A shielded cave was constructed and evaluated for use in analyzing low-activity environmental samples. Design of an Inexpensive, Flexible, Shielded Cave for Environmental Radioactivity Measurements Steven C. Dewey, Kimberlee J. Kearfott, Connor P. Henley, and Sara M. Bernal* Abstract: A graded-absorber radiation shield (cave) was designed with low cost and flexibility to lower background for counting environmental and other samples containing small amounts of radioactivity. Health Phys. 88(Supplement 2):S110 –S114; 2005 Key words: operational topic; shielding; en- vironmental assessment; gamma radiation INTRODUCTION A graded-absorber radiation shield was designed with low cost and flexibility to lower back- ground for counting environ- mental and other samples con- taining small amounts of radioactivity. The cave is com- prised of 300 lead bricks, which are staggered and arranged to provide 10 cm of shielding on all sides (Fig. 1). The lead shielding contains shims that are inserted to block inexact fits caused by using non-machined bricks. As a result, the shielding effectively stops most background gamma rays from entering the testing re- gion. However, characteristic x rays are produced in the lead as a result of photon absorption pro- cesses. To remove these photons, an inner layer comprised of 0.16 cm copper plate and 0.08 cm tin sheeting is added to the inside of the lead. The shield was con- structed in approximately 60 person-h and is movable due to its modular construction. The brick construction allows the flexibility for inserting HPGe de- tectors, requiring large external dewars or electrical cooling tubes, through holes that may be bored through individual bricks as needed. The interior space (Fig. 2) is approximately 176 L, allowing for the study of radioactivity in large objects, while the cost for materials is 30% lower than that of commercially purchased shields. The size allows multiple detectors to be placed in the shield for simultaneous or high efficiency measurements and al- lows them to be located a sub- stantial distance from the walls to further lower background. A slid- ing lid (Fig. 3) is incorporated into a separate steel stand to al- low for easy access to the interior while eliminating the need for careful leveling. Lead drapes are added over the intersection be- tween the lid and chamber body to further increase shielding. The performance of the shield is char- acterized by displaying back- ground gamma ray spectra inside and outside of the cave. METHODS Construction The cave was constructed with the intent of housing a germa- nium or sodium iodide detector; * Nuclear Engineering and Radiological Sciences, University of Michigan, Ann Arbor, MI 48109- 2104. Steven Dewey is currently working towards a Ph.D. in the area of radiological, environmental, and medical physics at the University of Michigan. Steve is board- certified in health physics and has a wide range of experience in the areas of non-ionizing radiation, diagnostic medical physics, industrial health physics and environmental health physics. Steve has served as a health physicist in the United States Air Force for the past 11 years and is currently an active duty Major. He has been stationed in Boston, San Antonio, Germany, and at the University of Michigan. Steve is currently working on the use of multiple geometry high purity germanium spectros- copy measurements to profile contamination spatial distributions in materials. Corre- sponding author for this article is Kimberlee Keartfott and her email is kearfott@umich.edu. Figure 1. Outside view of cave with lid in the open position. Figure 2. Inside view of the cave showing details of the interior construction includ- ing copper insides and flashing on cor- ners. Operational Topic S110 May 2005