Grapefruit and Chicken Salad

2 medium grapefruit
1/2 cup plain yogurt
1/4 cup mayonnaise
1 tablespoon minced fresh dill, or 1 teaspoon dried dill weed
salt and freshly ground pepper
1-1/4 pounds skinless, boneless chicken breasts, poached and cut into 1/2-inch pieces
1/2 cup chpped celery
2 tablespoons chopped pimiento (optional) springs of fresh dill for garnish

Cut each grapefruit in half crosswise. With a curved, serrated grapefruit knife, cut around each section to separate the fruit from the membrane. Remove the grapefruit sections and cut into bite-size pieces; set aside to drain. Scrape the remaining membrane from the shells and discard; reserve shells. In a medium-size bowl, combine yogurt, mayonnaise, dill, and salt and pepper to taste. Stir to blend well. Add grapefruit, chicken, celery, scallions, and pimiento; toss ligthly to blend. Cover and regrigerate until serving time. Wrap the grapefruit shells and refrigerate separately. To serve, scoop about 3/4 cup of chicken salad into each grapefruit sheel. Garnish with a sprig of dill.