution of i~ 50-pm, nanosecond tim- 
ing, radiation hardness, and measured count rate capability2 of - 1 x lo6  counts/mm2/sec. 
One ultimate goal of this project is the production of a MSGC detector suitable 
for use in the demanding environments of storage rings such as the Cooler and LISS. 
In particular, the MSGC offers a radiation-hardened alternative to the large area silicon 
detectors that are commonly used in Cooler e ~ ~ e r i m e n t s . ~  One particularly intriguing 
application, for example, would be "tagging" the atomic state of the pionium (a+n-) 
atom through detection of the X-ray emitted as the atom is formed. Another possible 
application might be using an MSGC array as the first detector element in the small angle 
spectrometer required for measurements of the charge-symmetry-violating 2H(d, T ' ) ~ H ~  
reaction. 
The significant accomplishment to date has been the successful operation of a detector 
fabricated in-house at the University of Louisville. A detector was reproduced from a 
commercially produced master photomask, and designed using an electrode configuration 
known to give good performance.4 This electrode structure has a pitch of 1 mm, with an 
active area of 40 mm x 40 mm (Fig. 1). The metal layer is 0.1 pm chromium, deposited 
on soda-lime glass. All anodes were coupled in common to an Ortec 124 preamp and 472 


An Absolute Measurement of Neutron Flux Using Calorimetry

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An Absolute Measurement of Neutron Flux Using Calorimetry

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